^The Evolution of the Southern California Uplift, 1955 Through 1976 U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1342 [ THE EVOLUTION OF THE SOUTHERN CALIFORNIA UPLIFT, 1955 THROUGH 1976 .....rr2Landsat image showing cumulative uplift within the central part of the southern California uplift during the period 1969.0-74.5. Contour interval, 0.05 m, dashed where approximately located; hachures denote area of closed low.The Evolution of the Southern California Uplift, 1955 Through 1976 By ROBERT O. CASTLE, MICHAEL R. ELLIOT, JACK P. CHURCH, and SPENCER H. WOOD U.S. CEOLOGICAL SURVEY PROFESSIONAL PAPER 1342 A description of the changing configuration of the southern California uplift from its inception to its partial collapse UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1984DEPARTMENT OF THE INTERIOR WILLIAM P. CLARK, Secretary U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director Library of Congress Cataloging in Publication Data The evolution of the southern California uplift, 1955 through 1976. (U.S. Geological Survey Professional Paper 1342) Bibliography: p.131-136 Supt.ofDocs.no.: 119.16:1342 1. Earth movements—California, southern. I. Castle, Robert Oliver, 1925- . II. Series: United States. Geological Survey. Professional Paper 1342. QE598.2.E93 1984 551.8'7 84-600305 For sale by the Distribution Branch, Text Products Section, U.S. Geological Survey, 604 South Pickett St., Alexandria, VA 22304CONTENTS Page Abstract_____________________________________________ 1 Introduction___________________________________________ 2 Acknowledgments ___________________________________ 4 Geologic framework_____________________________________ 5 Historical surface deformation _______________________ 11 Nontectonic deformation___________________________ 11 Artifically induced deformation ______________ 11 Vertical displacements associated with ground-water withdrawal and recharge---- 12 Vertical displacements associated with oilfield and gas-field operations------------- 13 Naturally induced deformation_________________ 14 Tectonic deformation______________________________ 14 Vertical-control data ________________________________ 14 Errors in height determinations------------------- 16 Systematic error______________________________ 16 Random error _________________________________ 18 Errors associated with continuing crustal deformation ________________________________ 19 Errors associated with imprecisely formulated orthometric corrections_____________________ 21 The reconstruction____________________________________ 24 The Ventura-Avila Beach line______________________ 26 The Ventura-Maricopa line________________________ 33 The Los Angeles-Mojave line----------------------- 43 The Los Angeles-San Bernardino line_______________ 59 The Orange-Barstow line___________________________ 62 Page The reconstruction—Continued The Quail Lake-Hesperia line__________________________ 67 The Mojave-Cottonwood Pass line_______________________ 70 The Colton-Mecca line_________________________________ 80 The Cottonwood Pass-Parker Dam line___________________ 89 The Lucerne Valley line ______________________________ 93 The Cottonwood Pass-Frink line________________________ 96 The Ocotillo-Ogilby line______________________________ 97 Selected stages in the evolution of the southern California uplift_______________________________________ 98 1955-59.0 ____________________________________________ 99 1959.0- 60.5_______________________________________ 99 1959.0- 61.4________________________________________ 99 1959.0- 62.0 ________________________________________102 1959.0- 65.5 ___________________________________ 105 1959.0- 69.0________________________________________ 107 1959.0- 70.0_______________________________________ 107 1959.0- 71.5_________________________________________110 1959.0- 73.0_________________________________________110 1959.0- 74.5_________________________________________112 1959.0- 77.0_________________________________________116 Other examples of aseismic uplift_________________________118 Seismicity associated with the southern California uplift__________________________________________________123 The origin of the southern California uplift______________123 Conclusion _______________________________________________130 References cited__________________________________________131 ILLUSTRATIONS [Plates are in case] Frontispiece. Landsat image showing cumulative uplift within the central part of southern California uplift. PLATES 1-5. Maps showing: 1. Major topographic features and naturally defined provinces of southern California 2. Generalized geology of southern California 3. Areas of fluid extraction in southern California 4. Subsidence attributed to fluid extraction in southern California 5. Principal routes and dates of levelings as used in this report 6-15. Profiles showing: 6. Changes in height along the Ventura-Avila Beach line 7. Changes in height along the Ventura-Maricopa line 8A,B. Changes in height along the Los Angeles-Mojave line 9. Changes in height along the Los Angeles-San Bernardino line 10. Changes in height along the Orange-Barstow line 11. Changes in height along the Quail Lake-Hesperia line 12. Changes in height along the Mojave-Cottonwood Pass line 13. Changes in height along the Colton-Mecca line 14. Changes in height along the Cottonwood Pass-Parker dam line 15. Changes in height along the Lucerne Valley, Cottonwood Pass-Frink, and Ocotillo-Ogilby lines 16. Map of southern California showing earthquakes of magnitude s4, January 1, 1932- December 31, 1976 VCONTENTS VI Page FIGURE 1. Index map of California showing area of study____________________________________________________________ 3 2. Map showing an early representation of the southern California uplift______________________________ 4 3. Schematic diagram showing effect of subsidence on observed elevations derived from discontinuous levelings________________________________________________________________________________________ 19 4. Map showing misclosures around the circuit Saugus-Lebec-Bakersfield-Mojave-Palmdale-Saugus based on levelings of 1926 and 1953/55 ___________________________________________________________ 24 5. Graph showing changes in mean sea level at Los Angeles with respect to San Diego__________________ 26 6-9. Maps showing misclosures around the circuits: 6. Gaviota-Surf-Pismo Beach-Buellton-Gaviota based on 1920 and 1927 levelings__________________ 28 7. Surf-Avila Beach-Harris-Gaviota-Surf, Surf-Avila Beach-Harris-Surf, and Surf-Harris-Gaviota-Surf based on 1956 and 1960 levelings__________________________________________________________ 29 8. Santa Barbara-Santa Maria-San Luis Obispo-McKittrick-Maricopa-Ventura-Santa Barbara based on 1948, 1956/57, 1934/35, 1939, and 1942 levelings_________________________________________ 30 9. Surf-San Luis Obispo-McKittrick-Maricopa-Ventura-Gaviota-Surf based on 1956/57, 1934/35, and 1960 levelings_____________________________________________________________________________ 32 10. Graph showing changes in orthometric height at bench mark I 30, Ventura____________________________ 34 11. Profiles showing changes in height along National Geodetic Survey monitor line southwest of Maricopa 36 12-15. Maps showing misclosures around the circuits: 12. San Pedro-Ventura-Ozena-Lebec-San Pedro based on 1920, 1934/35, 1942/43, and 1926 levelings_ 37 13. San Pedro-Oxnard-Ventura-Ozena-Lebec-Sandberg-Palmdale-Saugus-Los Angeles-San Pedro based on leveling carried out during the period April 1959-May 1961_______________________________ 40 14. Lebec-Ozena-Maricopa-Greenfield-Lebec based on 1959 leveling________________________________ 41 15. Castaic-Fairmont-Palmdale-Saugus-Castaic and Castaic-Sandberg-Fairmont-Castaic based on leveling carried out during the period March 1953-May 1961__________________________________ 41 16. Graph showing changes in orthometric height at bench mark S 32, Los Angeles________________________ 45 17. Map showing misclosure around the circuit San Pedro-Oxnard-Ventura-Ozena-Lebec-Sandberg- Saugus-Los Angeles-San Pedro based on leveling carried out during the period January 1968- April 1969________________________________________________________________________________________ 46 18. Map showing misclosure around the circuit San Pedro-Los Angeles-Saugus-Palmdale-San Bernardino-San Pedro based on leveling carried out during the period November 1972-November 1974_________________ 47 19. Map showing misclosure around the circuit Saugus-Castaic-Sandberg-Lancaster-Palmdale-Saugus carried out during the period November 1972-June 1974 ____________________________________________________ 48 20. Graph showing changes in orthometric height at bench mark J 52, Saugus_____________________________ 49 21. Map showing misclosure around the circuit Castaic Junction-San Fernando-Topanga Canyon- Ventura-Castaic Junction based on leveling carried out during the period March 1973-January 1975__ 50 22. Map showing misclosure around the circuit Saugus-Lebec-Grapevine-Bakersfield-Caliente-Mojave- Rosamond-Palmdale-Saugus based on leveling carried out during the period March 1972-February 1974 51 23. Graph showing changes in orthometric height at bench mark 3219 USGS, Vincent_______________________ 52 24. Graph showing changes in orthometric height at bench mark D 430, Palmdale__________________________ 53 25-28. Maps showing misclosures around the circuits: 25. Los Angeles-Burbank-Saugus-Palmdale-Mojave-Boron-Barstow-Oro Grande-Hesperia-Colton-Azusa- Los Angeles based on leveling carried out during the period March-November 1961_____________ 54 26. Palmdale-Rosamond-Mojave-Boron-Barstow-Oro Grande-Hesperia-Llano-Palmdale based on leveling carried out during the period February 1960-November 1961__________________________ 55 27. Azusa-Big Pines-Llano-Hesperia-Cajon Junction-Colton-Riverside-Azusa based on leveling carried out during the period February 1960-June 1962_______________________________________________ 55 28. Azusa-Los Angeles-Burbank-Saugus-Llano-Big Pines-Azusa based on leveling carried out during the period February 1960-June 1962__________________________________________________________ 56 29. Graph showing changes in orthometric height at bench mark Q 49, Mojave___________________________ 56 30. Graph showing changes in orthometric height at bench mark Boundary Monument 2, Lebec_____________ 57 31. Map showing misclosures around the circuits Castaic-Sandberg-Fairmont-Castaic and Castaic-Fairmont- Palmdale-Saugus-Castaic based on 1964 levelings___________________________________________________ 58 32. Profiles showing changes in height along primary vertical control line through La Canada___________ 62 33. Map showing misclosure around the circuit Azusa-Los Angeles-Burbank-Saugus-Palmdale-Llano- Big Pines-Azusa based on leveling carried out during the period November 1970-October 1971________ 63 34. Graph showing changes in orthometric height at bench mark E 43, Barstow__________________________ 65 35. Graph showing changes in orthometric height at bench mark D 39, Colton___________________________ 65 36. Profile showing postulated form of cumulative uplift developed between Colton and Hesperia between 1956 and the spring of 1961_______________________________________________________________________ 66 37. Map showing misclosure around the circuit Colton-Victorville-Lucerne Valley-Colton based on leveling carried out during the period September-November 1961--------------------------------------------- 67 38. Graph showing changes in orthometric height at bench mark 3409 USGS, Llano_________________________ 70CONTENTS VII Page FIGURE 39. Graph showing changes in orthometric height at bench mark E 41, Hesperia-------------------------------- 70 40. Schematic diagram illustrating effects of postulated 1944-61 tilts over unspecified reach between Daggett and Amboy_________________________________________________________________________________________ 73 41. Profile showing changes in height over a 15-km reach near Ludlow__________________________________ 74 42. Profiles showing changes in height between Kramer Junction and Barstow____________________________ 75 43^49. Maps showing misclosures around the circuits: 43. Barstow-Daggett-Amboy-Cadiz-Freda Junction-Cottonwood Pass-Twentynine Palms-Lucerne Valley-Victorville-Barstow and Barstow-Daggett-Amboy-Twentynine Palms-Yucca Valley-Lucerne Valley-Victorville-Barstow based on levelings carried out during the periods 1931-November 1961 and January 1944-November 1961, respectively____________________________ 76 44. Newberry Springs-Amboy-Twentynine Palms-Yucca Valley-Lucerne Valley-Newberry Springs and Amboy-Cadiz-Freda Junction-Cottonwood Pass-Twentynine Palms-Amboy based on levelings carried out during the periods January-April 1944 and 1931-April 1944, respectively________ 77 45. Included within the larger circuit Barstow-Amboy-Freda Junction-Cottonwood Pass-Yucca Valley- Victorville-Barstow based on levelings carried out during the period 1931-November 1961____ 78 46. Colton-Saugus-Palmdale-Rosamond-Moj ave-Boron-Barstow-Lavic-Amboy-Twentynine Palms-Mecca- Banning-Colton and Colton-Saugus-Palmdale-Llano-Hesperia-Colton based on levelings carried out during the period November 1972-July 1976 _____________________________________________ 83 47. Cabazon-Morongo Valley-bench mark ID (MWD)-Coachella-Cabazon based on leveling carried out during the period March 26, 1931-June 12, 1931_____________________________________________ 84 48. White Water-Yucca Valley-Twentynine Palms-Mecca-White Water based on leveling carried out during the period March 1974—September 1976 _____________________________________________ 86 49. San Pedro-La Canada-Palmdale-Llano-Hesperia-Colton-San Pedro based on leveling carried out during the period July 1973-April 1977_______________________-_____________________________ 87 50. Graph showing changes in orthometic height at bench mark H 516, Mecca_____________________________ 89 51. Map showing misclosure around the circuit Cottonwood Pass-Twentynine Palms-Amboy-Freda Junction- Cottonwood Pass based on leveling completed during the period March 1974—June 1976________________ 91 52. Map showing misclosure around the circuit Mecca-White Water-Yucca Valley-Twentynine Palms-Amboy- Freda Junction-Mecca based on leveling carried out during the period March 1974—September 1976 ___ 92 53. Graph showing changes in orthometric height at bench mark V 325, Lucerne Valley____________________ 94 54. Graph showing changes in orthometric height at bench mark R 41, Victorville______________________ 95 55-65. Maps showing height changes within the area of the southern California uplift: 55. 1955-59.0__________________________________________________________________________________ 100 56. 1959.0-60.5 __________________________________________________________________________________101 57. 1959.0-61.4 __________________________________________________________________________________103 58. 1959.0-62.0 ________________________________________________________________________________ 104 59. 1959.0-65.5 __________________________________________________________________________________106 60. 1959.0-69.0 _______________________________________________________________________________ 108 61. 1959.0-70.0 __________________________________________________________________________________109 62. 1959.0-71.5 __________________________________________________________________________________111 63. 1959.0-73.0 _________________________________________________________________________________ 113 64. 1959.0-74.5 __________________________________________________________________________________114 65. 1959.0-77.0 __________________________________________________________________________________117 66. Projections into north-south section showing height changes associated with both the early-20th-century uplift and the modern uplift______________________________________________________________________120 67. Graph showing changes in orthometric height at bench mark 3219 USGS, Vincent, since 1897/1902_____121 68. Graphs showing length changes along six lines within the central part of the southern California uplift during the period 1959-77 ________________________________________________________________________125 69. Schematic representation of layered lithosphere astride the plate boundary in southern California_128Any use of trade names and trademarks in this publication is for descriptive purposes only and does not constitute endorsement by the U.S. Geological Survey.THE EVOLUTION OF THE SOUTHERN CALIFORNIA UPLIFT, 1955 THROUGH 1976 By Robert O. Castle, Michael R. Elliot, Jack R Church, and Spencer H. Wood ABSTRACT The southern California uplift culminated in 1974 as a 150-km-wide crustal swell that extended about 600 km eastward and east-southeastward from Point Arguello to the Colorado River and Salton Sea, respectively; it was characterized by remarkably uniform height changes between 1959 and 1974 of 0.30-0.35 m over at least half of its 60,000-70,000 km2 area. At its zenith, the uplift included virtually the entire Transverse Ranges geologic province and parts of the Coast Ranges, San Joaquin Valley, Sierra Nevada, Basin and Range, Mojave Desert, Peninsular Ranges, and Salton Trough provinces. The aline-ment of the western part of the uplift closely paralleled the east-trending Transverse Ranges, whereas the southern flank of the eastern lobe roughly coincided with the west-northwesttrending San Andreas fault. The position and configuration of the uplift associate it with a singularly complex section of the boundary between the North American and Pacific plates that has certainly sustained major modification during the past 5 million years and probably during the past 1 million years. Surface deformation can be categorized as tectonic or non-tectonic. Nontectonic vertical displacements associated with the activities of man have overwhelmed natural compaction and areally significant soil expansion in the southern California area. Because tectonic displacements are implicitly defined as those that cannot be otherwise explained, those vertical movements that can be reasonably attributed to artificial processes have been subtracted from our reconstructed configurations of the uplift. Hence this reconstruction has necessarily included the assembly and evaluation of an enormous volume of data on oil-field operations, changes in ground-water levels, and measured subsidence (or rebound) associated with changes in the underground fluid regimen. Measured changes in height at various stages in the evolution of the uplift have been based chiefly on first-order levelings carried out between 1953 and 1976. Exceptions to this generalization consist largely of the results of pre-1953 surveys through the western Transverse Ranges and the eastern Mojave Desert. Errors in measured height differences derive from blunders, systematic survey errors, random survey errors, improperly formulated orthometric corrections, and intrasurvey movement; the last of these has created the most serious problems encountered in our reconstruction of the basic data. A variety of independent tests indicate that survey error associated with the utilized levelings was generally small and fell largely within the predicted random-error range. Moreover, the redundancy and coherence displayed by the entire data set provide convincing evidence of survey accuracy and the virtual absence of height- and slope-dependent error in particular. Our reconstructions of the changing configuration of the uplift derive chiefly from comparisons among sequentially developed observed elevations along the same route. Most of the observed elevations from which the vertical displacements were computed have been reconstructed with respect to bench mark Tidal 8, San Pedro, as invariant in height. Because the San Pedro tide station has been characterized by a history of modest relative uplift, vertical displacements referred to this station are biased slightly toward the appearance of subsidence. Where the observed elevations cannot be conveniently tied to Tidal 8, they have been referred to secondary control points whose history with respect to Tidal 8 can be independently established. Each of the lines of observed elevation changes provides, accordingly, a section athwart or along the axis of the uplift from which the changes in the configuration of the uplift can be roughly generalized. Because relatively few surveys were run in 1955, which we choose as a representative temporal datum, we have commonly incorporated the results of earlier or of somewhat later levelings as the equivalents of 1955 surveys. Although this procedure introduces a certain subjectivity, the probable equivalence between the results of these earlier or later surveys with those that would have been obtained had this leveling been carried out in 1955, usually can be independently tested. Wherever the calculated vertical displacements are based on comparisons between the results of levelings over different routes, the observed elevations have been orthomet-rically corrected to agree with those that would have been produced had each of these surveys been along the same route. The growth of the southern California uplift consisted of two well-defined spasms of positive movement, the second of which was closely followed by partial collapse. Our reconstruction, although it clearly errs in detail, indicates that the uplift, together with marginal and apparently ephemeral tectonic subsidence, nucleated in the west-central Transverse Ranges near Ozena, sometime between the spring of 1959 and the spring of 1960. The uplift expanded rapidly eastward (and probably westward as well), and by the fall of 1961 much of the Transverse Ranges and the Mojave Desert at least as far east as Twen-tynine Palms had risen by as much as 0.25 m. Between 1962 and 1972 the area included by the initially developed (1959-61) uplift sustained additional but clearly decelerating uplift accompanied locally by oscillatory displacements. Between 1972/ 73 and 1974 a second crustal spasm extended the uplift eastward to the Colorado River and elevated much of the eastern Mojave Desert by values that equaled or exceeded those developed within the western lobe. Between 1974 and 1976, at least the central part of the uplift sustained partial collapse that nowhere amounted to less than 50 percent of the cumulative uplift since 1959. Whether this collapse affected the entire uplift is conjectural, but we now recognize well-defined evidence of major down-to-the-north tilting that must have occurred within the eastern part of the uplift at some time between 1974 and 1976. Accumulating evidence indicates that nearly all the area included with the southern California uplift underwent similar uplift and partial collapse during the early part of the 20th century. Thus we infer that the recent uplift represents but a single event in an ongoing, more or less cyclic deformational process characterized by a period of about 50 years. Even though less than two full cycles are expressed in the geodetic 12 THE EVOLUTION OF THE SOUTHERN CALIFORNIA UPLIFT, 1955 THROUGH 1976 record, the cumulative rate of uplift near the center of the recent uplift probably has averaged about 5 mm/yr, a value that is roughly consistent with the uplift rates that have been deduced for the late Quaternary emergent marine terraces along the south flank of the Transverse Ranges. Although the evolution of the recent uplift is relatively well defined, its correlation with the regional seismicity is poorly defined. A comparison between the occurrence of southern California earthquakes of magnitude 5=4 during the period 1932 to 1976 with the 1974 configuration of the uplift demonstrates the existence of (1) relatively aseismic areas within the western lobe of the uplift (in the western Transverse Ranges), in the central part of the uplift (in the western Mojave Desert), and along an east-trending zone that extends into the eastern Mojave athwart the south flank of the uplift (north of the Salton Sea) and (2) localized concentrations of seismic activity along the flanks of the uplift. Moreover, 9 of the 10 largest earthquakes recorded within or around the area of the southern California uplift during the period 1932 to 1976 (the 1933 Long Beach, the 1941 Santa Barbara, the 1946 Walker Pass, the 1947 Manix, the 1948 Desert Hot Springs, and the four major 1952 Kern County shocks) occurred before the inception of the uplift in 1959 or 1960. The area embraced by the southern California uplift has been identified with geodetically defined horizontal strain, part of which may have accumulated as a major north-south con-tractional event that roughly coincided with the first spasm of uplift. Nonetheless, continuing contractional strain associated with regionally developed partial collapse argues that the uplift cannot be fully explained simply as the vertical expression of continuing north-south compression. Consideration of the two well-defined historical episodes of uplift and partial collapse indicate that the southern California uplift may be the product of decoupling and viscous flow beneath the seismogenic zone, presumably driven by continuing motion between the irregularly margined plates south of the great bend of the San Andreas fault. Because the magnitude of the maximum uplift associated with each episode was approximately the same, there may be some threshold value above which collapse (viscous flow) may ensue; the absence of total collapse may be a function of precollapse strain hardening within the postulated subseis-mogenic viscoelastic layer. INTRODUCTION Examination of the vertical-control record after the 1971 San Fernando earthquake disclosed a sharply defined tilt developed between 1961 and 1964 along a survey traverse extending west-south-westward from Palmdale across what has since been identified as the south flank of the southern California uplift (Castle and others, 1974; Castle and others, 1976). The surprisingly large magnitude of this tilt (about 0.17 m over a distance of 30 km), together with its rapid growth, subsequently provoked a systematic examination of the vertical-control record along a number of level lines athwart the San Andreas fault in southern California. These studies, together with other investigations of continuing crustal deformation in the western Mojave Desert (fig. 1) (Church and others, 1974), culminated in the first published representation of the southern California uplift (Castle and others, 1976). However, this representation (fig. 2) was deliberately conservative, and both the lateral and vertical dimensions were based on what was explicitly defined as a preliminary examination of the geodetic record within an area between and adjacent to the San Andreas and Garlock faults and eastward from Maricopa to Barstow. Because our initial reconstruction of the uplift was clearly skeletal, both with respect to the areal coverage and the volume of survey data that had been generated by various agencies operating in southern California, and because this earlier reconstruction incorporated several then-unverified assumptions, the resulting portrayal (fig. 2) was highly generalized. We have since attempted to assemble all the data that could be recovered through querying those southern California jurisdictions that are known to have carried out level surveys of at least third-order accuracy. We have, in addition, commissioned new leveling along several lines in order to better define the extent and history of the uplift. This report, accordingly, summarizes and synthesizes all the vertical-control data known to have been produced through 1976, thereby permitting a still-generalized but far more detailed description of the four-dimensional configuration of the southern California uplift than was heretofore possible. The results of the relatively detailed study presented here indicate that the southern California uplift may have been at least twice as long and far more complex—both in its geometry and its evolution—than could have been deduced from our earlier reconstruction. Although the eastern third of the uplift is much more poorly defined than is that to the west, owing chiefly to the relatively few repeated surveys east of long 117° W., by 1973/74 it probably ranged eastward to Arizona and east-southeastward to the Salton Sea. Similarly, synthesis of data from west of Maricopa shows that at its zenith the uplift extended as far west as the western end of the Transverse Ranges. Hence by 1973/74 the total length of this feature probably exceeded 600 km—roughly the distance between San Francisco and Los Angeles. Moreover, in spite of continuing uncertainties, certain aspects of its configuration can now be defined relatively precisely. For example, the southern boundary of the uplift lay generally south of the coastline eastward as far as Ventura, where it projected inland and south of the Santa Susana fault system; east of the San Fernando Valley this boundary turned south-INTRODUCTION 3 FIGURE 1.—Index map of California showing area of study that includes the southern California uplift.4 THE EVOLUTION OF THE SOUTHERN CALIFORNIA UPLIFT, 1955 THROUGH 1976 eastward, and thence eastward along the frontal fault system of the Transverse Ranges and the south branch of the San Andreas fault system, respectively. The evolution of the southern California uplift includes two well-defined spasms of uplift, the second of which was closely followed by partial collapse. The uplift apparently began in the west-central Transverse Ranges at some time between the spring of 1959 and the following spring; it spread rapidly eastward, and probably westward as well, such that by the fall of 1961 much of the Transverse Ranges and the Mojave Desert eastward as far as Twentynine Palms had been elevated by as much as 0.25 m. The second major spasm, probably confined largely to the period 1972/73-74, extended and amplified the uplift eastward to the Colorado River, elevating much of the eastern Mojave by values that equalled or exceeded those developed in the western lobe between 1959 and the fall of 1961. The distribution of the subsequent collapse, which could have begun as early as the end of 1974 but no later than 1976, remains poorly defined; it certainly encompassed the central part of the uplift and probably extended east-southeastward to the Salton Sea. Local oscillatory movement associated with the changing configuration of the uplift may prove to have been an integral part of its growth. Moreover, although our knowledge of its occurrence is very limited, apparently ephemeral downwarping along the leading edges of the propagating uplift has been recognized in enough places that it can now be regarded as a generally occurring aspect associated with the growth of the uplift. ACKNOWLEDGMENTS The synthesis presented here could not have been attempted without the generous assistance FIGURE 2.—An early representation of the southern California uplift based on a preliminary examination of the geodetic record by Castle and others (1976, p. 251).GEOLOGIC FRAMEWORK 5 and cooperation of many individuals and numerous public agencies operating in southern California. Nearly half of the utilized survey data derive from the field measurements of various southern California cities and counties. In particular, we thank the Los Angeles County Department of County Engineer, the County of Orange Office of County Surveyor and Road Commissioner, the County of Riverside Survey and Road Department, the County of San Bernardino Public Works Agency, the San Diego County Engineering Department, the Ventura County Department of Public Works, the City of Los Angeles Bureau of Engineering, the City of Riverside Bureau of Public Works, the California Department of Water Resources, the California Division of Mines and Geology, the California Department of Transportation, and the Metropolitan Water District of Southern California for the development and assistance in the acquisition of survey data utilized in this report. We are especially indebted to J. F. McMillan of the Los Angeles County Department of County Engineer and J. W. Raihle and R. E. Stone of the County of Riverside Survey and Road Department for the establishment within their respective jurisdictions of vertical control critical to the reconstruction described here. We also wish to thank our many colleagues in both the Geological Survey and the National Ocean Survey who have given so generously of their time. T. D. Gilmore, E. A. Rodriguez, C. C. Smith, M. E. Wilson, and J. L. Brown of the Geological Survey have provided technical assistance throughout this study. E. I. Balazs, C. L. Gilliland, N. L. Morrison, and F. L. Smith of the National Geodetic Survey have contributed immeasurably in data evaluation and advice and counsel on various geodetic problems. J. R. Hubbard of the Tides and Currents Division of the National Ocean Survey has provided sea-level measurements from several southern California tide stations that have proved particularly helpful in our analysis. We are especially indebted to Prof. Petr Vanicek of the Department of Surveying Engineering of the University of New Brunswick for his continuing geodetic counsel and for solutions to several problems fundamental to this investigation. Finally, we thank K. R. Lajoie, J. C. Savage, and W. R. Thatcher of the Geological Survey and Prof. B. M. Page of Stanford University for numerous suggestions and thoughtful reviews of an earlier version of this report. GEOLOGIC FRAMEWORK The geology of the area embraced by the southern California uplift is characterized by a complexity that may be unparalleled in North America. Hence, while a detailed consideration of the geology of this area is clearly beyond the scope of this report, any interpretation of the movements described here requires at least a rudimentary knowledge of the major geologic features, especially of the major tectonic elements that fall within and adjacent to the area of the uplift. For simplicity and brevity, these features are summarized here chiefly in map form. Moreover, because it is certainly germane to the origins of the uplift, we also present a brief sketch of the recent tectonic history of this area. Because the uplift shows several associations with the naturally defined physiographic or tectonic provinces of southern California (pi. 1), our description of the geologic framework is organized by province. The Coast Ranges province (pi. 1) is characterized by a generally northwest-trending structural and topographic grain. The rocks exposed at the surface in the southern Coast Ranges consist chiefly of clastic Mesozoic and Cenozoic sedimentary rocks that generally rest unconformably upon or are in fault contact with older highly deformed and mildly metamorphosed rocks commonly included with the Franciscan Complex (Jahns, 1954, p. 9). The deep crust and upper mantle beneath the central to southern Coast Ranges are apparently devoid of unusual features; Bateman and Eaton (1967, p. 1409-1413) show that the Mohorovicic discontinuity underlies this region at a “normal” depth of about 25 km, and that both crustal and upper-mantle P-wave velocities are characterized by more or less expectable values. The physio-graphically defined southern Coast Ranges province is transected along its northeastern edge by the San Andreas fault (pi. 2), which crudely defines the northeastern tectonic boundary of this province. The southern boundary, on the other hand, is obscurely and almost arbitrarily defined by the gradual change in topographic and structural trend from northwest to east-west. The San Joaquin Valley (pi. 1) has served as a vast depositional basin throughout much of Late Cretaceous and Cenozoic time (Jahns, 1954, p. 9-13). The southern end of the valley contains an enormously thick terrestrial section; south of Bakersfield these deposits, which comprise nearly half the section from basement to surface, are as much as 4,000 m thick (de Laveaga, 1952, p. 102-103). The6 THE EVOLUTION OF THE SOUTHERN CALIFORNIA UPLIFT, 1955 THROUGH 1976 depth to basement, moreover, increases from 4,000-5,000 m northwest of Bakersfield to as much as 9,000 m at the southern end of the valley (de Laveaga, 1952, p. 102-103). Most of the 4,000-m terrestrial section in the southern part of the valley consists of the Kern River Formation, which ranges in age from latest Miocene to early Pleis-tocene(?) and is probably no more than 8-9 m.y. old (Bartow and Pittman, 1983, p. D12-D13). Owing to the enormous thickness of the Kern River Formation in the southernmost part of the valley, it is likely that the southern San Joaquin Valley has sustained periods of accelerating tectonic subsidence during latest Neogene and (or) Quaternary time, although it is uncertain whether accelerating downwarping has characterized the Holocene history of this area. The western, and in particular, the southern margins of the San Joaquin Valley province are associated with severe deformation. We know nothing of the deep-crustal and upper-mantle structure beneath the southern end of the valley; nevertheless, if this region is in isostatic equilibrium, the great thickness of low-density materials suggests an arching of the Mohorovicic discontinuity similar to that shown by Bateman and Eaton (1967, p. 1411) along the eastern edge of the valley north of Bakersfield. The physiographically defined eastern boundary (pi. 1) of the San Joaquin Valley province lies only slightly downslope from the contact between the underlying crystalline basement and the eastward- thinning, unconform-ably overlying Cenozoic sedimentary deposits (Smith, 1964). The southern boundary is essentially coincident with a well-defined zone of thrust faulting (Jennings, 1973), whereas the southwestern boundary, as we have already observed, virtually coincides with the San Andreas fault. The Sierra Nevada province (pi. 1) is expressed as a “huge, asymmetric, westward-tilted block” that disappears to the west beneath the sedimentary rocks of the San Joaquin Valley (Jahns, 1954, p. 13). The province is underlain chiefly by Mesozoic plutonic rocks, “together with older metamorphic rocks that appear in most areas as inclusions, roof pendants, and screens in the igneous terrane” (Jahns, 1954, p. 13). The structural complexity of the Sierra Nevada block increases toward the south, particularly at the “triple point” with the San Joaquin Valley and Coast Ranges provinces (pi. 1). Much of this province, moreover, has been involved with major faulting during Pleistocene time, especially along its eastern and southeastern boundaries. The deep-crustal and upper-mantle structure of the Sierra Nevada province is rela- tively well known. Bateman and Eaton (1967, p. 1411-1413) show a low-velocity crustal root beneath the Sierra Nevada that depresses the Mohorovicic discontinuity to depths of as much as 50 km, an observation consistent with the seeming isostatic balance exhibited by this block (Oliver, 1960). The eastern boundary of the Sierra Nevada province virtually coincides with the generally high-angle Sierra Nevada fault zone (pi. 2), whereas the western boundary of this province is again defined by the zone where this giant, tilted block dips beneath the veneer of sedimentary deposits underlying the San Joaquin Valley. The southern or southeastern boundary of the Sierra Nevada province coincides almost precisely with the Garlock fault (pi. 2), against which it is sharply truncated. The small part of the Basin and Range province (pi. 1) that impinges on this study “is characterized by north-trending ranges, intervening valleys and basins, and an interior drainage” (Jahns, 1954, p. 13). Many or most of the individual ranges included with this province are clearly distinguishable fault blocks that show conspicuous differences in geology from block to block. Expressions of both Mesozoic and Tertiary deformation are abundant throughout the Basin and Range province; it is, however, the “widespread Quaternary faulting and warping reflected by many elements of the present topography” (Jahns, 1954, p. 13) that is especially significant here. While the deep-crustal structure beneath the Basin and Range province probably is generally similar to that beneath most of southern California, several large-scale crustal features associated with this province may be especially germane to the contemporary deformation described in this report. Thus the Mohorovicic discontinuity lies at a depth of about 30 km beneath the Basin and Range province (Bateman and Eaton, 1967, p. 1141), a depth indicative of a crust slightly thickened over that identified with the Coast Ranges but of roughly the same thickness as that associated with the Transverse Ranges, the Mojave Desert, and the Peninsular Ranges provinces (Hadley and Kanamori, 1977a, p. 1474). Similarly, Braile and others (1974) recognize a low-velocity layer between 10 and 15 km deep which they interpret as a zone of low rigidity underlying the Basin and Range province; however, because their conclusion is based on seismic refraction studies carried out 500-600 km northeast of the area described here, we cannot conclude with certainty that this low-velocity layer pervades the entire province. The western boundary of the Basin and Range province is, of course, well defined by its coincidence withGEOLOGIC FRAMEWORK 7 the Sierra Nevada fault zone, whereas the southern boundary is much less clearly defined. Eastward from the Sierra Nevada province to about the meridian of Baker, the southern boundary of the Basin and Range province closely coincides with the Garlock fault (pi. 2); still farther east, however, it tends to lose both its physiographic and tectonic identity and is almost arbitrarily defined. The Transverse Ranges province (pi. 1), as its name implies, owes its definition to a generally east-west topographic grain that transects the prevailingly northwest trends that characterize most of southern California; it is perhaps the most significant of the several provinces described here, for it has been spatially identified with the southern California uplift from its inception to its partial collapse. Although the Transverse Ranges province is treated as a single feature owing to its unique and disruptive trend, it may consist of two fundamentally different structural units separated by the San Andreas fault where it cuts through Cajon Pass north of Colton (pi. 2). The east-west grain of the Transverse Ranges is displayed not only in the physiography, but by faults, fold axes, other internal structural features, and major chemical trends as well (Jahns, 1954, p. 17; Baird and others, 1974). Upper Mesozoic and Tertiary sedimentary and volcanic rocks characterize the western Transverse Ranges; eastward, the rocks of this province are generally older and of a more crystalline aspect. Physiographic evidence of intense Quaternary deformation is widespread throughout the Transverse Ranges. Locally, as in the Ventura basin, the combined stratigraphic and structural records provide compelling evidence of major and probably accelerating deformation and associated denudation and sedimentation during Quaternary time (see, for example, Yeats, 1977, p. 296). Because the Transverse Ranges straddle the San Andreas fault, whatever may be responsible for the existence of these ranges seemingly either is independent of massive displacement on the San Andreas or has persisted or regenerated in spite of continuing right-lateral movement along this fault system. Accumulating evidence suggests that the deep-crustal and upper-mantle structural configuration beneath the Transverse Ranges may be unique. Hadley and Kanamori (1977a) conclude from an analysis of seismic traveltime data that a high-velocity upper-mantle ridge underlies much of the Transverse Ranges and, like the Transverse Ranges themselves, projects across the San Andreas with little apparent offset. In order to explain the uninterrupted persistence of this postulated mantle ridge (and perhaps the Transverse Ranges as well) athwart the San Andreas, Hadley and Kanamori (1977a) suggest that the crustal and mantle-plate boundaries diverge north of the Salton Sea in such a way that the mantle-plate boundary projects northwestward toward and beyond the eastern end of the Transverse Ranges. There is no direct evidence of decoupling between crust and mantle of the sort implicit in the Hadley-Kanamori model; nevertheless, the gravity high over the San Gabriel Mountains is consistent with the absence of a root beneath this elevated structural block (Hanna and others, 1975) and hence with decoupling beneath the San Gabriel Mountains. While the extreme northwestern boundary with the Coast Ranges province and the extreme southeastern boundary with the Mojave Desert province are arbitrarily defined, the margins of the Transverse Ranges elsewhere generally coincide with well defined faults: the San Andreas, the Pinto Mountain, the south-flanking frontal fault system, and the steeply dipping reverse fault system along the north side of the San Bernardino Mountains (pl. 2). The Mojave Desert province is generally defined as that great westward-pointing structural wedge of relatively monotonous physiographic aspect sandwiched between the Sierra Nevada and Basin and Range provinces on the north and the Transverse Ranges on the south (pl. 1). Of the several provinces described here, the Mojave Desert province is second only to the Transverse Ranges province in the clarity of its association with the southern California uplift. The Mojave Desert province is characterized by great geologic diversity. It consists largely of crystalline rocks ranging from Pre-cambrian to Mesozoic in age together with complex assemblages of middle and upper Cenozoic rocks deposited in apparently separate basins (Jahns, 1954, p. 13-17). The entire province apparently “was subjected to widespread erosion from late Mesozoic to middle Tertiary time, and, unlike the regions to the north, south, and west, it contains no lower Tertiary sedimentary rocks”; the “younger fluvial and lacustrine sediments indicate a complex history of basin formation that began in middle Miocene time and continued to the present” (Jahns, 1954, p. 17). The west-central part of the Mojave Desert province, in particular, is transected by a series of northwest-trending right-lateral faults, many of which show evidence of Quaternary activity (pl. 2). Moreover, the northwest- to north-northwest-trending boundary that separates an area of Quaternary faulting on the west from one of little8 THE EVOLUTION OF THE SOUTHERN CALIFORNIA UPLIFT, 1955 THROUGH 1976 if any Quaternary faulting to the east coincides roughly with the boundary that separates a region of conspicuous seismicity on the west from a virtually aseismic terrain on the east (Hileman and others, 1973, p. 64-65). This boundary, in fact, effectively divides the Mojave Desert province into two subprovinces of markedly contrasting tectonic character. The northwest-trending faults identified with Quaternary activity form one of the most distinctive features of the western Mojave, and many of them project into the San Bernardino Mountains of the eastern Transverse Ranges as gouge and breccia zones; however, none of these faults offset significantly the northern steeply dipping reverse faults that separate the Mojave Desert from the Transverse Ranges (F. K. Miller, oral commun., 1977). Although the Mohorovicic discontinuity lies at a near-normal depth of about 30 km, other deep-crustal and upper-mantle features associated with the Mojave Desert province depart from the usual. Thus, the high-velocity mantle ridge under the Transverse Ranges apparently projects northeastward into the western Mojave (Hadley and Kanamori, 1977a). Moreover, Hadley and Kanamori (1977b) also suggest a velocity reversal at a depth of 15-20 km that could be interpreted as similar in its origins and rheologic significance to that recognized by Braile and others (1974) in the Basin and Range province. The boundaries of the western subprovince of the Mojave Desert province coincide neatly with a series of well-defined faults or fault systems. Moreover, even within the eastern subprovince, the southern boundary of the Mojave Desert is roughly defined by the San Andreas system (pi. 2), and only in the northeastern part of the province is there neither a clearly defined tectonic nor physiographic boundary. The Continental Borderland province (pi. 1) is characterized by a northwest-trending ridge-basin configuration. It is “underlain chiefly if not entirely by Catalina Schist basement of Franciscan aspect” and has in this sense “both physiographic and geologic relevance” (Yerkes and Wentworth, 1965, p. 19). The Continental Borderland thus contrasts sharply with the adjacent Transverse Ranges and Peninsular Ranges provinces, both of which are devoid of basement rocks of this nature. Although Shepherd and Emery (1941) applied the term “Continental Borderland” to the entire offshore domain between the coastline and the continental slope, we have adopted the usage of Yerkes and Wentworth (a usage that includes only that part of the offshore tract lying generally south of the Channel Islands), simply because the Continental Borderland of Yerkes and Wentworth’s definition can be viewed as a tectonic entity. We have no direct knowledge of either the deep crust or the upper mantle beneath the Continental Borderland province; we infer, in any case, that the high-velocity ridge of Hadley and Kanamori (1977a) projects at least a short distance seaward into the Continental Borderland and that the Mohorovicic discontinuity shallows somewhat toward the Patton escarpment. The Continental Borderland shares a common border on the north with the western Transverse Ranges, where it roughly coincides with the north-dipping Santa Monica thrust or reverse fault system of Barbat (1958, p. 38). The eastern boundary of the Continental Borderland province is defined by the Newport-Ingle-wood zone and its southeastern projection, which takes it seaward south of Newport and thence inland again along the Rose Canyon fault north of San Diego (Jennings, 1975). The Peninsular Ranges province (pi. 1) is identified with a northwest-trending topographic grain truncated abruptly against the southern fault-bounded margin of the Transverse Ranges province. Jahns (1954, p. 19) has characterized the entire Peninsular Ranges province “as an uplifted and westward tilted plateau that has been broken into several large, elongate, subparallel blocks by major [northwest-trending] faults,” many of which have been active during Quaternary or at least later Cenozoic time. The Peninsular Ranges province is underlain largely by crystalline rocks of Paleozoic and Mesozoic age that show a number of seeming affinities with the Sierra Nevada province. The western margin of the Peninsular Ranges consists of a coastal plain underlain chiefly by “clastic marine and nonmarine strata of Upper Cretaceous, Tertiary, and Quaternary age, as well as by scattered volcanic rocks of Tertiary and Quaternary age” (Jahns, 1954, p. 19); this coastal-plain section, moreover, thickens sharply toward the boundary with the adjacent Transverse Ranges province to the north. The nature of the upper mantle and the depth of the Mohorovicic discontinuity beneath the Peninsular Ranges probably are roughly comparable to that elsewhere in southern California. According to Hadley and Kanamori (1977a, p. 1474), however, a relatively high-velocity (6.7 km/s) layer at the base of the crust thickens southward across the Transverse Ranges and persists into the Peninsular Ranges without apparent thinning. Although both the northern and western margins of the Peninsular Ranges province coin-GEOLOGIC FRAMEWORK 9 cide almost exactly with well-defined fault systems, the eastern boundary is less explicitly associated with major faults. It seems instead to step eastward to the north, where the northwest-trending faults through the eastern Peninsular Ranges province lose their surface expression beneath the thick sedimentary cover of the adjacent Salton Trough. The Salton Trough province (pi. 1), as its name implies, is a broad crustal depression, the surface of which lies in part below sea level; it is virtually identical with the Colorado Desert province of Jahns (1954, p. 11). The wedge-shaped region included within this province trends generally northwest and expands almost uniformly southeastward from the confluence between the south and east boundaries of the Transverse Ranges province and Peninsular Ranges province, respectively. Over-lying the basement complex beneath the Salton Trough is a sequence of chiefly nonmarine lacustrine and alluvial deposits that, together with various volcanic rocks, may be as much as 6 km thick (Biehler and others, 1964, p. 132); Muffler and White (1969, p. 170) have, in fact, described a 4.1-km stratigraphic section obtained from a well in the central part of the depression that seems to consist entirely of deltaic sediments of the Colorado River. The age of these deposits is conjectural, but they may be entirely Pliocene and younger (Sharp, 1972, p. 4-7). To the best of our knowledge, the presumably crystalline basement complex underlying the Salton Trough is unique within southern California. Specifically, seismic-refraction studies indicate that the velocity transition (at an average depth of about 5 km) between the sedimentary cover and the underlying basement is relatively smooth; it passes from less than 5 km/s through a zone about 1 km thick into basement velocities 2=5.65 km/s (Fuis and others, 1981). The basement in turn overlies what Fuis and others (1981) term the “subbasement,” the top of which is characterized by velocities of about 7.2 km/s. The relief on the top of the subbasement ranges through about 5 km within the study area alone; the high point, at a depth of about 10 km, occurs at or near the international border. A gravity model developed by Fuis and others (1981, fig. 11) suggests that the Mohorovicic discontinuity beneath the Imperial Valley occurs at an average depth of about 22-24 km, and that the subbasement is about as thick as the basement and sedimentary fill combined. Although the western margin of this province is irregularly outlined by the northwest-trending faults that transect the Peninsular Ranges and project southeastward into the Salton Trough, the eastern margin is relatively smooth and coincides approximately with the easternmost mapped strand of the San Andreas system (pi. 2). The rocks associated with the southern California uplift range from Early Proterozoic (1,750 m.y., Silver, 1971) to Holocene in age; nevertheless, because we are concerned here chiefly with an analysis of the historical deformation, we may legitimately lump the numerous mapped units into several broadly defined groups, provided that these generalizations do not impede reasonable interpretations of the evolution of the uplift. Accordingly, we have simplified the complex geologic section exposed at the surface by combining all of these rocks into three categories (pi. 2): (1) undifferentiated crystalline rocks composed chiefly of Early Proterozoic to Cretaceous units; (2) generally well-indurated unmetamorphosed sedimentary and volcanic rocks composed chiefly of Upper Cretaceous and Tertiary units; and (3) unconsolidated to poorly consolidated sedimentary deposits composed almost entirely of Quaternary units. Although this simplification (pi. 2) obscures the full (and very involved) geologic history of southern California, it provides a reasonable basis both for assessing the very recent geologic history and for distinguishing between movements of clearly tectonic origin from those attributable to artificial or other natural processes. We have similarly simplified the structural configuration by showing only those faults known or suspected to have been active during Quaternary time (pi. 2). Many of these same faults, of course, were active during pre-Qua-ternary time, but it is doubtful that the literally hundreds of mapped faults that show no evidence of Quaternary activity are germane to our analysis of the historic deformation. The tectonic history of southern California pertinent to this investigation can be said to have begun with the initiation of contractional strain athwart the present-day Transverse Ranges or, alternatively, with the inception of bending of the San Andreas fault north of Los Angeles. Precisely when either commenced is uncertain, but it is likely that the compressional stress system presently operating across the Transverse Ranges began no earlier than Pliocene time (Jahns, 1973). Moreover, whether this stress system was derivative from the bending of the San Andreas fault or vice-versa can only be inferred, but the two clearly are related. Powell (1981, p. 387) has suggested that as the regional stress system changed in orientation during the period 5-9 m.y. ago “the great keeled [Sierran10 THE EVOLUTION OF THE SOUTHERN CALIFORNIA UPLIFT, 1955 THROUGH 1976 and Peninsular Ranges] batholiths were rotated clockwise by right-lateral couples *** whereas the rootless Mojave Desert-Transverse Ranges block responded as an easily deformable ‘soft’ zone between the left-lateral couple of the rigid, rotating batholithic blocks.” However, while it is this postulated rotation that led to the bending, the spatial confinement of the bend is believed to have been inherited from an earlier deformational event that imparted a unique structural aspect to what we now recognize as the Transverse Ranges-Mojave block. Thus, according to Powell (1981, p. 374-378), at some point during early Cenozoic time “the Mojave Desert-Transverse Ranges block was shallowly underthrust by a relatively young, hot segment of oceanic crust, whereas the Sierra Nevada and Peninsular Ranges were underthrust at steeper angles by older, cooler oceanic segments.” It is this contrasting history between adjacent ter-ranes that led to the creation of the “soft” zone that both permitted and localized the bending of the main strand of the San Andreas fault (if not the plate boundary itself). Acceptance of the basic scenario outlined by Powell invites several corollary conclusions of varying tectonic significance: (1) The formation of the bend and its perhaps continuing accentuation during the period 5-9 m.y. ago or later, coupled with continuing right-lateral plate motion on the order of 60 mm/yr (Minster and Jordan, 1978, p. 5345), forced the generation of con-tractional strain across “crustal boundaries whose general east-west trends were established by earlier deformation” (Campbell and Yerkes, 1976, p. 555)—an observation that argues that the great bend in the San Andreas fault is more cause than effect. (2) As both rotation of the batholithic blocks and right-lateral translation continued along the North American-Pacific plate boundary, it imposed a left-lateral couple across or within the intervening block that resulted in the relatively recent inception of left-lateral movement on the Garlock fault. Indeed, Carter (1980) has argued that strike-slip displacement on the Garlock fault, which sums to about 60 km, originated in Pliocene time—and probably late Pliocene time. If the displacement on the Garlock fault and the growth of the present-day Transverse Ranges are linked, which seems likely if not compelled, the evolution of the Transverse Ranges probably occurred within a small fraction of Cenozoic time—perhaps during the past 2-3 m.y. (3) Comparison between the results of experimental modeling and the inferred recent tectonic evolution of southern California (Powell, 1981, p. 390) tend both to support Powell’s reconstruction and suggest mechanical explanations for what is observed or reasonably inferred. Experimentally induced deformation generated within an overlying and partially decoupled layer in response to right-lateral motion between adjoining blocks of the underlying layer and attendant underthrusting of one beneath the other closely matched the geologically inferred pattern and sequence of deformation described by Powell (1981, p. 387-390). Powell’s comparison suggests to us that horizontal or near-horizontal decoupling within the lithosphere has contributed significantly to the recent tectonic evolution of southern California. (4) Even if the postulated opposing rotations between the rooted batholithic blocks have ceased, the clearly persisting right-lateral motion between the North American and Pacific plates has compelled continuing left-lateral displacement on the Garlock fault (or similarly oriented faults within the North American plate originating in the region of the great bend). Specifically, if we assume that the Pacific plate is fixed, that the relatively discrete rupture that defines the surface trace of the San Andreas extends to a depth of about 15 km (the base of the seismogenic zone), and that decoupling between upper and lower layers of the lithosphere occurs at this or some greater depth (but probably above the high-velocity mantle ridge of Hadley and Kanamori, 1977a—see below), owing to the constraint imposed on the motion of the upper layer by the present position of the San Andreas, parallel trajectories of upper and lower layers north of the great bend probably have diverged south of the bend (whatever its location) during much or most of Quaternary time. North of the bend, eastward displacement of the upper layer accompanying this divergence would tend to horizontally load the upper layer. This loading, in turn, could be accommodated through east-west extension of the North American plate north of the bend together with left-lateral displacement along a zone originating at the bend and trending at a high angle to the San Andreas fault (whether the Garlock, the White Wolf, or any similarly positioned fault), a mechanism seemingly compatible with (if not identical to) one first proposed by McKenzie (1972, p. 175). There are several reasons for believing that the decoupling which plays an integral part in this model probably operates at some relatively modest depth within the crust (15-20 km) and, hence, that the lithosphere is a Theologically layered system: (1) The configuration of the high-velocity mantleGEOLOGIC FRAMEWORK 11 ridge described by Hadley and Kanamori (1977a) is difficult to reconcile with significant right-lateral slip through the full thickness of the lithosphere eastward from the Newport-Inglewood zone to the eastern Mojave Desert; accordingly, although we recognize the limited resolution of the data that permitted definition of this feature, it seems unlikely that major right-lateral displacement through this part of the upper mantle could have occurred while leaving so little evidence. (2) Were the lithosphere not multilayered, the loading effects proposed here could certainly have occurred as a result of slip at the base of the lithosphere. However, the Quaternary contractional effects within and around the margins of the Transverse Ranges would be much more difficult to explain if the upper- and lower-plate boundaries coincided (that is, if there were no decoupling between these layers); the very occurrence of major thrusts and reverse faults along both the northern and southern boundaries of the Transverse Ranges argues for a form of decoupling that is reasonably extended to depth along progressively shallowing surfaces (see, for example, Thatcher, 1976, p. 693). (3) The studies of Lachenbruch and Sass (1973, p. 192) argue that the heat-flow distribution along the San Andreas is compatible with a model that “attributes the [thermal] anomaly to mechanical heat generation in a broad shear zone between the North American and Pacific plates.” Acceptance of this internally consistent model requires that decoupling be generated at the base of the seismo-genic zone (Lachenbruch and Sass, 1973, p. 204). (4) Hadley and Kanamori (1978) have described as nearly horizontal the preferred fault planes for two small shocks that occurred near the base of the seismogenic zone within and adjacent to the aftershock zone of the 1971 San Fernando earthquake, an observation that suggested to them that the Transverse Ranges may form an evolving de-collement. The infrequency with which focal mechanisms of this sort have been detected probably is due to the relative infrequency of brittle failure at or near the base of the seismogenic zone. (5) Finally, observational evidence, coupled with various theoretical considerations outlined in the section on “The Origin of the Southern California Uplift,” indicates that decoupling not only can occur but should be occurring at relatively shallow crustal depths. We see no insurmountable arguments that refute the operation of plate-motion divergence as an explanation for the geologically recent left-lateral slip along the Garlock fault and the extensional strain effects recognized east of the San Andreas and north of the Garlock. However, this notion may carry within it the seeds of its own destruction. With continuing migration of the North American plate past the Pacific plate, pile-up between the lower layers is implied by the more sharply defined bends along the edges of the plate boundaries; it is this pile-up that may ultimately lead to a straightening of the entire system and a resultant narrowing of the shear zone that lies beneath the seismogenic zone south of the great bend. HISTORICAL SURFACE DEFORMATION Historical surface deformation within the area of the southern California uplift can be characterized as either tectonic or nontectonic. Nontectonic deformation is further divisible into artificially and naturally induced movement. Because the purpose of this report is the consideration of a certain category of tectonically derived surface movements, we have attempted to provide a basis for discriminating between tectonic movements and those of nontectonic origin. Our approach, which is both the most conservative and the only practical procedure open to us, has been to simply disregard those movements that are certainly or probably contaminated by a nontectonic signal, even though it requires that we discard a good deal of data that might have assisted in an assessment of the tectonic process. NONTECTONIC DEFORMATION Compaction, together with the much more subtly defined expansion of unconsolidated to incompletely consolidated basinal deposits produced through changes in the underground fluid-pressure regime, accounts for nearly all of the nontectonic surface deformation recognized in southern California. Surface movements of this derivation are, in fact, so significant in relation to all other types of nontectonic deformation that we may safely exclude from any further consideration the generally trivial and easily distinguished movements due to slope and free-face failures or to hydrocompaction. Because man-induced changes in the subsurface fluid-pressure regime have created the most dramatic examples of nontectonic deformation in southern California, this category of movement is examined first. ARTIFICIALLY INDUCED DEFORMATION Artificially induced surface deformation is clearly associated with both ground-water with-12 THE EVOLUTION OF THE SOUTHERN CALIFORNIA UPLIFT, 1955 THROUGH 1976 drawals and oil-field and gas-field operations throughout southern California (pis. 3 and 4). This type of deformation is most conspicuously expressed as differential subsidence (pi. 4), but it also includes elastic rebound, horizontal movement, faulting, and surficial Assuring (Church and others, 1974; Castle and Yerkes, 1976; Morton, 1977). However, even though all of these movements can be shown to be related, whether directly or indirectly, to fluid-pressure declines and resultant compaction at depth or to changes in ground-water levels, this discussion is restricted to a consideration of the vertical displacement field—that is, to differential subsidence and uplift. The application of consolidation theory to the analysis of differential subsidence associated with changes in the fluid-pressure regime has been summarized by Poland and Davis (1969). It begins with the acceptance of Terzaghi’s principle of effective stress, which states that within a porous, fluid-filled medium, p = p' + u, where p = total stress or pressure, p' = effective (grain-to-grain, intergranular, “solid”) stress or pressure, and u = fluid (pore-water, reservoir, neutral, internal) stress or pressure. In a confined water system in which the compressibility of the fluid is disregarded, unit head decline (which may be equated with fluid-pressure reduction) will produce an equal increase in effective pressure; in an unconfined water system any reduction in liquid level will produce an increase in effective pressure through loss of buoyancy, and the total pressure will decrease slightly owing to loss of fluid mass (Poland and Davis, 1969, p. 193-196). Because the overburden is supported by both fluid and effective pressure, a decrease in fluid pressure to a point approaching zero will increase the effective pressure to a value approaching the lithostatic pressure, whereas an increase in fluid pressure to a point approaching the lithostatic pressure will decrease the effective pressure to a value approaching zero. Reservoir compaction thus becomes a function of both the magnitude of the increased effective stress (or applied load) and the compressibility of the materials, whereas any expansion of the reservoir skeleton is a function of the magnitude of the reduced effective stress and the elastic component of the compressibility. Vertical Displacements Associated with Ground-Water Withdrawal and Recharge Easily measured subsidence has accompanied artificially induced water-level declines in many of the ground-water basins of southern California (pis. 3 and 4). The subsiding areas are typically underlain by unconsolidated to semiconsolidated alluvial or lacustrine clastic deposits of chiefly Quaternary age. Water is generally extracted from semiconfined and confined sand or gravel aquifers of low to moderate compressibility that are commonly interbedded with relatively impermeable and highly compressible clay-rich aquitards. In the absence of recharge, continuing extraction reduces pressure head within the aquifers and sets up pressure gradients across aquifer-aquitard boundaries, ultimately leading to the dewatering of the aquitards and the compaction of both aquifers and aquitards. The magnitude of this compaction and of any resultant surface subsidence depends, of course, on the thickness of the affected system. Several local ground-water basins have shown little if any subsidence in spite of major head declines (>30 m). Most of those basins in which there has been relatively little subsidence are characterized by coalescing fan deposits that consist of relatively coarse clean gravel generally devoid of silt and clay beds. Examples include the Raymond basin northeast of Los Angeles (Lofgren, 1971a) and much of the San Fernando Valley (pis. 3 and 4). Owing to limited preconsolidation produced through natural oscillations of the water table, there is generally some critical piezometric head decline at which compaction and resultant subsidence begin to accelerate in response to continuing exploitation. Although there are relatively few examples where there is good control on both head decline and changing elevation, significant subsidence in the semiarid southwestern United States typically begins following initial head declines of 15-20 m (Poland and Davis, 1969). During recharge, whether seasonal or long term, compaction commonly is almost completely arrested and slight elastic expansion and accompanying surface rebound may occur (Riley, 1969; Poland, 1969, p. 291). Water levels in the several ground-water basins of southern California have been affected by various combinations of secular meteorological changes, changing land use, artificial recharge practices, and importation of water, as well as withdrawals. Regional drought, combined with increased land development and ground-water withdrawals, produced major water-level declines in southern California during the periods 1924-36 and 1945-64. In many of the coastal basins, unusually high precipitation during the winters of 1965/66 and 1968/69, coupled with increased use of imported water, led to recoveries in water levels that wereHISTORICAL SURFACE DEFORMATION 13 apparently retained through at least 1974. However, water levels in the western Mojave Desert continued to decline through this period of general recovery (California Department of Water Resources, 1975a). We have summarized in plates 3 and 4 a current appraisal of water-level declines and associated differential subsidence in southern California. Although this pair of illustrations is largely self-explanatory, several points merit amplification. While much of the Los Angeles basin has sustained significant differential subsidence due to ground-water withdrawals, a large fraction of this subsidence is indistinguishable from that attributable to natural compaction or even to tectonic downwarping, and it is locally masked by subsidence associated with oil-field and gas-field operations (pis. 3 and 4). For example, the so-called La Cienega subsidence bowl, which lies along the northern edge of the basin about 10-12 km west of Los Angeles (pi. 4), probably is attributable to a combination of causes including ground-water withdrawal, tectonic downwarping, and, perhaps, oil-field operations (Castle and Yerkes, 1976, p. 10-11). Similarly, southeastward from Los Angeles along the axis of the basin, most of the localized differential subsidence (pi. 4) clearly is associated with ground-water extraction (pi. 3). Nevertheless, from 1968 to 1974, during a period of rising water levels that apparently began in 1962/63, a broadly defined area centering about 10-15 km south of Los Angeles continued to subside at about 10 mm/yr. While this continuing subsidence could be the product of aqui-tard dewatering and compaction attributable to the persistence of a pore-pressure gradient between aquifer and aquitard long after fluid pressures within the various aquifers had begun to recover, it is just as likely to be related to natural causes. The basinal complex extending westward from Colton to and beyond Ontario has undergone major water-level declines (pi. 3) that may have begun as early as 1904 (Mendenhall, 1908). Because the Quaternary stratigraphy and structural history of this area are so complex, most of the artificially induced subsidence is sharply localized; thus, unlike the situation in the Los Angeles basin, the occurrence of this subsidence has not generally constrained our reconstruction of the southern California uplift. Some fraction of this localized subsidence could be tectonic downwarping (analogous to that postulated to have occurred within the La Cienega subsidence bowl). However, because we usually are incapable of distinguishing between tectonic and artificially induced subsidence, we are forced to assume that all of the measured subsidence in this area is related to ground-water withdrawals. The well-defined subsidence in the Bunker Hill ground-water basin, which lies between the San Jacinto and San Andreas faults east of Colton (pi. 4), seems to be closely tied to the Quaternary stratigraphy. That is, even though substantial water-level declines have been recognized on both sides of the San Jacinto fault (pi. 3), the subsidence is largely restricted to the east block. This seeming inconsistency may be easily explained, however, for the section northeast of the San Jacinto fault is unlike that to the southwest and consists of a sequence of clay and silt layers interbedded with sands and gravels (Eckis, 1934, p. 160), a sequence that virtually invites compaction and subsidence as a result of even modest head declines. Rising water levels in the Bunker Hill basin during the period 1968/69-75 apparently arrested the subsidence that had characterized this area during the preceding decades. This apparent cause-and-effect relation suggests accordingly, that the previously recognized subsidence east of the San Jacinto fault cannot be attributed to phenomena other than ground-water extraction. Vertical Displacements Associated with Oil-Field and Gas-Field Operations The most dramatic examples of artificially induced surface deformation recognized in southern California are those associated with oil-field and gas-field operations (Castle and Yerkes, 1976). Compaction and resultant subsidence attributable to the extraction of oil and gas (and the water that generally accompanies petroleum production) are mechanically analogous to that associated with the production of water from confined aquifer systems. Similarly, fluid injection and attendant increases in reservoir fluid pressure not only tend to retard further compaction and subsidence, but can actually induce limited elastic rebound. Allen and Mayuga (1969), for example, interpret most of the 0.34 m of rebound in the Wilmington oil field east of San Pedro (pi. 3) during the period 1965-69 as the product of elastic expansion accompanying massive water flooding of this field. While other mechanisms may figure in rebound around the margins of producing oil fields (see, for example, Castle and Yerkes, 1976, p. 73-75), broadly distributed rebound associated with injection is generally attributable to decreased effective stress accompanying repressurization.14 THE EVOLUTION OF THE SOUTHERN CALIFORNIA UPLIFT, 1955 THROUGH 1976 Although fluid-pressure reductions and resultant increases in effective stress necessarily lead to a tendency toward compaction and surface subsidence, accumulating experience indicates that the occurrence of clearly measurable subsidence associated with oil-field and gas-field operations is generally restricted to a particular class of fields. Yerkes and Castle (1969, p. 56) show that subsidence is almost invariably identified with those fields characterized by production “from unconsolidated to poorly lithified and poorly sorted sands, generally Miocene or younger in age,” where the “median depths of production range from about 360 to 3900 m and exceed 1800 m in only four cases.” Owing in part to the small scale of the map (pi. 4), we show only the better documented and most impressive examples of subsidence associated with petroleum production. However, nearly every oil field shown on plate 3 meets one or more of the subsidence-susceptibility criteria listed above. Because both spatial and temporal coincidence between oil-field operations and subsidence have been demonstrated repeatedly (Castle and Yerkes, 1976), differential height changes within or around these fields (pi. 3) should be attributed to processes other than oil-field operations only in the presence of overwhelming evidence to the contrary. NATURALLY INDUCED DEFORMATION Natural compaction probably is continuing, in some measure, throughout the sedimentary basins of southern California. It may be an especially important process in those areas characterized by rapid deposition of fine-grained materials during recent geologic time—such as the central Los Angeles basin, the southern San Joaquin Valley, and the Oxnard Plain. However, although natural compaction and resultant surface subsidence probably have continued during historical time within much of the area embraced by the southern California uplift, it is generally very difficult to distinguish surface deformation of this type from that due to ground-water extraction. The Los Angeles basin, which contains about 1,400 m of Quaternary deposits alone and a combined upper Pliocene and Quaternary section over 3,500 m thick (Yerkes and others, 1965, pi. 4), is a case in point. Grant and Sheppard (1939) have outlined a trough of differential subsidence that increases progressively south-southeastward from Los Angeles along a zone that roughly coincides with the axis of the basin. Furthermore, compar- ison of 1968 and 1974 level surveys through the central part of the basin shows that subsidence during this period persisted at rates of about 10 mm/yr, whereas during the period 1962-76 ground-water levels in this same general area rose roughly 10 m. Hence, while there is a strong likelihood that the subsidence identified by Grant and Sheppard (1939) is due in part to natural compaction associated with rapid loading of the underlying section, we cannot be certain that it is not entirely man induced. That is, as suggested earlier, it is conceivable that the subsidence detected during the period 1968-74 is attributable to nothing more than the drainage of aquitards in response to earlier head declines in amounts significantly greater than the 1962-76 10-m water-level recovery. Similarly, while well-defined subsidence in the southern part of the Oxnard Plain (pi. 4) is reasonably attributed to natural compaction of Holocene la-goonal or marsh deposits (Castle and others, 1977, p. 220-225), the modest declines in ground-water levels recognized in this area (pi. 3) indicate that this subsidence is not necessarily due to natural phenomena. TECTONIC DEFORMATION Implicit in the preceding discussion is the notion that geodetically defined vertical-displacement fields of tectonic origin can be described in no more than an exclusionary sense—namely, as those vertical displacements that cannot be attributed, whether directly or indirectly, to natural compaction or to artificial processes. Accordingly, plates 3 and 4 provide a convenient guide for distinguishing between vertical movements of tectonic derivation and those of a probable or possible nontec-tonic origin. The value of these illustrations is enhanced, moreover, if they are used in conjunction with the physiographic map showing the natural provinces (pi. 1) and the generalized geologic map (pi. 2). Together, these two maps (pis. 1 and 2) provide a basis for assessing the form of those sedimentary basins that are especially susceptible to nontectonic deformation. Thus, in evaluating the changing configuration of the southern California uplift we have relied heavily on these data (pis. 1-4) in detecting (and discarding as irrelevant) those nontectonic vertical signals that appear in the profiles of height changes shown below. VERTICAL-CONTROL DATA The basic data used in the reconstructed height changes described in this report are drawn fromVERTICAL-CONTROL DATA 15 repeated level surveys between two or more bench marks. While the determination of any change in height difference between marks is a basically simple procedure, it is commonly complicated by the fact that measured height differences are both path and time dependent. Moreover, because most of the survey data that we have assembled here have been developed for engineering or cartographic purposes (where crustal stability among a widely spaced array of bench marks usually has been assumed), the thrust of our effort has been directed toward reconstructing and interpreting the leveling data in such a way that the effects of any crustal movement during a given level survey can be assessed and considered in calculating height changes referred to a common datum (or reference bench mark). Vertical-control surveys are divisible into several orders and classes of accuracy, each of which meets certain procedural and instrumental requirements (Federal Geodetic Control Committee, 1974). Although these requirements have been strengthened over the years, there have been no significant instrumental changes since 1916 (with the introduction of the invar rod) and relatively few procedural changes since 1925 (Rappleye, 1948a, p. 1-3, 7-9, 15-23; Federal Geodetic Control Committee, 1974, p. 9); hence the most recently established standards of the Federal Geodetic Control Committee provide a convenient basis for characterizing those requirements especially germane to the error estimates of the National Geodetic Survey. Moreover, although most all the height changes described in this report are based on comparisons between first-order level surveys (pi. 5), because we have used the results of lower-order surveys locally, the procedural specifications for each of the three orders of geodetic leveling are briefly summarized here. All first-order leveling is, by definition, double run over sections of 1-2 km. Balanced sights are required, and the maximum sight lengths permitted since 1974 are 50 m for class I and 60 m for class II surveys (Federal Geodetic Control Committee, 1974, p. 9). Prior to 1961, maximum sight lengths of 150 m were authorized, but their use was permitted “only under the most favored conditions” (Rappleye, 1948a, p. 7). In 1961, sight lengths were reduced to a maximum of 75 m; in 1964 they were further reduced to 50 m for all first-order surveys utilizing instruments other than the Fischer level (formal class distinctions were not introduced until 1974) (E. I. Balazs, oral commun., 1979). The rejection limits (or maximum permissi- ble closures) over individual double-run sections are 3 mm VK for class I surveys and 4 mm VK for class II surveys, where K is the distance in kilometers; the rejection limits for lines or loops are 4 mm VK and 5 mm VK for class I and class II surveys, respectively (Federal Geodetic Control. Committee, 1974, p. 9). The 3 mm VK section-rejection limit, which is the singularly significant requirement for class I leveling, was not introduced until the 1950’s; hence prior to the 1950’s first-order leveling can be said to have consisted of a single class characterized by a section-rejection limit of 4 mm VK. Second-order leveling may be either single or double run and is similarly divisible into two classes: class I leveling is in all cases double run, whereas class II work may be either single or double run (Federal Geodetic Control Committee, 1974, p. 9). Both class I and class II second-order surveys again require balanced sights, but the difference between forward and backward sights may be as much as 10 m. Maximum sight lengths currently range from 60 m for class I to 70 m for class II surveys (Federal Geodetic Control Committee, 1974, p. 9); they apparently have never exceeded 150 m (Rappleye, 1948a, p. 7), and because the U.S. Geological Survey (since 1928, at least) has permitted sight lengths no greater than 92 m for third-order leveling (Birdseye, 1928, p. 132), it is likely that the sight lengths used in second-order leveling rarely have exceeded 90 m. The rejection limits for both section closures and loop or line closures are given as 6 mm VK for class I second-order surveys and 8 mm VK for class II surveys (Federal Geodetic Control Committee, 1974, p. 9). Prior to 1974, and at least as far back as 1928, the rejection limit for all second-order leveling was 8.4 mm VK (Birdseye, 1928, p. 130; Rappleye, 1948a, p. 2-3; U.S. Geological Survey, 1966, p. 2). Third-order leveling is again divisible into single or double run, but because the procedural requirements are otherwise the same for both there is no formal “class” distinction for surveys of this order (Federal Geodetic Control Committee, 1974, p. 9). In practice, virtually no third-order leveling is double run unless a blunder or other major error is suspected. The maximum permissible sight length for modern third-order surveys is given as 90 m (Federal Geodetic Control Committee, 1974, p. 9). The older standards, moreover, were almost as stringent; since 1928 the Geological Survey has stipulated that maximum sight lengths not exceed 92 m except at river crossings or ravines (Birdseye, 1928, p. 132; U.S. Geological Survey, 1966, p. 20).16 THE EVOLUTION OF THE SOUTHERN CALIFORNIA UPLIFT, 1955 THROUGH 1976 The loop- or line-rejection limit currently is given as 12 mm VK (Federal Geodetic Control Committee, 1974, p. 9); this is the same maximum closure that has been in effect for third-order leveling since 1928 (Birdseye, 1928, p. 130; Rappleye, 1948a, p. 2-3). ERRORS IN HEIGHT DETERMINATIONS Errors in height differences, with the exception of usually easily detected blunders or “busts,” are attributable to four major sources: (1) systematic survey error, (2) random survey error, (3) surface deformation (bench-mark motion) during the course of a specified level survey, and (4) an imprecisely formulated orthometric correction (see below). The magnitude of both the second and last of these error sources generally can be closely estimated; systematic error and errors inherent in crustal deformation during the leveling are much more difficult to assess, and there is no single technique that permits their clear discrimination. SYSTEMATIC ERROR Although the distinction between systematic and random error is poorly defined, the recognition or suspicion of various types of systematic error has dictated many of the procedural requirements stipulated for geodetic leveling. Systematic leveling errors are of two general types: those that can be assessed through loop closures and those that are independent of closure. Because many of the lines considered here do not close a loop and because the larger misclosures that we have identified are attributable chiefly to deformation that occurred during completion of the respective loops, misclosures are usually inadequate indices of systematic error—at least in southern California. Systematic errors independent of closure are generally functions of topography and, hence, are either height or slope dependent. Thus, an undetected error in rod length can be expected to produce errors in measured elevation differences that are directly proportional to the error in rod length. For example, where the approximate elevation difference between marks is defined as Ae, an error of 0.03 mm in a 3-m rod would produce an error of AexlO-5—or about 24 mm in the maximum elevation difference (2,400 m) considered in this report. Similarly, “unequal refraction” (in which the refraction introduced in the foresight is unmatched by that in the backsight) may lead to significant gradient-dependent errors. Bomford (1971, p. 240-241) suggests that refraction errors may be as large as Ae x 2 x 10 4— or about 480 mm for the maximum elevation difference considered here— although they generally must be much less than this “worst case” situation. Because rod and refraction errors are height and slope dependent, they are most easily detected by searching for a correlation between topography and measured elevation discrepancies disclosed through repeated surveys. If the results of either survey are contaminated by significant rod error, the correlation between height and any seeming vertical displacement should be almost perfect. Although one could expect to see, for various geologic reasons, a rough correlation between height and apparent movement, the one-to-one correlations between height and apparent height changes associated with rod error are generally absent in the comparisons developed for this report. Hence, it seems unlikely that rod error has had any significant effect on the vertical signals described here. Refraction errors are less specifically correlated with terrain than are rod errors. That is, because refraction is a function of the atmospheric density along the line of sight, and thus is dependent on the vertical temperature gradient, short-term vagaries of climate superimposed on the normal diurnal and seasonal variations will tend to diffuse any correlation between slope and seeming height changes between surveys. Although the correlations are generally poor, several comparisons between height changes and topography within the area of the southern California uplift suggest that the apparent vertical displacements may be due in part to refraction error in one or the other of the comparative surveys. Nonetheless, because the associated misclosures are very small, these postulated refraction errors (if of any significant size) would have to have been almost precisely self-can-celling—a somewhat surprising conclusion in view of the path dependency of this error and that, in the general case, it should be expressed as a mis-closure. It is conceivable, of course, that the atmospheric refraction error is so overwhelmingly slope dependent that it tends to cancel, regardless of weather, length of day, and so forth. The implicit corollary—namely, that refraction errors have contaminated successively measured height differences to about the same degree—carries with it the conclusion that refraction errors could not have contributed significantly to the calculated vertical displacement values. (Because “balanced slopes” are certainly the exception rather than the rule, the generally small misclosures produced duringVERTICAL-CONTROL DATA 17 periods of tectonic quiescence (see below) suggest that refraction errors tend to randomize over distances of more than a few kilometers.) In any case, rigorous examination of the data assembled for this study has produced very little evidence that indicates that the vertical-control data have been seriously fouled by refraction error. Systematic error may be the most elusive problem we face in utilizing the results of repeated level surveys as indices of historical crustal deformation. Further complicating this problem is that various types of unrecognized or unsubstantiated systematic error may have contaminated these measurements. For example, spirit leveling may be characterized by a directional bias, a possibility suggested by the existence of the so-called sea-slope problem (Fischer, 1977). However, even though recent investigations (Castle and Elliott, 1982) suggest that the discrepancy between geodetic and steric (oceanographic) leveling cannot be dismissed as the product of a postulated direction-ally dependent systematic error in geodetic leveling, we infer that the north-south component associated with each successive survey over the same route has been approximately equally, if at all, contaminated. It is sometimes possible to search for the existence and significance of systematic error even if the physical source of the error cannot be specified. Specifically, systematic error may appear in the cumulative divergence between the forward and backward runs of a double-run line. Granted that we have investigated the cumulative divergence in only a handful of surveys utilized in this report, only one of those that we did examine showed divergence of a significant magnitude and character that could be interpreted as the product of systematic error; nonetheless, even this divergence was but a small fraction of the vertical signal disclosed through a comparison of the results of this particular survey against those of an earlier leveling. In the final analysis, the very strong likelihood that the large vertical signals described in this report have not been seriously distorted by systematic error rests heavily on circumstantial but very persuasive evidence, much of which is developed in detail in the body of the report. In the first place, replication of elevation determinations both before and after the large vertical displacements that are critical to our reconstruction, supports the argument that these determinations are free of measurably significant systematic error of whatever origin. Moreover, because the earlier set of concordant elevations is based on levelings that brack- eted the procedural change that halved the maximum permissible sight length, it is especially unlikely that any subsequent displacements can be easily dismissed as refraction-induced (sight-length-dependent) artifacts. Similarly, Mark and others (1981, p. 2792-2794) show that successively developed elevation differences between Saugus and Palmdale (fig. 2) probably are no more than trivially contaminated by either rod or residual refraction error. In particular, the Saugus-to-Palm-dale line is at once that part of the reach between San Pedro and Palmdale within which the south flank of the uplift is largely confined and which shows by far the clearest correlation between signal and terrain of any of those lines examined in this study. Yet levelings bypassing the Saugus-to-Palmdale line, over a variety of routes in which signal and topography are generally poorly correlated, have produced approximately the same pre- and post-uplift heights for marks in the Palmdale area as have those contemporary levelings propagated directly eastward from Saugus. Secondly, the generation of widely distributed large vertical signals (along five separate lines that occur within a zone more than 175 km in length) within a relatively tight time frame of only about 2 years argues that these signals are other than the products of survey error. In other words, interpretation of these broadly distributed signals as a time-constrained concatenation of errors would carry with it a probability approaching zero, for comparable signals (or errors) were undetected in this same general region during the decades before and after this 2-year interval. Thirdly, the detection of very subtly defined artificially induced movements (based on repeated levelings over lines where systematic error could be reasonably anticipated) that agree almost precisely with their predicted occurrence and configuration, testifies to the nearly error free nature of these measurements within and around the area of the southern California uplift (see, for example, Castle and others, 1974, p. 62, 65). Fourthly, the discovery of a similar uplift that apparently evolved during the early part of the 20th century suggests that the modern uplift represents but a single pulse in a continuing cyclic process (Castle and others, 1977) and supports by analogy the reality of both the southern California uplift and the validity of the measurements that permitted its identification. In other words, it would be very difficult to accept the contention that the remarkable correspondence in both the configuration and general history of these two episodes of uplift is simply the product of a18 THE EVOLUTION OF THE SOUTHERN CALIFORNIA UPLIFT, 1955 THROUGH 1976 cyclically distributed systematic error. Finally, accumulating geologic evidence (McCrory and Lajoie, 1977; Wehmiller and others, 1977a; Bull and others, 1979) indicates that the pattern of crustal deformation disclosed through the analysis of the historical geodetic record is consistent with the pattern of vertical displacements generated within the area of the southern California uplift during late Quaternary time. All in all, the preceding observations indicate that if the geodetically defined signals that permitted the identification and characterization of the southern California uplift are no more than measurement artifacts, chance coincidence has operated on a truly remarkable scale. RANDOM ERROR Random error developed during any double-run leveling may be assessed through statistical examination of the section closures, provided only that the sample population is sufficiently large. Double-run procedures are so specified that the expected random error in the measured elevation difference between any two bench marks a distance L apart is approximately normally distributed and hence proportional to L*. While the results of singlerun leveling are less amenable to statistical analysis, it is assumed that the random error in the measured elevation difference between any two marks is also normally distributed, if only because the procedures stipulated for both single- and double-run surveys are otherwise identical. This assumption is supported, moreover, by the experience of the Geological Survey in assessing the results of thousands of loop or line closures. The higher the order and, in general, the more recent the leveling, the greater the accuracy. First-order leveling procedures currently are such that for class I and class II surveys the estimated standard error—which can be treated as the equivalent of u, or the standard deviation over a unit distance—is 0.5 mm/km1 and 0.7 mm/km*, respectively (Federal Geodetic Control Committee, 1974, p. 3). However, the experience of the National Geodetic Survey with all first-order leveling indicates that o was about 1.5 mm/km* and 1.0 mm/km* during the periods 1917-55 and 1956-74, respectively (Vanlcek and others, 1980, p. 507). For simplicity, we have assumed in all of our calculations that o has in no case dropped below 1.0 mm/km*. Thus, one standard deviation in the measured elevation difference based on first-order leveling between two bench marks at opposite ends of a 600-km line—roughly the length of, the longest line considered in this report—would be about 37 mm had the survey been carried out between 1917 and 1955, whereas it would be about 24 mm had it been carried out since 1955. Second-order leveling specifications today are such that a is given as 1.0 mm/km* and 1.3 mm/ km* for class I and class II surveys, respectively (Federal Geodetic Control Committee, 1974, p. 3). Again, however, the experience of the National Geodetic Survey with all second-order leveling indicates that a was about 3.0 mm/km* and 2.0 mm/ km* during the periods 1917-55 and 1956-74, respectively (Vanicek and others, 1980, p. 507-508). Moreover, we have again assumed for purposes of this report that ct has not dropped below 2.0 mm/ km* for any second-order survey. Thus, one standard deviation in the measured elevation difference based on second-order leveling between two bench marks at opposite ends of a 150-km line— approximately the length of the longest second-order line examined in this report—would be about 37 mm had the leveling been carried out between 1917 and 1955, whereas it would be about 24 mm had it been carried out since 1955. Third-order leveling procedures currently are such that a is given as 2.0 mm/km* (Federal Geodetic Control Committee, 1974, p. 3). The experience of the U.S. Geological Survey, based chiefly on the results of loop and line misclosures developed from third-order leveling indicates that before 1956 a was close to 6 mm/km*, whereas since that time it has been about 4 mm/ km*. The specifications for third-order leveling are such, however, that these estimates almost certainly err on the conservative side. Nonetheless, we have extended this conservatism by assuming for the purposes of this study, that a has in no case dropped below 4 mm/km* for any third-order leveling. Thus, one standard deviation in the measured elevation difference based on third-order leveling between two bench marks at opposite ends of a 60-km line—nearly the length of the longest third-order line considered in this report—would be about 46 mm had the survey been run before 1956 and about 31 mm had it been carried out since 1955. Calculation of the standard deviation of the measured elevation difference between the end points of a single line composed of several segments of differing survey class or order (or simply of differing a) requires, in effect, that the standard deviation be calculated for each of these separately defined segments. Thus, in the most general case, m=-P A JA gdn, (4) where - WA = the potential difference between A and B and g = gravity at the leveling station or point of observation along the survey route (Heiskanen and Moritz, 1967, p. 161). The potential difference, WB - WA, is independent of path and is “the result of leveling combined with gravity measurements,” a true physical quantity. It is “basic to the whole theory of heights; even orthometric heights must be considered as quantities derived from potential differences” (Heiskanen and Moritz, 1967, p. 161-162). Because orthometric height differences and potential differences can be developed through the measurement of gravity along the level route, an orthometric correction can be derived, whereby AnAB can be corrected to a true orthometric height difference even though j^bn*HB-HA (5) (Heiskanen and Moritz, 1967, p. 162-169). The orthometric correction to be applied to Awab is given by Heiskanen and Moritz (1967, p. 168-169) as: ocAB=2 A V 7o bn + 9 a *Yo 7o Ha 9b~ lo 7o Hb, (6) where OCAB = the orthometric correction, g = gravity at the leveling station or point of observation along the survey route, gA = mean value of gravity along the plumbline between A and the geoid (a value that can be calculated from the geopotential number, CA(f gdn), at point A), gB = mean value of gravity along the plumbline between B and the geoid, and y0 = normal gravity at 45° latitude. An exact determination of the orthometric correction clearly requires an explicit knowledge of gravity along the path of integration. However, because g commonly is unknown in detail along the survey route, it generally has been approximated (at least in the United States) in the following way: gr = p45(l-oicos24> + 0cos22<]>-/c/?,), (7) where g45 is the normal acceleration of gravity at sea level at latitude 45° (980.624 cm/s), a and 0 are dimensionless constants (0.002644 and 0.000007, respectively), A; is a constant functionally dependent on the unit of height measurement, c}) is the latitude, and h is the orthometric “elevation” (Rap-peleye, 1948b, p. 157). Because the only variables in equation 7 are latitude, <)>, and orthometric height, h, it follows that an approximation for the orthometric correction, dh, to be applied to the observed elevation difference between any two nearby points, may be formulated in terms of h and 4>. That is, dh = -2ha. sin2<|> 1 + cos 2 d<$>, (8) where d§ is generally expressed in minutes of arc (Rappeleye, 1948b, p. 158-159). This correction can be further reduced to dh= — Cahd<\>, (9) where C0 is the factor 2a sin 2$ 1 + ( a — ?) cos2c)> sin 1', h is the average elevation of the instrument between two nearby points, and dc|> is the difference in latitude in minutes. C„ can be calculated for each minute of latitude; d is positive where the second point is north of the first (Rappeleye, 1948b, p. 159). Thus, the orthometric correction to be applied to AnAB finally can be approximated as OCAB = 2dh. (10) A Because the orthometric correction (OCAB) is very small with respect to the orthometric height, and because we are concerned here with changes in orthometric height (vertical displacements), changes in the orthometric correction associated with historic height changes may be dismissed as trivial. Nevertheless, because Awab and, hence, the magnitude of the orthometric correction are path dependent, we need to consider any differences in the magnitude of this correction both as functions of path and as functions of their formulation. That is, if the “orthometric closure,” which is simply the algebraic sum of the orthometric corrections between A and B and between B and A along two significantly different paths (or OCAB + OCBA,VERTICAL-CONTROL DATA 23 which, as a matter of convention, we consistently compute in a clockwise sense), is a large number with respect to the vertical signals reported here, the precision of its formulation may be critical to our reconstruction. The results of repeated levelings around the same loops support the argument that the orthometric closures in southern California are generally small. For example, the observed misclosures around the Saugus-Lebec-Bakersfield-Mojave-Palmdale-Saugus circuit based on 1926 and 1953/55 surveys were +0.0257 m and +0.0244 m, respectively (fig. 4)2 * *. Because these misclosures are nearly identical and fall well below the first-order rejection limit for this loop (about 0.07 m), neither should be dismissed as the product of compensating error. Were the precise orthometric closure around this loop (fig. 4) on the order of a decimeter, we would expect that at least one of these misclosures would approach this value. Because both of the measured misclosures are relatively small, the orthometric closure is itself probably very small. Moreover, since the addition of the orthometric correction based on normal gravity actually enlarges these misclosures (fig. 4), and because the measured misclosures are nearly identical, it is especially likely that a perfectly formulated orthometric correction around this loop is opposite in sign and probably departs from the value shown by no more than a centimeter or two. We have made similar comparisons around many of the loops shown in plate 5; with the exception of those cases in which one or the other of the surveys can be independently shown to have been contaminated by movement during the leveling, these comparisons all show similarly small, repeated misclosures. Hence, this subjective analysis argues that the orthometric closures in southern California are generally near or below the l-cx random-error level (roughly 0.026 m for the first-order misclosures around the Saugus-Lebec-Bakersfield-Mojave-Palmdale-Saugus circuit; see fig. 4). Alternatively, we can compare calculated orthometric closures based on observed gravity with those based on normal gravity. Computer-based numerical integrations around the circuits Los An-geles-Saugus-Mojave-Barstow-Colton-Los Angeles and Colton-Victorville-Lucerne Valley-Big Bear City-Colton (pi. 5) produce observed-gravity orthometric closures of +0.0282 m and +0.0376 m, re- 2Dates separated by slashes refer to surveys performed during the stated years. The convention “1953/55” indicates that leveling done during 1953 and leveling done during 1955 are combined and treated as a single survey. spectively. These compare with normal-gravity orthometric closures around the same loops of + 0.0142 m and +0.0276 m, respectively. The differences between these separately determined values are 0.0140 m and 0.0100 m, respectively, values which are well within the expected random error range for both loops. Because the observed-elevation data and observed-gravity data have not been generally available in the machine-readable form that permits computer calculation of the observed gravity orthometric correction, we have also produced a number of manually calculated numerical integrations based on an expression developed by Petr Vanicek (written commun., 1977). Vanicek’s expression provides for a correction to be added to the approximate (normal-gravity) orthometric correction, given the existence of a suitable Bouguer gravity-anomaly map—in this case, the new gravity map of California (Oliver and others, 1980). Thus, according to this formula, OGCy =-^(5AflfB-0.1119A/lij)? (11) where OGCy = the correction (in meters) to be added to the normal-gravity orthometric correction applicable to the measured elevation difference (8n) between two nearby points, i and j, = the mean height in m between i and j, G = 106 mGal 8Agf*f = the difference in Bouguer gravity values between i and j, 0.1119 = a constant in mGal/m, and A/iy = the height difference (in meters) between i and j. Based on this expression (equation 11), we have calculated: (1) observed-gravity orthometric closures around five loops that range from less than 200 to nearly 1,000 km in length and (2) repeatedly determined observed-gravity orthometric heights at eight widely separated marks developed from successive levelings over significantly different routes. The orthometric closures based on observed gravity differ from those based on normal gravity by amounts ranging from a few millimeters to slightly less than 0.04 m. The magnitudes of the discrepancies between the two sets of closures seem to be independent of circuit length, but they may be crudely correlative with the occurrence of steep gravity gradients. Comparisons of the eight sets of successively determined observed-gravity24 THE EVOLUTION OF THE SOUTHERN CALIFORNIA UPLIFT, 1955 THROUGH 1976 orthometric heights with those based on normal gravity show, as might have been anticipated, that the two categories of heights differ significantly (by as much as 0.2 m). However, the differences between the measured height changes developed from orthometric heights corrected for observed gravity versus those corrected for normal gravity are generally trivial and range up to about 0.03 m. The evidence developed in the preceding paragraphs indicates that, while orthometric heights based on observed gravity clearly are to be preferred over those based on normal gravity, use of the approximate (or normal-gravity) orthometric correction should result in no more than very slight errors in any of the comparisons described here. 36° 00' 36°00‘ j II9#00‘ II8"00‘ In fact, those bench marks that seem especially critical to our reconstruction, are just those marks at which we find the smallest discrepancies between orthometric heights based on observed gravity and those based on normal gravity. Accordingly, unless otherwise specified, the orthometric corrections and orthometric heights used or cited in subsequent parts of this report are based on normal rather than observed gravity. THE RECONSTRUCTION The basic data used in our reconstruction of the southern California uplift consist almost exclusively of corrected observed-elevation differences. --1--36#00‘ 36*00'-1 II9°00' II8°00‘ EXPLANATION MEASURED MISCLOSURE ORTHOMETRIC CORRECTION ORTHOMETRICALLY CORRECTED MISCLOSURE 0 10 20 30 40 50 KM 1 _i___l----------1----------1---------1----------1 119*00' A 34*00' 118*00' J 119*00' B 34*00' 118*00' J FIGURE 4.—Misclosures around the circuit Saugus-Lebec-Bakersfield-Mojave-Palmdale-Saugus based on levelings in 1926 (A) and 1953/55 (B). Data source for each survey segment indicated by National Geodetic Survey line number or by book or line number (if available) of originating agency; bench marks identify junction points at each end of the indicated segment or line; dates of leveling for each segment shown in parentheses. Measured misclosure is based on rod- and (commonly) instrument-corrected observed elevation differences derived, unless otherwise stipulated, from results of first-order leveling; orthometric correction based on normal gravity. Misclosures based on clockwise summations.THE RECONSTRUCTION 25 Corrections applied to the measured (field) elevation-differences consist of: (1) a temperature correction intended to account for any expansion or contraction of the rod with respect to the length of the rod at the calibration temperature, (2) a rod excess correction obtained through periodic calibrations and intended to compensate for differences between the nominal and actual lengths of the rod, and (3) relatively rarely (since it is generally tightly controlled through field procedures), an instrument correction (Rappleye, 1948b, p. 17-30). Although the observed data are not explicitly reiterated here as such, they are presented in a comparative format, chiefly as profiles of observed elevation changes (which can be equated with changes in orthometric height). These data could just as easily have been presented as tabulations of observed elevations and elevation changes; however, graphic representations provide a readily understood and much more useful way of showing how the uplift has evolved in different parts of the affected area. Each set of vertical-displacement profiles is accompanied by a terrain profile, a bar diagram specifying the interval during which each segment of leveling was completed, the order and rejection limit (if applicable) of the leveling, and the source of the data (by NGS line number if contained within their files). The successive elevations used in calculating the vertical displacements described here have been reconstructed, insofar as possible, with respect to bench mark Tidal 8, San Pedro (pi. 5).The reconstruction process consists simply of correcting the observed elevations by some constant along a continuous line of leveling, whereby the starting elevation is brought into conformity with the ending elevation at the junction between two connecting segments of continuous leveling. Hence, for any specifically defined leveling, the reconstructed elevations are ideally equivalent to those that would have been produced had they been based on continuous leveling emanating directly out of Tidal 8. Where the routes of successive levelings locally diverge, the difference in measured elevation differences implicit in leveling over different paths is disregarded if the orthometric closure around the divergent legs is less than 1 mm. Since the reconstructed elevations may vary by a millimeter or two (depending on the choice of mark at the junctions between segments of continuous leveling) there is little point in introducing a correction of even lesser magnitude. Bench mark Tidal 8 has been chosen as our primary reference point chiefly because it is virtually the only control point through which we may relate the observed elevations over the entire area of the southern California uplift. This bench mark is, in addition, adjacent to an automatic tide gauge (Berth 60) that has been in operation since 1924. Because continuous measurements at this gauge show that this site has been characterized by relatively negative sea-level changes with respect to most of the other primary tide stations in California (Hicks and Shofnos, 1965, p. 24-25; Hicks and Crosby, 1974, p. 4-5), height changes referred to Tidal 8 probably are biased against the detection of uplift. For example, comparison of the mean sea-level record obtained at Berth 60 with that obtained from San Diego (the nearest long-term, continuously operating station) shows that mean sea level at Berth 60 has been generally falling with respect to San Diego (fig. 5) or, alternatively, that Tidal 8 has been rising with respect to the San Diego tide station. A simple linear regression for the full period of comparison (1927-75) indicates that San Diego has been subsiding with respect to Tidal 8 at about 1.3 ± 0.1 mm/yr, a figure that is only slightly less than the long-period sea-level rise of about 1.5 ±0.3 mm/yr established for the conterminous United States (Hicks and Crosby, 1975). The remarkably close correspondence between these two values suggests that the San Diego tide station has been rising at such a slow rate with respect to any arbitrarily defined invariant datum that it can be treated as if it were tectonically stable. This conclusion is, in fact, supported by uplift rates developed from dated marine terraces, which show that San Diego has been rising at about 0.2 mm/yr (with respect to present sea level) during late Quaternary time (Wehmiller and others, 1977b, table 13). Thus, we may safely infer that over any recent 15-year interval, “absolute” uplift of 18-27 mm could be completely masked were it based on successive surveys referenced to Tidal 8 as invariant. We have chosen 1955 as our primary reference datum chiefly because it falls within a period (1953-58) that probably was characterized by tectonic quiescence over most of the area embraced by the southern California uplift. Nevertheless, even if we are incorrect in this assessment, we are obliged to choose some datum of about this vintage simply to ensure that we have described as completely as possible those vertical displacements that have been involved in the evolution of the uplift. Most of that part of southern California with which we are concerned was not covered by first- or second-order leveling during 1955 (pi. 5); hence, we have been forced to resort to several artifices in order26 THE EVOLUTION OF THE SOUTHERN CALIFORNIA UPLIFT, 1955 THROUGH 1976 to at least approximate the cumulative vertical displacements since 1955. In the absence of 1955 leveling along a primary vertical control line, we have generally adopted the most recent pre-1955 datum as an equivalent base; the validity of this procedure usually can be appraised through various independent tests. If we can show that the indicated survey route sustained (or probably sustained) significant movement during the interval between the earlier leveling and 1955, or if the most recent pre-1955 datum predates 1955 by more than a quarter of a century, we have rejected any assumed equivalence with a 1955 datum. Alternatively, we have from place to place adopted the earliest post-1955 datum as the equivalent of one that would have been produced in 1955, provided that the collective evidence supports this postulated equivalence. We have, however, been reluctant to accept post-1959 datums as 1955 equivalents simply because we are virtually certain that the deformation associated with the uplift began no later than 1960 and may have begun by the end of 1959. Finally, there are a number of cases where an equivalence between either a pre- or a post-1955 and a 1955 datum cannot be directly determined, and the acceptability of such an equivalence ultimately depends on the resulting coherence of our interpreted reconstruction over the entire area of the uplift. The histories of vertical movement along each of a number of primary control lines are separately described in the following sections. Displacements recognized along relatively short lines or spurs, where the reconstructed elevations and height changes can be tied to Tidal 8 through the primary control line, are discussed together with the associated primary line. Special procedures, required assumptions, and problems involved in the generation of particular reconstructed elevations or height changes are also considered in the line discussions. Because each successive leveling along a given line generally has been accompanied by new monumentation, because there has almost inevitably been some destruction of marks between levelings, and because elevation changes between levelings would not otherwise be clearly evident, each set of profiles has been reconstructed with respect to a series of progressively later datums. Presentation of the data in this form tends to minimize any ambiguity yet preserves as much detail as possible. THE VENTURA-AVILA BEACH LINE GENERAL REMARKS Calculated height changes between Ventura and Avila Beach (pi. 6) depend principally on the results of only two levelings. The earlier leveling consists of a combination of 1956 and 1960 surveys; the second was completed during the period 1970/71. Because significant vertical displacements probably occurred during junction intervals that accompanied both of these surveys, each of the resulting sets of observed elevation differences is almost certainly distorted. That is, a simple comparison of the survey data that disregards the likely occurrence of movement at Surf duringthe 1956-60 junction interval and that at Point Conception during FIGURE 5.—Changes in mean sea level (Ah) at Berth 60, Los Angeles (San Pedro), with respect to Municipal Pier, San Diego. Based on unpublished data of the Tides and Currents Division of the National Ocean Survey (J. R. Hubbard, written commun., 1977).THE RECONSTRUCTION 27 the 1970-71 junctioning produces a seeming 0.23-m uplift of Avila Beach that is clearly inconsistent with the sea-level records and apparently an artifact of the reconstruction. The probable occurrence of intrasurvey movement during the 1956/60 and 1970/71 levelings effectively limits useful comparisons to two parts of the main line: the section between Ventura and Gaviota and that between Surf and Avila Beach. Because the results both of repeated levelings and of discontinuous sea-level measurements indicate that the Santa Barbara tide station remained virtually invariant with respect to San Pedro during the interval 1920-60, 1939-48 surveys emanating from Santa Barbara can be treated as the equivalent of those propagated directly out of Tidal 8. Thus the results of the 1939/42/48 leveling tied to the reference tidal bench mark at Santa Barbara provide a reasonable preuplift datum for the sections between both Ventura and Gaviota and Gaviota and Los Olivos. Similarly, because the vertical displacement history of the Avila Beach tide station with respect to San Pedro can be assessed through differencing of the sea-level means, and because we are concerned here with height changes rather than with heights as such, the 1956 observed elevations provide an equally useful preuplift datum between Avila Beach and Surf. A comparison of the results of the 1960 leveling against the preuplift datum shows that the uplift reached a maximum of about 0.07 m at Carpinteria and diminished westwardly to almost zero in the area of Gaviota. Two independent assessments indicate major tectonic subsidence at Surf during the interval 1956-60. Because the larger value is based on what we believe to be an unsupportable presumption of stability along the entire line during the 1970/71 leveling, subsidence at Surf of about 0.16 m, suggested by a 1960 misclosure on the 1956 height of Surf, is thought to be the more accurate estimate. By 1970, the uplift had increased to roughly 0.10 m within the reach between Carpinteria and Gaviota and the tectonic subsidence had diminished to about 0.08 m at Surf and to still lesser values northward to Avila Beach. DETAILED RECONSTRUCTION The Ventura-Avila Beach line consists of a primary control line between Ventura and Avila Beach, together with a single spur between Gaviota and Los Olivos incorporated in an alternative representation of the vertical movement history along this line (pi. 6). Starting elevations at Ven- tura are based on continuous first-order levelings between Tidal 8 and Ventura in 1960 and 1970/71. The 1960 starting elevation at Ventura derives from leveling between Tidal 8 and Ventura during the period March-May 1960 (NGS lines L-17850 and L-17847). Continuation of the “1960” datum northward from Surf to Avila Beach is based on 1956 leveling and an assumption of invariance between 1956 and 1960 at the junction bench marks at Surf. The 1970/71 starting elevation at Ventura is based on leveling between Tidal 8 and Ventura during the period October 1970-March 1971 (NGS line L-22292). The 1970 elevations between Pismo Beach and Avila Beach are based on the results of 1956 leveling, coupled with the assumption that the elevation differences remained invariant over this 10-km reach between 1956 and 1970. The latest pre-1956 datum between Ventura and Avila Beach (or Pismo Beach) is based on 1920 leveling and is, as such, the only possible alternative to a combined 1956/60 datum as an approximation of a hypothetical 1955 primary datum. Even though its use violates our operating principle that this datum be no earlier than 1930, a 1920 datum is clearly preferable in the sense that it certainly predates any deformation associated with the initial development of the southern California uplift, whereas the 1956/60 datum probably does not. Nevertheless, the period 1920-56 includes the 1925 M = 6.3 Santa Barbara earthquake and the 1927 M = 7.5 Lompoc (or Point Arguello) earthquake, both of which occurred along or adjacent to this survey route (Richter, 1958, p. 534; Gawthrop, 1975, p. 8—9, 9-14). Morever, the relatively large misclosure around the 1920/27 circuit Gaviota-Surf-Pismo Beach-Buellton-Gaviota (fig. 6) suggests significant crustal deformation in this area during the period 1920-27. Accordingly, because we have no way of assessing the preseismic, coseismic, or even postseismic vertical movements that may have been associated with these shocks, we are especially reluctant to identify the 1920 elevations along this line with a hypothetical 1955 datum. Comparison of the results of the 1970/71 surveys against a combined 1956/60 datum discloses modest uplift along the generally east-trending coastline between Ventura and Point Conception, together with sharply increasing uplift where the coast turns northwestward at Point Conception (pi. 6A). The 1960-70/71 uplift increased from virtually zero at Ventura to 0.05-0.06 m immediately east of Carpinteria and ranged generally between 0.05 m and 0.10 m westward from Carpinteria to about bench mark G 1050 near Point Conception. Northward28 THE EVOLUTION OF THE SOUTHERN CALIFORNIA UPLIFT, 1955 THROUGH 1976 from Point Conception, the apparent uplift increased to about 0.17 m at bench mark W 536 at Surf. Continuation of the comparison beyond Surf indicates that the 1956/60-70/71 uplift seemingly persisted (together with locally developed differential subsidence attributable to fluid extraction; see pi. 3), rising gradually toward the northern end of the line to a maximum of about 0.23 m. An alternative reconstruction is suggested by several relatively large misclosures based on 1956 and 1960 levelings in the western Transverse Ranges (fig. 7). The 1956-60 misclosures, both of which are well above first-order limits, are consistent with crustal instability and are indicative of down-to-the-west tilting between Buellton-Los Olivos and Surf of 0.09-0.13 m during the period 1956-60. This probable instability, accordingly, precludes the use of the results of the 1960 surveys as SAN LUIS OBISPO 0 10 20 30 40 50 KM 1 -1-----1-----------1__________I____________I___________I I2I°00' 120*00' FIGURE 6.—Misclosure around the circuit Gaviota-Surf-Pismo Beach-Buellton-Gaviota based on 1920 and 1927 levelings. See figure 4 for adopted conventions and explanation of symbols. a primary datum—that is, one that roughly matches the datum that would have been generated through 1955 leveling along this same route. The development of an alternative primary datum derives from the recognition of relative stability at Santa Barbara during the period 1920-74 and sea-level measurements at the Avila Beach tide station as a basis for establishing elevations with respect to Tidal 8. Thus, the elevations of bench mark 0 28, Santa Barbara, with respect to Tidal 8, are: 1920—3.0915 m (the observed elevation at 0 28 has been reduced by 3 mm to account for the difference between the orthometric correction based on the 1920 interior route and that associated with the two subsequent levelings which followed nearly identical routes along the coast; it is assumed that bench mark I 33, San Pedro, remained invariant with respect to Tidal 8 during the period 1920-26) 1960—3.0911 m 1970/71—3.1278 m (NGS lines 74203, 82583, L-17847, L-17850, L-21366, L-21537, L-21729, and L-22292). Because 0 28 remained virtually invariant with respect to Tidal 8 during the period 1920-60 (a conclusion supported by discontinuous sea-level measurements at Santa Barbara which show that Santa Barbara subsided less than 0.01 m with respect to Tidal 8 during the period 1933/34-74; J. R. Hubbard, National Ocean Survey, written commun., 1977), leveling emanating from 0 28 between 1920 and 1960 may be treated as if it had emanated directly from Tidal 8. Accordingly, the results of 1942 and 1948 levelings originating at 0 28 should closely approximate those that would have been obtained had these surveys originated at Tidal 8. Because the 1942 surveys extended eastward only as far as Carpin-teria, we have used the results of 1939 leveling to develop a pre-1960 datum between Carpinteria and Ventura. Use of this 1939/42/48 datum as the approximate equivalent of a hypothetical 1955 datum is supported by a relatively small misclosure around the 505-km loop Santa Barbara-Santa Maria-San Luis Obispo-McKittrick-Maricopa-Ven-tura-Santa Barbara based on leveling carried out over the full interval 1934-57 (fig. 8). Morever, were it not for various differential displacements that almost certainly occurred at several junctions along this route, this misclosure (fig. 8) could have been even less. (For example, differential subsidence at I 30 during the period 1934-39 or at M 569 during the period 1939-42 would increase any positive clockwise misclosure.) Hence, we have pro-THE RECONSTRUCTION 29 visionally accepted the combined results of the 1939, 1942, and 1948 levelings emanating from bench mark 0 28 as the approximate equivalent of a 1955 datum emanating from Tidal 8. Although there are no data permitting the development of a pre-1960 datum between Gaviota and Surf (other than those obtained from the 1920 leveling), a 1956 datum between Avila Beach and Surf can be reconstructed through a combination of levelings between Tidal 8 and Avila Beach and sea-level measurements at both Avila Beach and San Pedro. Thus, the 1970/71 elevation of bench mark Tidal 11, Avila Beach, is determinable through the results of more or less continuous 1970/ 71 leveling between Tidal 8 and bench mark X 25 and 1956 leveling between X 25 and Tidal 11, where it is assumed that the elevation difference between X 25 and Tidal 11 remained unchanged between 1956 and 1970 (NGS lines L-15972 and L-22292). The 1956 elevation of Tidal 11 with respect to Tidal 8 SAN LUIS OBISPO AVILA BEACH is based on a comparison of annual sea-level means that show that the Avila Beach tide station subsided 21.4 mm between 1956 and 1970 (J. R. Hubbard, National Ocean Survey, written commun., 1977). The random error alone in the reconstructed 1956 elevations between Avila Beach and Surf probably is at least 50 percent greater than that based on leveling that proceeded directly out of Tidal 8. Morever, some uncertainty is occasioned by the probable noise level inherent in the differencing of sea-level measurements over the 300-km distance between San Pedro and Avila Beach. Nevertheless, both of these errors are probably in the centimeter range and the only necessary procedural assumptions are: (1) that the elevation difference between X 25 and Tidal 11 remained invariant during the period 1956-70/71, (2) that any changes in eustatic sea level have been expressed equally at both tide stations, and (3) that any changes in salinity, temperature, and other factors SAN LUIS OBISPO AVILA BEACH 0 10 20 30 40 50 KM I2I°00' I----34°00' /I It.'-' .J I21°00' I----34"00' I C.\J J FIGURE 7.—Misclosures around the circuit Surf-Avila Beach-Harris-Gaviota-Surf (A) and the circuits Surf-Avila Beach-Harris-Surf and Surf-Harris-Gaviota-Surf (B) based on 1956 and 1960 levelings. See figure 4 for adopted conventions and explanation of symbols.30 THE EVOLUTION OF THE SOUTHERN CALIFORNIA UPLIFT, 1955 THROUGH 1976 that affect the length of the water column can be disregarded over the 300-km distance between San Pedro and Avila Beach. Accordingly, the reconstructed 1956 datum should roughly approximate a 1956 (1955-equivalent) datum based on leveling emanating from Tidal 8. The alternative representation of elevation changes between Ventura and Avila Beach (pi. 6B) provides further insight into the history of vertical movement in the western Transverse Ranges and contradicts, in part, the simple comparison between the results of the 1970/71 leveling against a 1956/60 datum (pi. 6A). Westward from Ventura, uplift during the period 1939/42/48-60 increased sharply from about -40 mm to roughly 70 mm at Carpinteria, declined to a nearly uniform figure of 40-50 mm between Carpinteria and Santa Barbara, and dropped still further between Santa Barbara and Gaviota to about 20 mm (pi. 6B). Cumulative vertical displacements during the period 1939/42/48-70/71 increased westward from about -40 mm at Ventura to well over 100 mm at Carpinteria, and held at or slightly below 100 mm between Carpinteria and Gaviota (pi. 6B). The height changes between Surf and Avila Beach shown in the alternative reconstruction (pi. 6B) differ strikingly from those shown on profile A. Specifically, rather than uplift increasing northward from Surf during the period 1956-70, as shown in profile A, the alternative reconstruction H 119°00' I2I°00' I---34°00‘ 34° 00 II9°00' I*-----• FIGURE 8.—Misclosure around the circuit Santa Barbara-Santa Maria-San Luis Obispo-McKittrick-Maricopa-Ventura-Santa Barbara based on 1948,1956/57,1934/35,1939, and 1942 levelings. See figure 4 for adopted conventions and explanation of symbols.THE RECONSTRUCTION 31 indicates that this reach sustained tectonic subsidence that accumulated more or less uniformly southward from Avila Beach. Similarly, a comparison of 1960 elevations based on leveling originating at Tidal 8 against the reconstructed datum indicates that Surf sustained tectonic subsidence of about 0.25 m during the period 1956-60. Moreover, even though major uplift is inferred to have occurred at Surf between 1960 and 1970/71 (pi. 6A), it was apparently insufficient to restore Surf to its preuplift (1956) height. Hence, while the 1956-60 tectonic subsidence at Surf seems surprisingly large (pi. 6B), it is certainly not impossible. Although we are convinced that the alternative reconstruction (pi. 6B) is a much more realistic and generally more consistent appraisal of height changes along the Ventura-Avila Beach line than that shown on profile A, the cumulative evidence suggests that certain aspects of this reconstruction (especially the 0.25-m 1956-60 subsidence at Surf) may be significantly in error. For example, if it is assumed that the western Transverse Ranges were characterized by crustal stability during the period 1934-57, unqualified acceptance of the alternative reconstruction coupled with the invariance of bench mark I 30 between 1939 and 1960 (see section on “The Ventura-Maricopa Line”), argues that the clockwise misclosure around the loop Surf-San Luis Obispo-McKittrick-Maricopa-Ventura-Gaviota-Surf based on 1960,1956,1957,1956/57, and 1934/35 levelings should be about -0.25 m. Moreover, if allowance is made for the probable tectonic subsidence of 0.06 m at bench mark H 326 (fig. 8) during the 1952 Kern County earthquake (see section on “The Ventura-Maricopa Line”), this postulated misclosure would enlarge to about -0.3 m. In fact, however, the actual misclosure is only about -0.06 m (fig. 9). Several explanations may account, in whole or in part, for this seeming contradiction. (1) To the extent that it may indicate an error in the represented misclosure (fig. 9), an imprecisely formulated orthometric correction seems an especially plausible partial explanation for this apparent discrepancy. Not only might the orthometric closure be a good deal smaller than shown here, it might even be opposite in sign. Moreover, this possibility is consistent with the positive misclosure around the somewhat shorter loop that excludes the results of the 1960 leveling (fig. 8). That is, given the occurrence of 0.06 m of tectonic subsidence at H 326 during the 1952 earthquake, this misclosure (fig. 8) might otherwise have been even larger; algebraic reduction of the orthometric correction could reduce even the displacement-cor- rected misclosure to a value well within first-order limits (0.0899 m). Nevertheless, all of our experience in this area indicates that errors in the orthometric closure in excess of 0.05 m must be very rare. Hence, while rigorous calculation of the orthometric closure (based on observed gravity) could lead to a corrected misclosure involving the results of the 1960 leveling (fig. 9) of as much as -0.11 m, a value more in keeping with that predicted by the alternative reconstruction (pi. 6B), it could account for only a small fraction of the approximately 0.2-m discrepancy. (2) It is conceivable that the junction marks around that part of the loop north and east of Surf (fig. 9) sustained artificially induced differential displacements between connecting levelings. This possibility is challenged, however, by the relatively small misclosure that excludes the results of the 1960 leveling (fig. 8). (3) Finally, the indicated discrepancy may, in fact, be more apparent than real. Specifically, clockwise around the nearly congruent circuits shown in figures 8 and 9, the misclosures are based on the results of the same levelings between bench marks G 740 and I 30. Thus, if we assume that the actually measured elevation difference over the relatively short reach between N 761 and G 740 produced through 1948 leveling between N 761 and U 64 and 1956 leveling between U 64 and G 740 is the same as that that would have been obtained through 1948 leveling between N 761 and W 536 and 1956 leveling between W 536 and G 740, and if we disregard the trivial difference in the orthometric correction around these two loops (figs. 8 and 9), the misclosure around the exterior loop must have enlarged by —0.1651 m during the period 1956-60. This inferred change in the misclosure is much more consistent with the tectonic subsidence deduced from the alternative reconstruction (pi. 6B), and while it could have occurred for whatever reason, it is presumably attributable to tectonic activity within the reach between bench mark I 30 and W 536 during the period 1956—60. The validity of the reconstructed 1956-60 height changes at Surf (pi. 6B) ultimately depends on the validity of the 1970/71 height difference between Tidal 8 and the Avila Beach tide station, a value that is currently suspect. Balazs and Douglas (1979) present fairly convincing evidence that the 1968/71 San Francisco-San Pedro height difference, based on leveling that includes the results of the 1970/71 survey between X 25 and Tidal 8, may be in error by 0.5 m or more. Thus there is a reasonable basis for assuming that a sizable fraction of this error occurred between Avila Beach and San32 THE EVOLUTION OF THE SOUTHERN CALIFORNIA UPLIFT, 1955 THROUGH 1976 Pedro. And, in fact, recent investigations (Castle and Elliott, 1982, p. 7012-7021) have shown that a 0.2-m displacement could easily have occurred (and probably did occur) at Point Conception during a 3-month interruption in the 1970/71 “continuous” leveling. Accordingly, the indicated 1956-60 tectonic subsidence at Surf is almost certainly in error, whereas the 1956-70 displacements between Avila Beach and Surf are much less subject to question (pi. 6B). That is, while all of the possible explanations set forth in the preceding paragraph may have contributed in some measure to the discrepancy between the orthometrically corrected misclosure (fig. 9) and the represented 1956-60 height change at Surf (pi. 6B), the change in mis- closures between 1956/57 and 1960 (figs. 8 and 9) probably closely approximates the actual 1956-60 tectonic subsidence at Surf. Hence the representation shown on plate 6B probably exaggerates the 1956-60 tectonic subsidence at Surf by about 0.1 m. Height changes along the Gaviota-Los Olivos spur (pi. 6B) are based on comparisons between the results of 1948 (1955-equivalent) leveling emanating from 0 28, 1956 (1955-equivalent) leveling emanating from Avila Beach (NGS lines L-15972 and L-15975), and 1960 leveling emanating from Tidal 8. While the apparent subsidence of U 64 of 4.2 mm during the period 1948-60 contrasts sharply with the much greater (orthometrically corrected) subsidence of 113.4 mm between 1956 and 1960, both 35° Od—( II9°00' 121*00' L 34*00' 34° OCf 119*00' J FIGURE 9.—Misclosure around the circuit Surf-San Luis Obispo-McKittrick-Maricopa-Ventura-Gaviota-Surf based on 1956/57, 1934/35, and 1960 levelings. See figure 4 for adopted conventions and explanation of symbols.THE RECONSTRUCTION 33 values indicate that the southern California uplift could not have extended significantly west of Los Olivos by 1960. Moreover, the relatively tight closure around the 505-km circuit that involves both the 1948 and the 1956 levelings (fig. 8) suggests that the discrepancy between the 1948-60 and the 1956-60 subsidence of U 64 cannot be attributed solely to uplift of U 64 between 1948 and 1956. Hence we again suspect that this discrepancy is a measure of the inaccuracy of the reconstructed 1956 elevations (based on a 1970/71 tie between Tidal 8 and Avila Beach and the 1956 leveling originating at Avila Beach). That is, the 1956-60 tectonic signal at Los Olivos probably was virtually zero. THE VENTURA-MARICOPA LINE GENERAL REMARKS Assessments of the vertical displacements along the Ventura-Maricopa line (pi. 7) are complicated by the absence of an unambiguous preuplift datum and the probable occurrence of intrasurvey movement during one of the three surveys along the full length of this line. Although we have provisionally adopted a combined 1934/35 Ventura-Maricopa and 1942/43 Ozena-Frazier Park datum as the approximate equivalent of a hypothetical 1955 datum, it is virtually certain that the northern end of this line sustained at least modest coseismic deformation during the 1952 Kern County earthquake. Accordingly, in calculating post-1955 uplift along this line, use of the pre-1952 datum requires that we subtract out, wherever possible, any coseismic vertical displacements associated with this earthquake. The near invariance of the southern end of the Ventura-Maricopa line during the 1952 earthquake, together with the results of 1953 leveling into the northern part of this area, provides a basis for a reasonably good characterization of the 1952 coseismic movements along much of this line. Specifically, immediately north of Ozena, the 1934/35 heights were as much as 0.10 m above the 1953 postearthquake heights, whereas northward toward Maricopa this height difference diminished to about 0.06-0.07 m and, less certainly, to values approaching zero southward toward Wheeler Springs. Eastward from Ozena to Frazier Park, on the other hand, the coseismic subsidence inverted to uplift of about 0.06 m in the Lebec area. Thus, during the period 1953-59/60 aseismic uplift of about 0.21 m accumulated in the Ozena area and uplift of nearly this magnitude apparently persisted northward toward Maricopa, yet diminished southward to no more than 0.08-0.10 m in the Wheeler Springs area and eastward to roughly 0.18 m at Lebec. However, the exclusion of equivocally defined 1953-57 post-seismic adjustment along the northern end of the line suggests that the 1955-59/60 uplift may have been somewhat less than these actually measured values. By 1968, the heights along most of the line had increased by 0.08-0.10 m over those that obtained in 1959/60, and the uplift reached a cumulative maximum at Ozena of about 0.31 m. Finally, comparisons of the results of 1974 surveys (limited to the southern half of the main line) against a 1968 datum indicate that a significant change in the displacement pattern, expressed as 0.05 m of tectonic subsidence over a 15-km reach between Ventura and Wheeler Springs, apparently occurred during this latest interval. An alternative reconstruction of the data is suggested by the nearly certain stability of central Bakersfield during the period 1953-59 and a 0.29-m misclosure interpreted as the product of intrasurvey movement during the course of 1959-61 levelings through the west-central Transverse Ranges. Accordingly, because the 1953 heights of the Bakersfield marks are based on levelings tied directly to Tidal 8, the 1959 leveling emanating from Bakersfield may be treated as the equivalent of that propagated directly out of Tidal 8. Contrary to our expectation, this alternative reconstruction indicates that our basic presumption of stability at Ozena is invalid, and that Ozena actually rose 0.16 m between the 1959 and 1960 levelings into (or out of) Ozena. This same reconstruction also indicates that Lebec sustained tectonic subsidence of 0.11-0.14 m duringthe period 1959-60/61 and, hence, that a 1959-60/61 up-to-the-west tilt between Lebec and Ozena accounts for nearly the entire 1959-61 misclosure. Acceptance of this alternative reconstruction effectively divides the vertical displacement history along the Ventura-Maricopa line into two parts: one extending northward and eastward from Ozena to Maricopa and Lebec, respectively, and a second extending southward from Ozena to Ventura. The chronology of the uplift and partial collapse is much more clearly documented along the southern half of the line, whereas the magnitude of the uplift (through 1968) is less ambiguously described along the northern half. The ultimate effect of this preferred reconstruction is to reduce the calculated maximum aseismic uplift along the Ventura-Maricopa line from 0.31 m to about 0.28 m.34 THE EVOLUTION OF THE SOUTHERN CALIFORNIA UPLIFT, 1955 THROUGH 1976 DETAILED RECONSTRUCTION The Ventura-Maricopa line consists of a primary vertical-control line between Ventura and Maricopa, together with a single spur between Ozena and Frazier Park (pi. 7). Starting elevations at Ventura are based both directly on a series of virtually continuous first-order levelings between Tidal 8 and Ventura and indirectly on repeated first-order surveys between Tidal 8 and bench mark 0 28, Santa Barbara. The 1934 starting elevation at bench mark I 30, Ventura (pi. 7), is based on the results of a combination of surveys of various vintages. Because bench mark 0 28 remained nearly invariant with respect to Tidal 8 during the period 1920-60 (see section on “The Ventura-Avila Beach Line”), and because the observed elevation difference between bench marks 0 28 and D 29, Carpin-teria (pi. 6), increased by less than 12 mm between 1920 and 1942 (NGS lines 74203 and L-9449), 1939 leveling emanating from D 29 may be treated as if it had originated at Tidal 8. Accordingly, combining the 1939 elevation difference between D 29 and I 30 (NGS line L-8470) with the 1920 elevation of D 29 (NGS lines 74203 and 82583—rather than with the more complexly determined 1942 elevation based on leveling emanating from 0 28, a determination that would increase the length of the Tidal 8-D 29 survey route by about 40 km—produces a 1939 observed elevation for I 30 of 5.2464 m. The reconstructed 1920 and 1960 orthometrically compatible observed elevations of I 30 are given as 5.3078 m and 5.2157 m, respectively (NGS lines 74203, 82583, L-17847, and L-17850); because the resulting 1939 interpolated elevation of I 30 is only about 0.01 m greater than the 1939 reconstructed elevation (5.2589 m versus 5.2464 m), it supports the validity of the reconstruction. Nonetheless, the 1934 starting elevation of bench mark I 30 finally depends on an assumed invariance of this mark between 1934 and 1939. The likelihood that I 30 in fact remained unchanged in height during this period is significantly enhanced by its relative stability during the full period 1939-73, and especially during the period 1939-60 (fig. 10). The 1960 starting elevation is based on leveling between Tidal 8 and Ventura completed during the period March-May 1960 (NGS lines L-17847 and L-17850). The 1968 starting elevation has been reconstructed from surveys carried out during the period February-September 1968 (NGS lines L-21366, L-21537, and L-21729). The 1974 starting elevation of the junction bench mark at Ventura is based on leveling completed during the period March-October 1973 (NGS lines L-23693, L-23697, L-23701, L-23709, and L-23891) and the presumed invariance of this mark during the period 1973-74. The 1953 orthometrically compatible starting elevation at Maricopa is based on the results of 1953/55 leveling propagated directly out of Tidal 8 (NGS lines L-14796, L-14799, and L-15577). The 1957 starting elevation at bench mark H 326 is based on an assumption of stability at this mark during the period 1953—57. Several lines of evidence indicate that much or most of the Ventura-Maricopa line remained relatively free of regional deformation from 1934 through at least 1957 and, hence, that the reconstructed 1934/35 observed elevations roughly approximate those that would have been developed from the results of 1955 leveling originating at Tidal 8. Implicit in this judgment is the probable invariance of junction bench mark K 174 (fig. 8) during the interval between the 1934 and 1935 levelings; the 1934-35 stability of this mark seems especially likely since there is no evidence of physical disturbance (which would be revealed as a step in the profiled elevation changes), nor is there any indication that K 174 might have sustained any artificially induced or coseismic displacements during this period (pis. 3 and 4; Hileman and others, 1973, p. 16-17). Arguments suggestive of relative stability along the Ventura-Maricopa line during the period 1934-57 include: (1) Between 1934 and 1939, bench mark M 173 sustained differential uplift with respect to 1 30 of no more than 15-16 mm (pi. 7). Moreover, because I 30 subsided at an average rate of about 2 mm/yr during the period 1920-60 (fig. 10), it is YEAR FIGURE 10.—Changes in orthometric height at bench mark I 30, Ventura. The 1939 height is based on 1920 leveling between bench mark 133, San Pedro, and bench mark D 39, Carpinteria, and on assumptions of invariance between O 28, Santa Barbara, and bench mark Tidal 8 during the period 1920-60 and between 0 28 and D 39 during the period 1920-39. The 1973 height is based on an assumption of invariance between bench mark I 30 and adjacent bench mark P 1100 during the period 1968-73. See text for details. One-standard-deviation error bars show conventionally estimated random error only.THE RECONSTRUCTION 35 likely that even this modest tilt is attributable in part to compaction-induced subsidence beneath I 30. In any case, it is reasonably certain that the 20-km reach between I 30 and M 173 experienced little, if any, deformation (exclusive of the artifi-cally induced differential subsidence centering on the Ventura oil field—see pis. 3 and 4 and Buchanan-Banks and others, 1975, p. 118,123-124) during the period 1934-39. (2) The vertical displacement history of bench mark I 30 (fig. 10) indicates that at least the southern end of this line sustained little tectonic displacement between 1939 and 1960. (3) A comparison of the results of 1935 levelings against 1957 levelings between bench marks H 326 and Q 326 shows very little evidence of regional tilting along this 20-km segment at the northern end of the Ventura-Maricopa line during the whole interval 1935-57 (pi. 7). (4) The results of repeated levelings along the National Geodetic Survey monitor line athwart the San Andreas fault (fig. 11) indicate that this section of the Ventura-Maricopa line remained virtually free of regional deformation during the period 1935-59. Although reconstruction of these data with respect to bench mark N 326 indicates that measurable movement occurred between 1935 and 1938 (fig. 11A), this movement seems to have been confined largely to the area of the monitor line, for the more inclusive section between bench marks H 326 and Q 326 was devoid of significant tilting during the period 1935-57 (pi. 7). In any event, the relative stability of the monitor line during the interval 1938-59 is clearly evident, and such deformation as did occur between 1935 and 1959 was certainly much less than subsequent deformation recognized along this line (fig. 11). (5) The misclosure around the 505-km loop based on the results of first-order levelings during the 23-year interval 1934-57, which includes the results of the 1934/35 leveling between I 30 and H 326, is only 6 mm over first-order limits (fig. 8). Moreover, the indicated orthometric correction around this loop may be significantly in error (see section on “The Ventura-Avila Beach Line”); utilization of a correction based on observed gravity could diminish this misclosure to an even smaller value. Nonetheless, this relatively small misclosure (fig. 8) suggests that the western Transverse Ranges sustained no more than modest height changes during the period 1934-57. The same set of arguments that supports the equivalence of the reconstructed 1934/35 elevations between Ventura and Maricopa with a hypothetical 1955 datum also supports the use of the 1942/ 43 reconstructed elevations between Ozena and Frazier Park as the approximate equivalents of those that would have been obtained had the leveling been carried out in 1955. The 1942/43 starting elevation at Ozena is necessarily based on the reconstructed 1934/35 observed elevation of junction bench mark 3450 (Tri-Co) (pi. 7) and the acceptance of the vertical stability of this mark during the interval 1934/35-42/43. The likelihood that the entire Ozena-Frazier Park spur, including 3450 (Tri-Co), remained free of significant vertical displacement during the interval 1934/35-42/43 is supported by the very small misclosure around the loop San Pedro-Ventura-Ozena-Lebec-San Pedro completed during the period 1920-42/43, which includes the results of both the 1934/35 surveys between Ventura and Ozena and the 1942/43 surveys between Ozena and Lebec (fig. 12). The sense of this small misclosure, moreover, is consistent with the similarly slight subsidence at Ventura (bench mark I 30) during the period 1920-39 (fig. 10). The likelihood that at least the eastern end of the Ozena-Frazier Park spur remained invariant during the period 1934/35-42/43 is reinforced by the elevation history of bench mark E 54, Lebec (pi. 7). Thus, the 1926 observed elevation (1097.5131 m) of bench mark E 54 based on first-order leveling originating at Tidal 8 (NGS lines 82466, 82583, and 82598) very nearly matches the reconstructed observed elevation (1097.4877 m) or the orthometrically compatible elevation (1097.4869 m) based on 1942/43 leveling via Ozena. Furthermore, had the 1934/35 starting elevation at I 30 been based on a 1934 interpolated elevation, rather than on a reconstructed 1939 elevation, the difference between the 1926 and 1942/43 elevations of E 54 would have been reduced by 16 mm. Thus, the very small misclosure incorporating the results of the 1934/35 and the 1942/43 surveys (fig. 12) and the excellent correspondence between the reconstructed 1926 and 1942/43 elevations of bench mark E 54 support both the likelihood of tectonic quiescence in the western Transverse Ranges between 1926 and 1942/43 and the validity of the reconstructed 1934/35 and 1942/ 43 elevations along the Ventura-Maricopa line. However, whether the inferred quiescence along the Ozena-Frazier Park spur persisted significantly beyond 1942/43 is much less certain. In spite of the various indications of crustal stability cited in the preceding paragraphs, there is fairly compelling evidence of at least modest deformation along the Ventura-Maricopa line during the period 1934-55, particularly in the area between Wheeler Springs and the San Andreas fault and eastward from Ozena to Frazier Park (pi. 7):Ah, IN MILLIMETERS 36 THE EVOLUTION OF THE SOUTHERN CALIFORNIA UPLIFT, 1955 THROUGH 1976 I- East-west- -Northeast- CO CD U c o ro ro >» * X TD C < o “D O - o CD 111 > < £|— Oh-h- UJ CO CD cr cd < < CD O < o CD < 0.30m< 1974 1944/61 9/4 (0.30-Z)m 1944 1961 (0.30-Z) m A 0.30 m< B FIGURE 40.—Postulated 1944-61 tilts over unspecified reach between Daggett and Amboy coupled with 1961-74 tilts required to preserve the elevation difference between these locales during the period 1944-74. Shaded area indicates cumulative 1961-74 uplift between arbitrary point A and Amboy. Z indicates displacement of uniform tectonic downwarping. A, 1944-61 down-to-the-west tilt associated with 1961-74 down-to-the-east reversal (dashed line). B, 1944-61 down-to-the-east tilt associated with 1961-74 down-to-the-west reversal (dashed line).74 THE EVOLUTION OF THE SOUTHERN CALIFORNIA UPLIFT, 1955 THROUGH 1976 £ o _l o ID CO CO CO CO CVJ 00 CD CD 00 00 Is- Is- Is- Is- X “3 £ >- CD II6°I5' 34045'--------- II6°00' ------34°45' FIGURE 41.—Height changes (A/i) over a 15-km reach near Ludlow. Based on a comparison between the results of 1959 third-order leveling against the 1944 datum (pi. 12), where bench mark H 728 is assumed to have remained invariant. 1959 observed-elevation differences courtesy of California Department of Transportation.THE RECONSTRUCTION 75 very little differential movement during the periods 1927-39, 1939-61, and 1927-61. Moreover, if the differential subsidence developed within the Hinkley Valley ground-water basin (see pis. 3 and 4) is subtracted from these profiles (fig. 42), the reach between bench marks E 43 and 2368 remained virtually invariant during the period 1939-61. West of bench mark 2368, especially during the period 1927-39, the elevation differences tended to diverge. Nevertheless, even though the elevation difference between Barstow and Kramer Junction increased about 0.04 m during the period 1939-61 (fig. 42), and though this figure is equivalent to about four standard deviations in the predicted discrepancy between two successive measured elevation differences between E 43 and Q 68, it still suggests a very small tilt over this 50-km distance (<1 |xrad). Moreover, because Kramer Junction lies along the eastern edge of an area of continuing tectonic activity, a part of which may have been involved with the 1952 Kern County earthquake, we would be surprised to see a complete absence of differential movement along the western end of this line. Hence, the relatively limited differential movement between Barstow and Kramer Junction during the periods 1927-39,1939-61, and especially 1927-61 (the last two of which include the 1944 leveling epoch) suggests that the area extending some indeterminate distance eastward from Barstow remained similarly free of regional tilting during the period 1939-61. FIGURE 42.—Height changes (Ah) between Kramer Junction and Barstow with respect to bench mark E 43, Barstow. Based on the results of first-order levelings by the National Geodetic Survey (NGS lines L-l, L-8531, and L-18230). A third technique used in assessing the regional stability along the line of the 1944 leveling between Daggett and Cottonwood Pass depends on an examination of misclosures developed from the results of leveling of varying vintage within and including the period 1944-61. Although, as we noted earlier, small misclosures are less than infallible indices of tectonic stability during the period of leveling around a single circuit, examination of an entire network tends to strengthen (or refute) conclusions based on closures. Two classes of misclo-sure have been used in this assessment: those derived exclusively from first-order surveys and those derived from a mix of the results of first- and third-order levelings. This assessment suggests that little, if any, regional tilting occurred within the area of the 1944 leveling during the period 1931-61. All but one short section of the largest of the loops shown here are based exclusively on first-order levelings completed between 1931 and March-April 1961 (fig. 43)—that is, prior to the propagation of the southern California uplift into the central or eastern Mojave. The one exception to this generalization consists of a 35-km survey segment between Victorville and Lucerne Valley that was completed during the fall of 1961, well after the uplift had extended into this area. Nevertheless, because the elevation difference between the ends of this segment remained virtually invariant between 1935 and October-November 1961, and because at least three-quarters of this line (defined by the extent of surveys common to both 1953 and 1961) retained its vertical integrity between 1953 and October-November 1961 (see section on “The Lucerne Valley Line”), substitution of this short segment of postuplift leveling for nonexistent 1944-60 first-order work should have a negligible effect on any resulting misclosures. The largest single loop considered here is based on leveling extending over the full period 1931-61; it is characterized by an observed misclosure of + 0.0466 m and an orthometrically corrected misclosure of -0.0110 m around a 633-km circuit (fig. 43). The smaller (403-km) loop, Barstow-Daggett-Amboy-Twentynine Palms-Yucca Valley-Lucerne Valley-Victorville-Barstow, is identified with an observed misclosure of only -0.0175 m and an orthometrically corrected misclosure of -0.0482 m. Misclosures around both of these loops are well within first-order limits. Moreover, although the smaller loop shows a larger misclosure, we suspect that the measured (observed) misclosures are nearly as reliable as indices of both stability and76 THE EVOLUTION OF THE SOUTHERN CALIFORNIA UPLIFT, 1955 THROUGH 1976 measurement precision as are the “corrected” (and, in the case of the smaller loop, larger) closures, which may be artifacts due to imprecisely formulated orthometric corrections. The first-order closures defined by the 1944 and 1931-44 levelings (fig. 44) provide some indication of the pre-1961 stability of the central and eastern parts of the area traversed by the 1944 surveys. The misclosure around the 384-km loop Amboy-Cadiz-Freda Junction-Cottonwood Pass-Twenty-nine Palms-Amboy (fig. 44) suggests relative stability during the period 1931-44 and for some indeterminate period beyond 1944. On the other hand, the 331-km loop defined solely by the 1944 leveling (fig. 44) shows a misclosure strongly indicative of either measurement error or crustal movement during the 4-month 1944 survey period. Because the closures around the larger first-order loops that include all but the Lucerne Valley-New-berry Springs segment are well within first-order limits (fig. 43), it is virtually certain that any crustal deformation that might account for the large 1944 misclosure must have occurred along this line between January and April of 1944. However, profiles of height changes that compare the results of various later surveys against a 1944 datum show no evidence of localized deformation at either end of the Newberry Springs-Lucerne Valley line (see pi. 12 and section on “The Lucerne Valley Line”). Moreover, the occurrence of aseismic tilting of this magnitude within this particular 4-month period seems most unlikely. Alternatively, the misclosure around the 1944 circuit may be due to measurement error. First-order leveling procedures are so specified that busts are extremely uncommon; nevertheless, they do occur, and a good deal of evidence indicates that the large misclosure around the 1944 loop (fig. 44) is due to a simple measurement error between Lucerne Valley and Newberry Springs: (1) The elevation differences between two adjacent bench marks, 1607 and 1604, common to both the 1944 leveling and a 1935 third-order survey (CWA line G-36) differ by 0.0822 m. Substitution of the 1935 FftSS Figure 43.—Misclosures around the circuits Barstow-Daggett-Amboy-Cadiz-Freda Junction-Cottonwood Pass-Twentynine Palms-Lucerne Valley-Victorville-Barstow and Barstow-Daggett-Amboy-Twentynine Palms-Yucca Valley-Lucerne Valley-Victor-ville-Barstow (dashed line); based on levelings carried out during the periods 1931-November 1961 and January 1944-No-vember 1961, respectively. See figure 4 for adopted conventions and explanation of symbols.THE RECONSTRUCTION 77 elevation difference between these two marks, which are located toward the northern end of the line, would reduce the 1944 observed misclosure from -0.1194 m (fig. 44) to -0.0372 m, well within first-order limits. (2) Examination of the 1944 field books that include the survey data connecting bench marks 1607 and 1604 shows a 0.091-m discrepancy between TBM 16V and BM 1607. The original “back-of-the-rod” reading is given as 1.18 ft; this reading was later changed to 0.88 ft. If the original value were correct, the elevation difference over this section would be reduced by 0.091 m and the observed misclosure would drop to - 0.0284 m. (3) The relatively large misclosures defined by the two small adjacent loops that include the 1944 Lucerne Valley-Newberry Springs segment are al- most perfectly balanced (fig. 45). The balanced nature of these misclosures is consistent with a measurement error of about 0.12 m in the 1944 leveling between Lucerne Valley and Newberry Springs. Although the balanced aspect of this misclosure is conceivably attributable to a January-April 1944 down-to-the-north tilt, this postulated tilting is clearly ad hoc, in the sense that the tilt vector would have to have virtually coincided with the survey route in order to preserve the balanced misclosures. The preceding evidence, accordingly, convincingly demonstrates that the 1944 misclosure around the loop Newberry Springs-Amboy-Twen-tynine Palms-Yucca Valley-Lucerne Valley-Newberry Springs (fig. 44) is almost entirely attributable to an error of about 0.09 m in the Lucerne II7°00 35°00 - 115° 00 NEWBERRY SPRINGS 0 10 20 30 40 50 KM 1 ___1_____I___________l__________l___________I___________l FREDA JCT COTTONWOOD PASS FIGURE 44._Misclosures around the circuits Newberry Springs-Amboy-Twentynine Palms-Yucca Valley-Lucerne Valley-New- berry Springs and Amboy-Cadiz-Freda Junction-Cottonwood Pass-Twentynine Palms-Amboy; based on levelings carried out during the periods January-April 1944 and 1931-April 1944, respectively. See figure 4 for adopted conventions and explanation of symbols.<1 oo 35*00' 115*00' Figure 45.—Misclosures around various circuits within the larger circuit Barstow-Amboy-Freda Junction-Cottonwood Pass-Yucca Valley-Victorville-Barstow developed from various combinations of the results of first-and third-order levelings. Based on surveys carried out during the period 1931-November 1961. See figure 4 for adopted conventions and explanation of symbols. THE EVOLUTION OF THE SOUTHERN CALIFORNIA UPLIFT, 1955 THROUGH 1976THE RECONSTRUCTION 79 Valley-Newberry Springs segment. Correction for this error would reduce the misclosure around this loop to about 0.03 m. Accepting this correction, all of the closures defined by the results of first-order leveling extending from 1931 through the beginning of 1961 are well below first-order limits (figs. 43 and 44) and are clearly consistent with crustal stability within the area of the 1944 leveling during the period 1944-61. A series of small loops, developed from the results of 1931, 1944, 1956, and 1961 first-order levelings and 1953 and 1955/56 third-order surveys (fig. 45), supports the preceding generalization. Although third-order leveling provides a much less sensitive vehicle for the detection of crustal deformation than does leveling of a higher order, the results of these surveys (all of which were completed well after the 1944 leveling, yet somewhat before 1961) define a relatively tight network over the area of the 1944 first-order leveling. Specifically, all but one or two of these loops (fig. 45) are identified with both balanced and very small corrected misclo-sures. While the loops that include the first-order 1944 survey between Lucerne Valley and Newberry Springs are relatively large, correction for the virtually certain 0.09-m error in this line would reduce these misclosures to about +0.03 m and — 0.03 m, respectively. The relatively large misclosure (-0.0815 m) defined by the Bagdad-Amboy-Old Dale-Twentynine Palms-Bagdad loop is almost certainly due to cumulative measurement error in the third-order line; this particular line includes the largest end-to-end elevation difference of any third-order line considered here and is, as such, the most subject to systematic error. The exceptionally small closure around the adjacent loop probably is the result of balanced errors in the third-order surveys that define this loop. The misclosures around the easternmost of the loops, which include the results of 1931 leveling, are relatively well balanced, particularly if allowance is made for the size of each of these circuits. Moreover, if it is assumed that the closure error in the first-order circuit that includes these loops (fig. 44) is evenly split between the two, subtraction of this error would increase the northern misclosure by 0.02 m and decrease the southern misclosure by 0.02 m. The resulting “corrected” misclosures would thus become nearly perfectly balanced and would indicate a measurement error of only about 0.06 m in line PV 274 (or a 1931/44-55/56 down-to-the-east tilt of this same magnitude). In short, it is clear that the misclosures developed from this network of combined first- and third-order surveys suggest, both by their generally small magnitudes and balanced nature, that this area remained free of significant (regionally defined) tilting from 1931 or 1944 through at least 1953 and probably through 1955/56. We have considered three lines of evidence, no one of which is necessarily compelling, in order to assess the crustal stability during the period 1944-61 of the area traversed by the 1944 leveling. Taken together, however, this evidence provides an excellent basis for concluding that there was very little regional deformation during this period and that we may treat the results of the 1944 levelings as the equivalent of those that would have been produced had these levelings been carried out during the early spring of 1961. Although height changes along the Mojave-Cot-tonwood Pass line since 1961 (1955-equivalent) are largely self-explanatory, they are sufficiently dramatic that several comments seem in order. Probably the most startling feature revealed by these profiles of elevation changes (pi. 12) is the extent of the large cumulative uplift that apparently occurred between the spring of 1961 and 1974. From our preliminary studies of this phenomenon (Castle and others, 1976), we fully expected that the uplift would close off somewhat east of Barstow and certainly west of Amboy. It appears instead, however, that the uplift between Mojave and Amboy rarely fell below about 0.28 m, and that it actually increased toward the east-southeast (pi. 12). The growth of the uplift along the Mojave-Cot-tonwood Pass line was both irregular and, at least in part, oscillatory. A comparison of the results of the 1974 surveys against a 1972/73 datum shows that there was a modest deflation between Mojave and Kramer Junction during the interval 1972/73-74, whereas east of Boron clearly defined up-to-the-east tilting was associated with about 0.12 m of uplift at Barstow (pi. 12). The axis around which this tilting occurred is very nearly coincident with bench mark 2509, which in turn is centered on a zone of differential uplift that developed between 1961 and 1972/73. Earlier studies (Church and others, 1974) show that this uplift coincided with a more broadly defined band that sustained about 0.5 m of differential uplift during the preceding (1939-61) survey epoch. Hence, we infer a genetic connection between this seemingly persistent feature and the tilt axis between Boron and Barstow. This evolutionary complexity is equally evident in the history of movement along the Barstow-Bry-man spur (pi. 12). Between 1956 (1955-equivalent— see section on “The Orange-Barstow Line”) and the fall of 1961 the entire reach between Bryman and80 THE EVOLUTION OF THE SOUTHERN CALIFORNIA UPLIFT, 1955 THROUGH 1976 Barstow rose about 0.15 m; it continued to rise during the succeeding period, 1961-68, by 0.06 m and 0.08 m at Bryman and Barstow, respectively. During the period 1968-73, on the other hand, a conspicuous reversal occurred which resulted in tectonic subsidence of about 0.08 m at Barstow, as contrasted with virtual stability at Bryman. Because it was consistent with a pattern of movement based on geodetic control emanating from Tidal 8, we initially interpreted the 1968-73 down-to-the-north tilt as an up-to-the-south tilt coupled with invariance at Barstow during the period 1961-72/ 73 (Castle and others, 1976, p. 252). While this earlier judgment is now recognized as clearly incorrect, it had little effect on the published reconstruction, which purports to show the minimum cumulative uplift developed between 1959 and 1974 and depended heavily on other evidence. Although the 1961-74 cumulative uplift eastward from Barstow was relatively uniform over most of the Mojave-Cottonwood Pass line, several significant exceptions challenge this generalization. The inflection centering on bench mark S 3 near Newberry Springs (pi. 12) marks the western end of a 0.05 m tilt between S 3 and V 727 (about 6 grad). Moreover, because this differential movement lay athwart the active right-lateral Pisgah fault, it is reasonably interpreted as an expression of strain accumulation along this fault. South of bench mark K 725, cumulative uplift increased well above the 0.28-0.30 m that prevailed north and west of this point to a maximum value of over 0.40 m at Twentynine Palms. Between Twentynine Palms and the end of the line, the magnitude of the uplift ranged within narrow limits and dropped off by only about 0.05 m between bench marks N 724 and 2H (MWD). This very small decrease implies a very steep gradient in the 1961-74 isobases of equal height change between Cottonwood Pass and Mecca (see section on “The Colton-Mecca Line”). Finally, although there is a vaguely defined association between the differential uplift developed between K 725 and 2H (MWD) and the relatively elevated Pinto and Hexie Mountains, the correlation between uplift and terrain is certainly very poor (pi. 12). Hence, it is unlikely that the differential uplift south of K 725 is unreal and attributable simply to systematic survey error. The pattern that emerges from even this fragmentary reconstruction of height changes along the Mojave-Cottonwood Pass line suggests an almost wavelike west-to-east propagation of uplift during the period 1961-74. That is, between the spring and fall of 1961, an initially large uplift per- vaded this area at least as far east and north as Barstow. This was followed by additional although lesser uplift between 1961 and 1968 which increased south to north and perhaps west to east as well. Similarly, sometime after 1968, but before 1972/73, collapse occurred within the quadrant defined by Barstow at the center and Boron and Bryman along its western and southern margins, respectively. Between 1972/73 and 1974, further uplift overcame and in fact exceeded this localized subsidence eastward from Boron to and beyond Barstow and was accompanied by modest downwarping in the area between Boron and Mojave. Because this second major surge of uplift occurred between 1972/73 and the following winter, we infer that the eastwardly increasing increment of uplift, produced sometime after 1968 but before 1974 (compare pi. 12 and sections on “The Lucerne Valley Line” and “The Colton-Mecca Line”), probably developed during this same interval (1972/73-74). THE COLTON-MECCA LINE GENERAL REMARKS Because the Colton-Mecca line (pi. 13) forms a spur off the Orange-Barstow line, many of the problems associated with the assessment of height changes along the Orange-Barstow line apply to this line as well. In addition, reconstructed height changes along the Colton-Mecca line are complicated by the fact that the successively developed heights are based on only two virtually continuous levelings (the 1931 and 1968) emanating directly from Tidal 8, and one of these (the 1931) probably is seriously contaminated by intrasurvey movement. Moreover, while the results of 1956 leveling can be reconstructed as a reasonable equivalent of a hypothetical 1955 datum at the extreme western end of the line, the fragmentary development of the 1956 data, together with the otherwise indirect ties with Tidal 8, precludes use of the 1956 surveys as a generally appropriate preuplift datum. Since the 1956 data cannot be used for this purpose, the results of the 1931 leveling provide the only potentially suitable alternative preuplift datum. However, three independent lines of evidence indicate that the 1931 leveling was error ridden, owing chiefly, in our judgment, to intrasurvey deformation. Arguments suggestive of major errors in the 1931 leveling include unusually large (above limits) misclosures against the primary 1931 leveling developed from nearly temporally equivalent levelings, a large discrepancy between the 1931 and 1944/61 reconstructed heights for the junctionTHE RECONSTRUCTION 81 bench mark at Cottonwood Pass, and measurably significant subsidence of a primary junction bench mark south of Riverside sometime between the summer of 1931 and the following winter. Thus, because the 1931 leveling produced, at the very least, ambiguous heights, we are left without a suitable preuplift datum over nearly all of the main line. A similar problem attaches to the use of the latest of the postuplift levelings—that is, the results of the 1974/76 survey. Nearly all of the most recent leveling between Colton and Mecca and White Water and Twentynine Palms was carried out in 1976, and the results of these surveys have been provisionally tied to the 1974 height of the junction bench mark at Colton. However, a good deal of collectively compelling evidence indicates that the Colton-Mecca line sustained major tectonic collapse sometime between 1974 and 1976. Thus, even though there are several ways whereby we can estimate the cumulative vertical displacements along this line during the interval 1968-76, height changes that occurred between 1968 (or 1956) and 1974 can be developed along only a few segments (based in part on the use of the 1976 misclosure on the results of the 1974 leveling between Twenty-nine Palms and Mecca) and at the junction points between the 1974 and 1976 leveling at Mecca and Twentynine Palms. In spite of the disjointed and limited reconstruction of accurate heights along the Colton-Mecca line, several significant sets of height changes emerge from even these partial comparisons. Specifically, comparisons of the results of subsequent levelings against a 1956 datum indicate that the area extending from Mecca northwestward to Indio and southward toward Truckhaven experienced modest but well-defined tectonic subsidence between 1956 and 1968. This subsidence was apparently overcome by 1974, during the 1968-74 0.13-0.18-m uplift of Mecca. Similarly, through a combination of the results of the 1974 and 1976 surveys, we can also show that virtually the entire line between White Water and Twentynine Palms probably sustained uplift of no less than several tenths of a meter during the interval 1968-74. Finally, the results of the 1976 leveling, together with the recognition of about 0.13 m of subsidence at Colton during the period 1974-76, indicate that by 1976 the main line between Colton and Mecca had collapsed well below the 1968 datum, and that much of the line probably collapsed below a hypothetical preuplift datum as well. DETAILED RECONSTRUCTION The Colton-Mecca line consists of a primary vertical-control line between Colton and Mecca, together with a spur between White Water and Twentynine Palms, a second very short spur extending northeastward from Indio, and a third short spur between Mecca and Truckhaven (pi. 13). The starting elevations are taken, insofar as possible, directly from the temporally equivalent junction bench mark elevations along the Orange-Barstow line. The 1931 starting elevation is based on nearly continuous leveling between Tidal 8 and bench mark M 71, Banning, completed during the period July(?) 1931-March 1932 (NGS lines L-386 and L-7407). The 1931 observed elevation of M 71 has been orthometrically corrected to agree with the observed elevation which would have been obtained had this leveling followed the primary route through Riverside and Colton eastward to Banning. The 1956 starting elevations are based on local datums—an assumption of stability, in other words, at bench marks D 39, P 517, S 70, and H 516 during the periods 1956-61, 1956-76, 1931-68, and 1956-68/69, respectively. The 1969/73 White Water-Twentynine Palms datum depends on an assumption of relative stability along this line during the period 1969-73 (see section on “The Lucerne Valley Line”), together with a starting elevation based on the 1968 elevation of bench mark 603-68; the 1976 starting elevation of 603-68 is based on 1975/76 leveling extending eastward from Colton (pi. 13). Owing to uncertainties in both the 1931 and 1956 heights toward the eastern end of the Colton-Mecca line, all height changes along the Indio spur are referred to bench mark S 70 (pi. 13). Although we have profiled observed elevation changes here against a 1931 datum (pi. 13), chiefly because this datum was developed from the most recent preuplift surveys between Tidal 8 and Indio completed within a reasonably short period (March 1931-March 1932; NGS lines L-386 and L-7407), the results of the 1931 surveys regrettably cannot be equated with those that would have been developed in 1955 (or the spring of 1961). Moreover, not only have we been unable to establish a satisfactory preuplift (1955-equivalent) datum along this particularly critical line, observed elevations developed from the 1931 surveys probably form a generally invalid datum, a judgment that has nothing to do with the quality of the measurements. Rather, comparisons between the results of disconnected pre- and post-1931 surveys show that the region traversed by the Colton-Mecca line was82 THE EVOLUTION OF THE SOUTHERN CALIFORNIA UPLIFT, 1955 THROUGH 1976 characterized by significant crustal mobility during the period 1928-56. Moreover, it seems likely that nearly all of the differential movement implied by comparisons with the 1931 datum is tectonic, for none shows any evident association with known areas of fluid extraction (compare pis. 4 and 13). Hence, while the most obvious conclusion that could be drawn from the results shown on plate 13 is that the cumulative movement during the periods 1931-68 and 1931-74/76 was conspicuously negative, this conclusion is suspect and contrasts significantly with conclusions developed from the results of post-1931 levelings. Several lines of evidence indicate that the results of the 1931 surveys cannot be viewed as a reliable basis for the development of “true” or instantaneously established elevations with respect to Tidal 8. Specifically, we conclude that the junction bench mark at White Water rose about 0.15 m and by as much as 0.16 m during the periods 1961-68 and 1968-74, respectively, and that the height of Mecca increased by as much as 0.18 m during the period 1968-74 (see below and section on “The Lucerne Valley Line”), a conclusion that suggests that the 1931 heights along the Colton-Mecca line must have been almost unbelievably high. Similarly, simple comparisons between 1928 and 1931 observed elevations (with respect to Tidal 8) indicate that the Banning area sustained about 0.07 m of uplift during the period 1928-31. Because this area was elevated about 0.4 m during the period 1902-28, and probably largely during the period 1902-14 (Wood and Elliott, 1979, p. 254-256), it seems unlikely (but certainly not impossible) that continuing major uplift could have persisted into the period 1928-31. A further indication that the 1931 heights (as reconstructed here) are of questionable validity emerges from a comparison between the 1931 and 1974 observed elevations at Cottonwood Pass. The 1931 elevation of 2H (MWD) based on leveling via Riverside, Banning, and Indio, is given as 526.7318 m, whereas the 1974 elevation based on leveling via Palmdale, Mojave, and Twentynine Palms is given as 526.7638 m, for a difference of only 0.0320 m. Application of orthometric corrections based on normal gravity would increase these figures to 526.7386 m and 526.7843 m, respectively, and thus increase the 1931-74 difference slightly to 0.0457 m. Use of orthometric corrections based on observed gravity would further modify this difference by no more than a few millimeters, for the observed gravity orthometric closure around nearly identical paths (fig. 46) differs from that based on normal gravity by only 7 mm. Accordingly, because a comparison between the 1944/61 and 1974 heights of 2H (MWD) based on leveling over the same route indicates that this mark rose about 0.35 m (see section on “The Mojave-Cotton-wood Pass Line”), it suggests either that the 0.35-m figure is based on an improper reconstruction or that the 1931 observed elevation is invalid. In fact, abundant evidence indicates uplift of 2H (MWD) of 0.30-0.35 m (and certainly no less than 0.25 m) (see sections on “The Los-Angeles-Mojave Line” and “The Mojave-Cottonwood Pass Lines”). Moreover, to contend that there was virtually no uplift of this mark during the period 1961-74 directly contradicts the results of the 1931-76 comparison between Cottonwood Pass and the Arizona border, which shows that differential uplift of 2H (MWD) with respect to Parker Dam could have been no less than and probably a good deal more than 0.27 m during the period 1931-74 (see section on “The Cottonwood Pass-Parker Dam Line”). Although various indirect arguments indicate that reconstruction of the results of the 1931 surveys has almost certainly produced an invalid (exaggerated) observed elevation for bench mark 2H (MWD), an explanation of the source of this invalid determination is of more than academic importance to this study. That is, if the discrepancy between the 1931 and the 1944/61 elevations of 2H (MWD) cannot be explained, it casts great doubt on the 1972/73-74 eastward propagation of the southern California uplift implied by the comparison shown on plate 12. Several lines of evidence indicate that the discrepancy between the 1931 and 1944/61 observed elevations of 2H (MWD) is attributable chiefly to the occurrence of crustal deformation during the 1931/32 leveling eastward from Tidal 8: (1) The observed misclosure around a narrowly defined loop between Cabazon and Indio, derived from first-order leveling carried out during the spring of 1931, is given as - 0.0684 m (fig. 47). Orthometric corrections based on normal gravity reduce this closure to 0.0669 m. However, because this circuit lies astride the San Andreas fault, an orthometric correction based on observed gravity has also been calculated; this recalculated value (-0.0148 m) enlarges the misclosure to - 0.0832 m. Hence, the corrected misclosure implies a down-to-the-west tilt of more than 0.08 m between ID (MWD) and 1442 USGS during the interval bracketed by the levelings of March 26-May 15, 1931, and May 31-June 12, 1931 (fig. 47), a remarkably large tilt developed over a remarkably short period. (2) Several rela-MOJAVE r3 tI9*00 b3 ROSAMOND SAUGUS L-23691 13-7/73) 04-04830 AMBOY BORON 35*00'- I (0/73-2/74) ^_______ BARSTOW 43^ L-23227 (2-3/74) \(-23685 12-10/74) *101-151 \L-2367I 00/73-2/74) f*0-1526 i . .0163 LLANO I-33A ________________V* 1669 1-2360) (7-8/73) L 1046 •---, '"1___^ HESPERIA ♦0.0311 m \ r. ' ♦ .0194 | ^ .0505 J *!L\ - L-23632 (9-10/73) ’ "-9/74) 11-72 \ >-)0/73)—jL^----f^»iL,J'7_^41--^'L24Q83 (7/76, ° 570 COLTON~^\i-«9/4 (7-8/75) __j_ 4^8 499 BANNING 10 20 30 40 50 KM TWENTYNINE PALMS FIGURE 46.—Misclosures around the circuits Colton-Saugus-Palmdale-Rosemead-Mojave-Boron-Barstow-Lavic-Amboy-Twentynine Palms-Mecca-Banning-Colton and Colton-Saugus-Palmdale-Llano-Hesperia-Colton (dashed line); based on levelings carried out during the period November 1972-July 1976. See figure 4 for adopted conventions and explanation of symbols. oo os THE RECONSTRUCTION84 THE EVOLUTION OF THE SOUTHERN CALIFORNIA UPLIFT, 1955 THROUGH 1976 tively small circuits between White Water and Cajon Pass that either involve or closely bracket the 1931 leveling produce misclosures ranging from 0.0847 m (NGS lines L-ll and L-991) to 0.1594 m (NGS lines 82464 and L-5334; USGS line B 6637). Disregarding measurement errors, orthometric corrections, and any differential movement between Riverside and Colton during the period 1928-34, the smallest of these misclosures (based exclusively on first-order leveling) is consistent with down-to-the-west tilting of more than 0.08 m between the 1928 and the 1931 surveys—probably carried out entirely during the summer of 1931 (W. Edwards, Metropolitan Water District of Southern California, oral commun., 1977). (3) The observed elevation of bench mark M 38 of 242.3681 m, used here as a starting elevation for the 1931 leveling eastward to Indio, is based on leveling between Tidal 8 and M 38 via Orange carried out largely during the period February-March 1932. The observed elevation of M 38, based on leveling between Tidal 8 and M 38 via Los Angeles and Ontario, carried out largely during the period August-Decem-ber 1931, is given as 242.4266 m. Hence, while we have again made no attempt to incorporate any orthometric correction in this comparison, the M0R0NG0 VALLEY 34°00' II7-00' CABAZON 0 10 20 30 40 30 KM COACHELLA 117"00‘ 1— 33“00‘ II6°00' FIGURE 47.—Misclosure around the circuit Cabazon-Morongo Valley-bench mark ID (MWD)-Coachella-Cabazon based on leveling carried out during the period March 26, 1931-June 12, 1931. This misclosure enlarges to -0.0832 m where the orthometric correction is based on observed gravity. See figure 4 for adopted conventions and explanation of symbols. 0.0579-m difference between these two “1931” elevations argues that M 38 probably subsided about 0.05 m sometime between the summer of 1931 and the following winter. The preceding evidence strongly suggests, accordingly, that crustal collapse, probably proceeding from east to west, occurred during the course of the 1931 leveling. For example, simply substituting the alternative elevation differences disclosed by the two described misclosures involving the 1931 leveling diminishes the 1931 observed elevation of 2H (MWD) to 526.5787 m and, hence, increases the 1931-74 uplift of this mark to 0.1851 m (0.1988 m, if orthometric corrections based on normal gravity are added to the observed elevations). Regardless of the validity of this particular reconstruction of the 1931 observed elevation of 2H (MWD), it now seems certain that crustal deformation during the 1931 leveling virtually guaranteed the generation of an invalid height for this mark; whether the resulting height could be expected to have been higher or lower than the “true” height is a function of the distribution of the 1931 leveling in both space and time relative to the concomitantly evolving deformation. The likelihood that the actual 1931 height of 2H (MWD) is closely approximated by the orthomet-rically corrected results of the 1961/44 leveling into this mark via Los Angeles, Mojave, Barstow, and Amboy (see sections on “The Los Angeles-Mojave Line” and “The Mojave-Cottonwood Pass Line”)— and, hence, that 2H (MWD) in fact rose about 0.35 m between the beginning of 1961 and 1974—is convincingly supported by the results of 1926/27/31 levelings into this mark through southwestern Arizona. The 1931 height developed from leveling via Los Angeles, Mojave, Barstow, and Amboy is again based on an assumption of invariance between Daggett and Cottonwood Pass during the period between 1931 and March-April 1961 (see section on “The Mojave-Cottonwood Pass Line”). The 1931 height developed from leveling through southwestern Arizona is based on one correction and one assumption. Because the 1926/27 leveling emanated eastward from San Diego, the tie between Tidal 8 and San Diego is based on a combination of 1931/32 and 1932/33 levelings that have been corrected for about 0.01 m of compaction-induced subsidence during the 1931/32-1932/33 junction interval at Santa Ana (Castle and Elliott, 1982, p. 7005, 7014-7015). Similarly, the establishment of a 1931 height for 2H (MWD) based on the 1926/27 leveling through southwestern Arizona has required that we assume that the height difference between the starting mark (M 57), adjacent to the San DiegoTHE RECONSTRUCTION 85 Municipal Pier tide station, and bench mark 22 Q, about 20 km east of Parker Dam (see section on “The Cottonwood Pass-Parker Dam Line”), remained invariant during the period 1926/27-31. This assumption is strongly supported by the near certain tectonic stability of the Municipal Pier tide station during historical time (Castle and Vanicek, 1980, p. 292; K. R. Lajoie, oral commun., 1983) and the geologically inferred stability at the site of bench mark 22 Q along the Bill Williams River in southwestern Arizona (Suneson and Lucchitta, 1983, p. 1006-1008). Based on the stated assumptions and correction, the 1931 observed elevation of 2H (MWD) obtained from the 1961/44 leveling via Los Angeles, Mojave, Barstow, and Amboy is 526.4073 m (NGS lines L-11067, L-11069, L-11115, L-18230, L-18296, L-18299, and L-18364), whereas that obtained from the 1926/27/31/32/33 leveling via southeastern California and southwestern Arizona is 526.4620 m (NGS lines 82606, 82625, 82632, L-386, L-570, and L-7407). Orthometric corrections based on normal gravity produce 1931 heights for this mark of 526.3512 m and 526.3966 m, respectively. Where the orthometric corrections are based on observed gravity, the separately determined 1931 heights for 2H (MWD) are given as 526.4707 m and 526.4877 m, respectively. While this exercise provides no assurance that the area between Daggett and Cottonwood Pass remained free of deformation during the period between 1931 and the beginning of 1961, it does demonstrate that 2H (MWD) remained virtually fixed in height between 1931 and at least as late as 1944 and that the uplift at Cottonwood Pass during the period 1931-1974 is much more accurately described through use of the results of the 1961/44 leveling via Los Angeles, Mojave, Barstow, and Amboy than it is by those obtained from the 1931/32 leveling via Riverside and Banning. Height changes profiled against a 1956 datum involve an assumption of invariance at bench marks D 39, P 517, S 70, and H 516 during the periods 1956-61,1956-76,1931-68, and 1956-68/69, respectively (see above). Although this assumption is clearly invalid in terms of the stability of these marks with respect to Tidal 8, it provides a basis for showing differential movement significant to the evolution of the southern California uplift. An alternative reconstruction of elevation changes along the short segment eastward from D 39 (shown by the dashed-line representation, p. 13) is drawn from a reconstruction of the 1956 height of D 39 based on 1956 surveys between Colton and Barstow. Because the stability of Colton during the period 1956-61 is doubtful, whereas Barstow is believed to have remained relatively stable between 1926/27 and the spring of 1961, the alternative reconstruction is preferred (see section on “The Or-ange-Barstow Line”). Although the 1968 leveling was completed during a period of relative quiescence, the 1974/76 surveys extended over a 3-year period (1973-76) during which considerable deformation occurred, such that a comparison of the results of the 1974/76 surveys against a 1968 datum probably differs significantly from that which would have been obtained had the later leveling been run entirely in 1974. That is, while the 1968-74/76 comparison may approximate the cumulative movement with respect to Tidal 8 between 1968 and 1976, this cumulative representation cannot be used as a basis for estimating the maximum uplift developed along this line since 1968. Similarly, while comparisons of the results of the 1976 leveling against the combined 1969/73 datum along the White Water-Twentynine Palms spur suggest no more than about 0.12 m of uplift at Yucca Valley, 0.08 m at Twentynine Palms, and virtually zero uplift at White Water, these values are almost certainly less than the uplift developed at these points between 1968/69 and 1974. Probably the most direct way of showing the extent to which the 1974 heights along the Colton-Mecca line had changed by 1976 is through the examination of the 1968-74 height changes at several junctions along this line that can be determined independently of those shown on plate 13. (1) The 1968 observed elevation of bench mark H 516, Mecca, is given as -54.9160 m; the 1974 elevation of H 516, based on leveling via Mojave, Barstow, and Amboy, is given as -54.7450 m. Application of orthometric corrections based on normal gravity increases these values to -54.9100 m and -54.7217 m, respectively. Owing to the length (~ 1,000 km) and breadth of the circuit defined by the 1974-68 leveling, orthometric corrections based on observed gravity have also been calculated; these corrections further increase the 1968 and 1974 heights to -54.8990 m and —54.7190 m, respectively. Accordingly, the 1968-74 uplift at H 516 probably was very close to 0.1800 m (or 0.1300 m— see section on “The Los Angeles-Mojave Line”). Because a comparison between the 1968 and 1974/76 observed elevations of H 516 (-54.9160 m and - 54.8977 m, respectively) indicates that this mark rose by only 0.0183 m during the period 1968-74/76 (pi. 13), H 516 apparently subsided by about 0.16 m (or 0.11 m— see above) during the interval 1974-76, provided, of course, that the starting elevation86 THE EVOLUTION OF THE SOUTHERN CALIFORN IA UPLIFT, 1955 THROUGH 1976 of D 39 remained unchanged during this same period. A down-to-the-east tilt developed between Colton and Mecca during the period 1974-76 is suggested by the +0.1689 m clockwise misclosure around the 896-km Colton-Saugus-Mojave-Amboy-Mecca-Colton loop defined by the “1974”-76 surveys (fig. 46) and, hence, tends to corroborate the 1974-76 subsidence of Mecca deduced from a direct comparison with the 1968-74 uplift. The smaller misclosure (0.1184 m) around the 1974-76 loop that bypasses the 1972/73 Saugus-Palmdale leg (fig. 46) suggests that the 1974-76 differential movement between Colton and Mecca may have been somewhat less than 0.17 m. (2) The 1968-74 uplift of the junction bench mark (603-70) at White Water may be estimated by assuming either that the elevation difference between White Water and Mecca remained invariant between 1968 and 1976, or that the elevation difference between White Water and Twentynine Palms remained invariant between 1974 and 1976. Acceptance of the first of these assumptions indicates that White Water (specifically, bench mark 603-70) rose by 0.1800 m during the period 1968-74—that is, by an amount equal to that sustained at H 516 during the same interval—as contrasted with cumulative uplift at White Water between 1968 and 1974/76 of only 0.0136 m (pi. 13). The second or alternative assumption is supported by the misclosure around the circuit White Water-Yucca Valley-Twentynine Palms-Mecca-White Water defined by the 1974/76 leveling (fig. 48). Thus, the misclosure based on observed elevation measurements around this circuit is only 0.0558 m, and although this figure is enlarged to 0.0768 m through the inclusion of the orthometric closure based on normal gravity, a manually computed correction based on observed gravity changes the clockwise orthometric closure from +0.0210 m to -0.0186 m and, hence, reduces the misclosure to only 0.0372 m. Even though this misclosure rigorously describes no more than the relative deformation between Twentynine Palms and Mecca, it suggests that the area included by this loop sustained very little differential movement during the period 1974/76. Proceeding with the postulated assumption of invariance between White Water and Twentynine Palms, the 1974 observed elevation of bench mark Z 1250, Twentynine Palms, based on leveling via Mojave, Barstow, and Amboy is given as 591.2618 m, whereas the 1974/76 observed elevation of this mark based on leveling via Colton, White Water, and Yucca Valley is given as 591.1604 m. Corresponding, orthometrically corrected heights based on normal and observed gravity, re- spectively, are 591.2498 m and 591.2920 m (1974) and 591.1529 m and 591.1455 m (1974/76). Use of the orthometrically corrected figures based on observed gravity indicates that Z 1250 collapsed 0.1465 m between 1974 and 1976. Hence, the 1974 heights of Z 1250 and, by extension, of 603-70 were at least 0.1465 m greater than those produced by the 1976 leveling eastward from Colton (see below). These calculations argue that the 1968—74 uplift of bench mark 603-70 was equal to the sum of 0.0136 m (the cumulative uplift between 1968 and 1974/76) and 0.1465 m, or 0.1601 m. Because these alternative assumptions produce estimates of the uplift at 603-70 that agree very closely (0.1800 m versus 0.1601 m), we conclude that White Water probably rose at least 0.16-0.18 m during the period 1968-74. The corollary of this conclusion is that bench mark 603-70 probably subsided 0.14-0.16 m between 1974 and 1976—provided, of course, that D 39 remained invariant during the period 1974-76. The preceding estimates of uplift at White Water between 1968 and 1974 and tectonic subsidence at TWENTYNINE YUCCA VALLEY PALMS 116°00' 33-00' -L- FlGURE 48.—Misclosure around the circuit White Water-Yucca Valley-Twentynine Palms-Mecca-White Water based on leveling carried out during the period March 1974-September 1976. This misclosure is reduced to + 0.0372 m where the orthometric correction is based on observed gravity. See figure 4 for adopted conventions and explanation of symbols.THE RECONSTRUCTION 87 White Water, Twentynine Palms, and Mecca during the period 1974-76 (as well as of cumulative movement along the entire line between Colton and Mecca between 1968 and 1974/76) have been based on the previously stipulated but unverified assumption of invariance at bench mark D 39 during the interval 1974-76. While evidence of cumulative uplift of D 39 during the period 1974-76 is, at best, equivocal, there is excellent evidence that D 39 actually subsided by as much as 0.13 m during the full interval 1974-76/77. The clockwise misclosure based on observed elevation measurements around the circuit San Pedro-La Canada-Palmdale-Llano-Hesperia-Colton (1976/77)-San Pedro (1973/74) is given as —0.1624 m (fig. 49). Orthometric corrections based on normal and observed gravity reduce |—35'00' II9° 00‘ this misclosure to -0.1428 m and -0.1270 m, respectively. However, even though we are reasonably certain that D 39 subsided about 0.13 m between 1974 and 1976/77, we are uncertain when this collapse occurred with respect to the completion of the 1976 leveling eastward to Mecca. The evidence now before us indicates that tectonic subsidence of the Los Angeles-La Canada area may have occurred as early as the fall of 1974 and almost certainly no later than July 1976 (see section on “The Los Angeles-San Bernardino Line”). Similarly, massive subsidence of both Mecca and Twentynine Palms (with respect to D 39), together with demonstrable collapse of much of the eastern Mojave that apparently occurred between 1974 and the summer of 1976 (see above and section on “The Cottonwood 35°00‘ —| II7°00' 0 10 20 30 40 50 KM 1 -1---1---------1---------1_________I__________I PALMDALE Figure 49.—Misclosure around the circuit San Pedro-La Canada-Palmdale-Llano-Hesperia-Colton-San Pedro based on leveling carried out during the period July 1973-April 1977. This misclosure is reduced to -0.1270 m where the orthometric correction is based on observed gravity. See figure 4 for adopted conventions and explanation of symbols.88 THE EVOLUTION OF THE SOUTHERN CALIFORNIA UPLIFT, 1955 THROUGH 1976 Pass-Parker Dam Line”), suggests that the crustal disturbance that produced the subsidence of D 39 had largely run its course by the spring of 1976. Finally, R. C. Jachens (written commun., 1977) has shown through precise gravity ties between Riverside and Glendale (adjacent to La Canada) that gravity changes at Glendale with respect to Riverside were +18 ± 4 p,Gal during the period June 1976-May 1977 (two gravity meters) and +6 ± 6 p,Gal during the period October 1976-May 1977 (three gravity meters). These observations, accordingly, are at least consistent with the probable cessation of massive subsidence by September of 1976. The implications of as much as 0.13 m of tectonic subsidence at Colton between 1974 and the spring of 1976 are, at the very least, disturbing. Specifically, if the 1974-76 subsidence of D 39 is disregarded, the already impressive determinations of tectonic collapse at White Water, Mecca, Twenty-nine Palms, Amboy, Cottonwood Pass, and Frink, based on an assumption of invariance at D 39 during the same interval (see above and sections on “The Cottonwood Pass-Parker Dam Line” and “The Cottonwood Pass-Frink Line”), may be underestimated by as much as 0.13 m. Moreover, because we have discovered no evidence to suggest that any significant fraction of the subsidence of D 39 occurred after the completion of the 1976 leveling eastward to Mecca, whereas there is both direct and indirect evidence of the collapse of Colton between 1974 and the summer of 1976, the profiles of height changes based on comparisons of the results of the 1976 surveys against the 1968 and 1969/73 datums (pi. 13) probably should be dropped by 0.1270 m. In other words, the 1968-76 cumulative vertical displacements between Colton and Mecca and between White Water and Twentynine Palms may have been uniformly negative. The preceding discussion demonstrates, at the very least, that the profiled height changes along the Colton-Mecca line (pi. 13) are the most equivocal of any considered to this point and could, in the absence of any other information, seriously mislead us. Thus, the height changes measured against a 1931 datum are certainly suspect, at least to the extent that this datum could hardly be selected as a 1955-equivalent. Similarly, while the relative movements over the full period 1968-76, with respect to virtually any control point along this line, can be assessed and accepted, there is no direct evidence indicating that this period actually harbored a dramatic aseismic vertical oscillation. Nevertheless, and in spite of the ambiguity inher- ent in the results of all but the 1968 leveling, these reconstructions provide additional insight into the history of vertical movement along the south flank of the southern California uplift. Although the profiled height changes along the Colton-Mecca line (pi. 13) are ambiguous and even misleading, they provide a basis for several useful generalizations, particularly when coupled with additional information. For example, although the 1931 datum is clearly suspect, it is very unlikely that the true 1931 heights fell below the 1976 heights between Banning and Indio; hence, if the 1931 heights even approached those that obtained in 1955, it is equally unlikely that post-1955 uplift persisted along the main line through 1976. Similarly, height changes since 1956 along the western end of the main line probably are closely approximated by the dashed-line representations (pi. 13). Again, however, the post-1956 uplift is incompletely described by the cumulative movements profiled here (see section on “The Orange-Barstow Line”). Moreover, while we have plotted the 1956-68 and 1956-76 height changes along the eastern end of the line with respect to local datums (pi. 13), estimates of height changes with respect to Tidal 8 during the period 1956-68 (or 1956-76) can be determined through a knowledge of the height history of any bench mark included in these several surveys. Thus, if it is accepted that H 516, Mecca, rose 0.0023 m with respect to Tidal 8 during the interval 1955/56-68 (fig. 50), in order to show the 1956-68 displacements with respect to Tidal 8, the 1956-68 profiled height changes along the main line should be datum-shifted by -0.0887 m, those along the Indio spur by - 0.0557 m, and those along the Mecca-Truckhaven spur by + 0.0023 m. In other words, bench marks S 70, P 517, and 1C (MWD) are interpreted as having subsided 0.0557 m, 0.0887 m, and 0.1177 m, respectively, between 1956 and 1968. Moreover, this tectonic subsidence apparently increased southward from Mecca as well. That is, again accepting the 0.0023 m of uplift at H 516 with respect to Tidal 8 during the interval 1955/56-68 (fig. 50), representative bench marks C 517 and G 577 sustained height changes of -0.0466 m and - 0.0835 m, respectively, during this same interval. Perhaps significantly, the form of the differential subsidence developed north of Mecca between 1956 and 1968 apparently persisted through 1976, regardless of whether Mecca remained invariant between 1968 and 1976 (fig. 50A) or actually subsided about 0.13 m (fig. 50B). Finally, even though comparisons against the 1968 datum permit the determination of no more than cumulative heightTHE RECONSTRUCTION 89 changes and, therefore, obscure such intraperiod movements as may have occurred between 1968 and 1976, localized differential movements can be determined with considerable confidence. For example, the narrow zone of differential uplift centering on bench mark H 525 is very well defined and is certainly real; because this uplift cannot be reasonably attributed to elastic rebound accompanying recharge of the Bunker Hill ground-water basin (pi. 4), it is likely that it is an expression of strain accumulation on the Banning fault near its junction with the San Jacinto fault (pi. 2). THE COTTONWOOD PASS-PARKER DAM LINE GENERAL REMARKS The Cottonwood Pass-Parker Dam line (pi. 14) is in effect a continuation of the Mojave-Cottonwood Pass line. It has not been treated as such, however, because the described signal is based on the results of only two levelings, the latest of which (the 1976) was produced subsequent to the 1974-76 tectonic collapse that pervaded the Colton-Mecca line and apparently extended into the Cottonwood Pass area as well. Accordingly, the up-to-the-west tilt based on the results of the 1976 leveling between Parker Dam and Cottonwood Pass is almost certainly a minimum expression of the 1955-74 uplift along this line. The results of the 1976 leveling between Cottonwood Pass and Parker Dam probably differ significantly from the observed elevation differences that would have been obtained had this survey been completed in 1974; however, it is much less likely that the 1931 elevation differences along this line are significantly different from those that would have been obtained in 1955. This conclusion is supported explicitly by the very small misclosure involving both 1931 and 1944 leveling around the circuit Cottonwood Pass-Amboy-Freda Junction-Cottonwood Pass, by the previously developed evidence of regional stability within the area extending east-southeastward from Daggett to Cottonwood Pass between 1944 and the early spring of 1961, and by the excellent correspondence between the 1926/27/31/32/33 and 1944/61 heights for the Cottonwood Pass junction mark developed from levelings through southwestern Arizona and the western Mojave Desert, respectively. The results of a variety of measurements indicate that the entire region traversed by the Cottonwood Pass-Parker Dam line, including the area extending westward from Freda Junction to Amboy, sustained major tectonic deformation during the period 1974-76. Moreover, virtually all of the evidence at our disposal indicates that the Cottonwood Pass junction mark subsided as much as several decimeters with respect to Tidal 8 during the period 1974-76. The implications of this tectonic subsidence are several fold. For example, if it is accepted that Parker Dam remained essentially invariant with respect to Tidal 8 during the period 1931-76, an assumption for which there exists at least permissive support, the 1931-74 uplift of Cottonwood Pass could easily have been about 0.44 m and perhaps as much as 0.57 m; these values clearly exceed the 1944/61-74 0.36-m uplift developed from comparisons along the Mojave-Cottonwood Pass line. The occurrence of a large up-to-the-west tilt (0.44-0.57 m) between Parker Dam and Cottonwood Pass could be equally well explained by a combination of uplift at Cottonwood Pass coupled with to cc X CD Hi x -54.7000 -5 4.7500 -54.8000 -54.8500 -54.9000 -54.9500 -55.0000 -55.0500 -55.1000 o in O in O in O in in <0 CD 1^ N 00 0) 0> o> 0) 0) 0) 5 981.3000 h U Z 981.2500 h* 5 981.2000 ui x 981.1500 981.1000 981.0500 981.0000 980.9500 m o m in o in o m o m o m o m o in o o> o o CVJ CNi IO IO in in <0 h- 00 GO 0> 0> O) o> 0> 0> 0> 0> a> o> o> 0) YEAR FIGURE 67.—Changes in orthometric height at bench mark 3219 USGS, Vincent, with respect to bench mark Tidal 8, San Pedro, since 1897/1902. The 1897/1902 height is based on double-rodded third-and second-order levelings of the U.S. Geological Survey. The 1914 height is based on second-order (double-run) leveling of the U.S. Geological Survey. All other heights are based on the results of first-order levelings of various agencies. One-standard-deviation error bars show conventionally estimated random error only. Orthometric corrections based on observed gravity.122 THE EVOLUTION OF THE SOUTHERN CALIFORNIA UPLIFT, 1955 THROUGH 1976 about the same magnitude as those identified with the later uplift. Moreover, the partial collapse of the early uplift is similar in chronology, if not in form, to that which followed the 1974 culmination of the recent uplift (figs. 66 and 67). In fact, it is not unlikely that the uplift across Cajon Pass described by Gilluly (1949, p. 562-565) is characteristic of the general form of the south flank of the residual early uplift—that is, the uplift that persisted following the partial collapse that must have begun no later than 1926 (figs. 66A and 67). Granting the similarities between the earlier and later uplifts, there remain several notable differences that can be loosely described as second-order variations. The most significant of these differences is the much more pervasive involvement of the Peninsular Ranges in the earlier uplift (fig. 66; Wood and Elliott, 1979). Indeed, the significant propagation of this earlier uplift into the Peninsular Ranges south of Los Angeles (fig. 66A) suggests that these vertical displacements may be as much the product of collapse of the northern edge of the Peninsular Ranges as uplift of the Transverse Ranges. While this interpretation would neither remove nor diminish the measured tilt along the south flank of the early-20th-century uplift, it implies that the represented displacement field (fig. 66A) distorts the vertical-movement history with respect to a tectonically invariant control point. However, several short-term occupations of the San Pedro tide station during the second half of the 19th century (Wood and Elliott, 1979, p. 258) argue that this has not occurred to any significant degree. Specifically, the results of these measurements indicate that the sea-level trend at this tide station remained virtually unchanged through the growth and partial collapse of the early uplift. The earlier uplift apparently differed as well in its mode of collapse. That is, as can be deduced from direct inspection of the profiled height changes, the collapse of the modern uplift was expressed as a remarkably uniform down-to-the-north tilt that extended at least as far north as Rosamond (fig. 66B). The earlier uplift, on the other hand, was characterized by a much more irregular pattern of collapse, and its southern margin was virtually devoid of tectonic subsidence (fig. 66A). Moreover, while there is some indication that the deflation of the recent uplift may have begun in the western Transverse Ranges and spread rapidly eastward, the earlier uplift apparently sustained major collapse in the western Transverse Ranges and the western Mojave block well in advance of any collapse in its eastern reaches. Specifically, if we assume that the reported 0.6-m uplift of the northern Salton trough was at its maximum in 1928 (Wood and Elliott, 1979, p. 255)—and it is difficult for us to reject this assumption—major collapse of the western lobe (fig. 66A) must have begun at least 2 years before any significant subsidence had occurred in the eastern part of the uplift. There is, in addition, at least permissive evidence that the collapse of the eastern uplift probably persisted at least into 1931 (see Wood and Elliott, 1979, p. 254 and section on “The Colton-Mecca Line”). Finally, conspicuous downwarping along the south flank of the uplift, such as occurred between 1955 and 1976 (figs. 65 and 661?), almost certainly did not accompany the collapse of the earlier uplift. It is, in fact, this distinction that most clearly differentiates these two episodes of uplift and partial collapse. The occurrence of an earlier episode of uplift within the same general area as that occupied by the recent southern California uplift suggests that the recent uplift represents but a single event in a continuing and more or less cyclic tectonic process. Although the historic record embraces less than two full cycles, if we assume the later cycle to be representative, it suggests a period of about 50 years. Use of this value indicates that the cumulative uplift rate in the Palmdale area during the past century has been about 5 mm/yr (fig. 67), a figure that is certainly in acceptable agreement with the uplift rates deduced from studies of emergent marine terraces along the south flank of the western Transverse Ranges (Wehmiller and others, 1977; Weh-miller and others, 1979), and hence along the south flank of the uplift. In the larger geologic context, moreover, the areas of maximum cumulative uplift and maximum cumulative collapse are generally consistent with their geologic locale. For example, based on repeated geodetic measurements, the high desert and western Transverse Ranges and at least parts of the San Bernardino Mountains have been identified as areas of relatively high cumulative uplift rates (figs. 65 and 67) that roughly coincide with areas of geologically determined high uplift rates during Holocene or late Quaternary time (Wehmiller and others, 1977; Bull and others, 1979; Wehmiller and others, 1979; Herd, 1980, p. 15— 16). Similarly, the geodetically determined maximum cumulative collapse (or negative uplift) rates are at perhaps their greatest in the central Salton trough (fig. 50; Wood and Elliott, 1979, p. 255), which is precisely the environment in which one would expect continuing cumulative tectonic subsidence.THE ORIGIN OF THE SOUTHERN CALIFORNIA UPLIFT 123 SEISMICITY ASSOCIATED WITH THE SOUTHERN CALIFORNIA UPLIFT The rapidly accelerating vertical displacements identified with the evolution of the southern California uplift (figs. 55-65) invite an almost intuitively obvious comparison with the instrumentally recorded earthquakes in this same general area. Accordingly, we have compared the distribution of earthquakes of magnitude 4 and greater during the period 1932-76, inclusively, with our preferred interpretation of the cumulative uplift developed during the period 1959-74.5 (pi. 16). Shocks of less than magnitude 4 have been excluded chiefly because they probably contributed very little to the elastic strain release in this area. Moreover, while we recognize that much more rigorous comparisons could (and probably should) be made between the changing pattern of seismicity and the evolving uplift, our purpose here is simply to draw attention to several of the evident relations between the uplift and the spatially associated earthquakes that occurred during the interval 1932-76. Perhaps the most valid generalization that can be made in this connection is that the uplift and the seismicity of this area are surprisingly poorly correlated. The generally uncorrelated nature of these two phenomena would be even more striking were we to exclude from consideration those earthquakes that preceded the uplift. Specifically, virtually all of the earthquake activity along the New-port-Inglewood zone (pi. 16) can be attributed to the 1933 Long Beach earthquake and associated aftershocks (Hileman and others, 1973, p. 11, 15-20). Similarly, a large part of the seismicity south of Santa Barbara, north of Cantil, east of Barstow, and in the area centering on White Water is clearly identified with the 1941 Santa Barbara, the 1946 Walker Pass, the 1947 Manix, and the 1948 Desert Hot Springs earthquakes, respectively (Hileman and others, 1973, p. 11,34-39). Finally, and perhaps most importantly, nearly all of the shocks developed along the northwest flank of the uplift can be related to the 1952 Kern County earthquake (Hileman and others, 1973, p. 34-41). In short, although a significant part of the 1932-76 seismicity occurred after 1959, more than half of the seismic energy produced during this period probably preceded the inception of the uplift. Apart from the pre-1960 earthquake activity, the seismicity associated with both the growth and partial collapse of the uplift is localized largely within (1) the Santa Barbara Channel westward from Ventura, (2) the Saugus-San Fernando area, (3) the San Bernardino Mountains area, and (4) various areas along the trend of the San Jacinto fault (pi. 16). Perhaps even more apparent, however, is the occurrence of several large, generally aseismic areas within and around the uplift. Specifically, the western lobe of the uplift (exclusive of the Saugus-San Fernando area) remained virtually free of earthquakes during the period 1960-76. In addition, and even if the pre-1960 earthquakes are taken into consideration, the area projecting eastward across the southeast flank of the uplift into the eastern Mojave has been almost totally free of seismic activity. The apparent spatial independence between the uplift and the temporally associated seismicity could be interpreted as indicating that the two are genetically dissociated. It could be argued with equal Conviction, moreover, that the identified “seismic gaps,” particularly around the flanks of the uplift, represent zones of significant elastic-strain accumulation indicative of impending and conceivably major earthquake activity. However, at least one and perhaps both of these generalizations probably are wide of the mark. That is, there is no evidence to conclude other than that both the seismicity and the apparently aseismic deformation are equally valid expressions of the orogenic process. Simply because we are unable to show that one is somehow directly derivative from the other does not preclude an ultimately demonstrable relation between the two. Similarly, there is as yet no clearly defined basis for assuming that the aseismic areas within and around the uplift should be targeted as sites for major seismic activity in the near term—although we should add that this is certainly a reasonable possibility. Nonetheless, and even though we recognize at least one major difference between the two historically defined episodes of uplift that may bear on this problem, what little we know of the early-20th-century uplift argues by analogy that we need not necessarily expect to see these seismic gaps filled by large earthquakes. THE ORIGIN OF THE SOUTHERN CALIFORNIA UPLIFT The southern California uplift is, at best, an imperfectly understood phenomenon. Nevertheless, various scholarly speculations dealing with the origin of this feature appeared almost immediately after its recognition (Thatcher, 1976; Hadley and Kanamori, 1977b, p. 1474-1477; Kosloff, 1977; Wyss, 1977a; Castle, 1978, p. 7; Savage and Prescott, 1977;124 THE EVOLUTION OF THE SOUTHERN CALIFORNIA UPLIFT, 1955 THROUGH 1976 Rundle and Thatcher, 1980). However, several of these hypotheses were generated in advance of significant constraints that have since been placed on any theory of the origin of the uplift; thus the range of reasonable explanations has been correspondingly narrowed and, in a certain sense, further complicated. Specifically, any model that attempts to explain the origin of the uplift must consider the episodic or spasmodic growth of the uplift and its subsequent collapse, the apparently cyclic nature of this phenomenon, the surprisingly short period identified with each cycle, and the generally aseis-mic nature of the uplift and subsequent collapse. Moreover, any viable explanation of the uplift must also accommodate its apparent involvement with the Transverse Ranges (or, alternatively, with the double bend of the San Andreas fault) and both the measured and inferred contractional strain athwart the uplift. Although we have seriously considered all of those explanations for the origin of this feature of which we are aware, we obviously favor some over others; hence this discussion focuses on what we perceive to be the most soundly based suggestions and criticisms formulated to date. Shortly after the recognition of the southern California uplift, but prior to the identification of its partial collapse, Thatcher (1976) concluded from an analysis of repeated triangulation surveys that the impulsive inception of the uplift was accompanied by horizontal strain rates roughly four times greater than those normally associated with the San Andreas system. Thus, according to Thatcher (1976, p. 691-692), anomalous shear straining during the periods 1952-63 and 1959-67 probably destructively interfered with the normal pattern in such a way that the compressive stress axes were rotated into azimuths roughly perpendicular to the axis of the uplift. This anomalous pattern suggested to Thatcher (1976, p. 693) that the uplift could be explained as the product of aseismic slip along a virtually horizontal megathrust diving beneath the Transverse Ranges or, alternatively, decoupling between the asthenosphere and a relatively thin lithosphere. Savage and Prescott (1979) subsequently challenged this model, chiefly because (1) geodimeter lines that roughly parallel the maximum compressive axes identified by Thatcher (1976, p. 693) seemed to lengthen during the critical period 1956-63 and (2) they could detect no evidence of anomalous horizontal straining accompanying the partial collapse of the uplift during the period 1974-77. Rundle and Thatcher (1980) have since proposed a modified megathrust model that is seemingly consistent with Thatcher’s (1976) earlier analysis and may overcome several of the earlier objections. The Rundle-Thatcher model presupposes a layered system extending into the asthenosphere, in which each layer is characterized by significantly different time constants. Thus, according to Rundle and Thatcher, relatively rapid slip along a shallowly dipping megathrust within the lithosphere would be accompanied by correspondingly rapid elastic uplift along the leading edge of the upper plate. The viscoelastic asthen-ospheric response to this rapid slip would tend to produce relatively slow uplift overlying the slip zone that would persist until such time as equilibrium was restored. Collapse rapid enough to overcome the asthenospheric recovery could occur within a porelastic layer characterized by relatively short time constants and extending to a depth of perhaps no more than 5 km. The deformation of the porelastic layer is attributed in turn to fluid diffusion, although other mechanisms might be postulated. Unfortunately, the complexities of the Rundle-Thatcher model preclude a simple synopsis; hence we have attempted here to outline only its more basic aspects in order to demonstrate its general consistency with Thatcher’s (1976) earlier hypothesis. While the post-1973/74 partial collapse persists as perhaps the most enigmatic feature associated with the evolution of the uplift, the seemingly divergent positions of Thatcher (1976) and Savage and Prescott (1979) may be less irreconcilable than the data suggest. Specifically, the occurrence of an unusual horizontal strain event superimposed on the secular strain pattern could explain the association between the inception of the uplift and the anomalous shear straining identified by Thatcher (1976), yet at the same time show at least partial consistency with the data reported by Savage and Prescott (1979). For example, lines 43, 59, and 61 (fig. 68) are characterized by apparent strain histories that Savage and Prescott (1979, p. 172-173) largely dismiss as the products of measurement error, but which we contend are consistent with discontinuous migration of slip (in both space and time), both athwart the axis of the uplift and along a horizontal or subhorizontal surface underlying the uplift. Owing to the relatively short length of these lines (with respect to the width of the uplift) and the discontinuous nature of the postulated slip events, we see no reason why such events should necessarily be expressed at the surface as shortening—even though we would expect that a contractional trend should be evident along the av-IN MILLIMETERS THE ORIGIN OF THE SOUTHERN CALIFORNIA UPLIFT 125 INDEX MAP YEAR FIGURE 68.—Measured length (L) less a constant nominal length (L„) as a function of time for each of six geodimeter lines in the west-central Transverse Ranges. Corrections have been applied for suspected systematic errors introduced as a result of earlier survey procedures. Error bars show one-standard-deviation figures for plotted points; for solid circles the radius of the plotted point is about one standard deviation. The vertical line in 1971 represents the time of occurrence of the San Fernando earthquake. After Savage and Prescott (1979, figs. 3 and 5).126 THE EVOLUTION OF THE SOUTHERN CALIFORNIA UPLIFT, 1955 THROUGH 1976 erage line during the full interval 1959-74. Nonetheless, the seemingly oscillatory strain (contraction alternating with extension) shown by lines 59 and 61 between the beginning of 1960 and the early spring of 1961 (fig. 68) is especially intriguing (the intermediate measurement is missing along line 43). This period is included within the interval defined by the collapse of the Lebec area (sometime after the spring of 1959 but no later than the beginning of 1960) and its subsequent recovery (no later than the end of 1961) (see sections on “The Ventura-Maricopa Line” and “The Los Angeles-Mojave Line”). The significance of this observation is not that we can associate the tectonic collapse (or later recovery) with either extensional or con-tractional strain along either of these lines, but rather that reversals are disclosed in both the vertical and horizontal regimes during the same general period in the same general area—and that these reversals are reasonably interpreted as expressions of inchworm-like migration of slip along an essentially horizontal surface underlying the identified area. Similarly, repeated measurements along line 43 (roughly coincident with the northern end of the Ventura-Maricopa line) and line 61 (extending north-northeast off the Saugus-Grapevine spur of the Los Angeles-Mojave line) indicate that both lines experienced contractional strain sometime between 1963 and 1968 (fig. 68), a period during which a well-defined pulse of uplift occurred along the Saugus-Grapevine spur (see section on “The Los Angeles-Mojave Line”). Measurements along line 59 (extending north-northeast off the Saugus-Palmdale segment of the Los Angeles-Mojave line) indicate that comparable contractional strain occurred along this line sometime between the spring of 1961 and 1968, and conceivably as early as the middle of 1961 (fig. 68). Because the vertical-displacement histories of the marks in the Palmdale area indicate that the pre-1971 uplift occurred largely between the spring and fall of 1961 (figs. 23 and 24), it is likely (although obviously not demonstrable) that the uplift and the indicated contractional strain along line 59 occurred simultaneously. Significantly, moreover, the 100-mm 1961-68 (or March-October 1961) shortening along line 59 (Savage and Prescott, 1979, p. 172-173) is consistent with that predicted by the megathrust model. On the other hand, there is relatively little indication of strain along those lines (60, 62, and 63) that roughly parallel the axis of the uplift. This is especially true of line 62 (fig. 68), which shows little if any strain above noise level and—unlike line 63—is not apt to have been influenced by de- formation preceding or accompanying the San Fernando earthquake (Savage and Prescott, 1979, p. 173-174). Finally, provided only that the collapse of the uplift (whether in response to fluid flow within a postulated porelastic layer or to some other phenomenon) does not demand a major relaxation of the contractional strain athwart the uplift, reinstitution of a secular pattern is hardly inconsistent with partial collapse; that is, anomalous straining, of whatever nature, need not necessarily accompany the collapse. Accordingly, ase-ismic slip along a horizontal or subhorizontal surface at depth, as suggested by the triangulation data (Thatcher, 1976), remains a plausible explanation for the uplift, if not for its subsequent collapse. Our position, we suspect, is clearly revealed in the preceding remarks: the existing data strongly support the argument that the southern California uplift was produced through slip (or decoupling) along a virtually horizontal surface, much as first suggested by Thatcher (1976). Our preferred explanation (it would be presumptuous to identify it as a model) proceeds from one that was first proposed and subsequently reiterated in very similar form by Lachenbruch and Sass (1973; 1981) to explain the heat-flow distribution across the San Andreas fault. We assume the existence of a brittle seismogenic layer overlying a viscoelastic or ductile layer extending to the base of the lithosphere (fig. 69A); we further assume that these two layers are partially decoupled through a thin low-viscosity layer that extends over at least the width of the subseismogenic shear zone that marks the boundary between the North American and Pacific plates. The postulated existence of the low-viscosity layer obviously is speculative, but its occurrence is at least consistent with the modest velocity reversal at 15 to 20 km suggested by Hadley and Kanamori (1977a). The configuration of the San Andreas fault implied by this representation (fig. 69A) departs significantly from the conventional characterization of this fault, which assumes that it extends to the base of the lithosphere as a more or less discrete surface. That is, if one accepts the Lachenbruch-Sass model, a section through the San Andreas has the form of an inverted “T” whose stem represents the vertical (or transcurrent) part of the fault, bottoming at the base of the seismogenic layer, and whose cross represents the horizontal (or decoupled) part of the fault. Implicit in this characterization is the likelihood that the horizontal part of this structure tends to dominate the San Andreas fault, especially in the locked section.THE ORIGIN OF THE SOUTHERN CALIFORNIA UPLIFT 127 The Lachenbruch-Sass model—at least in its unmodified form—assumes that simple shear is continuous across the entire subseismogenic plate boundary. It also assumes that right-lateral shear straining associated with this continuous movement is symmetrically distributed with respect to the medial line that traverses the length of the boundary zone, falling off to zero at either edge of the boundary zone. Because the velocity of any point within the subseismogenic shear zone is, by definition, unimpeded, whereas that for a corresponding point within the overlying brittle zone is constrained (Lachenbruch and Sass, 1973, p. 198, fig. 4Cb), a horizontal shearing traction, t* (fig. 69A), is necessarily imposed on the base of the brittle layer (or, alternatively, on the base of some arbitrarily designated section within the low-viscosity layer). Adoption of this characterization of the San Andreas fault (fig. 69A) carries with it implications significant to the origin of the southern California uplift. Specifically, “the seismogenic layer could offer appreciable resistance to plate motion even though stress on the main fault were negligible; [thus] the principal resisting surface would be the horizontal base of the seismogenic layer” (Lachenbruch and Sass, 1980, p. 6219). Hence, provided that Th rises to values large enough to overcome the shearing resistance along the horizontal fault, the brittle or seismogenic layer may simply ride out over the subseismogenic zone (fig. 69B). Moreover, because of the geometric and strength conditions that govern movement on the San Andreas fault where it traverses the Transverse Ranges, slip along the horizontal fault need not necessarily be accompanied by movement along the vertical or main fault. That is, since continuing contractional strain across the San Andreas fault between the two chief bends in the system is enhanced with respect to that both north and south of this reach, increasing normal stress across the vertical fault will increase the frictional resistance to movement throughout and immediately beyond this section of the fault, whereas the shear stress along the main fault, T, probably remains at a minimum with respect to that elsewhere within the seismogenic layer (Lachenbruch and Sass, 1973, p. 198, fig. 4Cc). Hence the interaction between these stresses effectively bonds the main fault over short periods. However, those factors that probably led to this welded-like condition in no way preclude movement along the main fault (it would be absurd to argue otherwise since it is known to have sustained large displacements during historical time); that is, pro- vided that the horizontal shearing traction across the horizontal fault rises to some critical value, slip may occur along both parts of the fault. The variation in t,, as a function of both position and time is controlled by a variety of factors. Perhaps the most obvious of these is the velocity distribution across the subseismogenic layer, which is ultimately dependent on interplate motion. That is, the velocity must increase gradually from zero at the edge of the boundary zone to a value along the opposite edge that matches the velocity of the plate motion itself. Similarly, the shearing resistance must vary widely across the width of the boundary zone, but it may be controlled chiefly by the rheologic properties that characterize the postulated low-viscosity layer. Accordingly, if the relative velocities for points within the subseismogenic zone fall off toward the edges of the plate boundary, whereas the shearing resistance increases toward the edges, failure conditions along the horizontal fault will, in general, tend to develop toward the central part of the boundary zone. Failure (where > the shearing resistance) may be achieved much more readily and, hence, much more frequently than one might intuitively infer. For example, if the shortening (90 ±20 mm/yr) between Quincy, Calif., and San Diego (fig. 1) reported by Smith and others (1979) and Smith (1980) is even approximately correct, relative motion of 110 mm/ yr between the North American and Pacific plates is certainly a reasonable expectation. Thus, for corresponding points on opposite sides of the low-viscosity layer midway across the plate boundary, differential movement over a period of 50 years could easily rise to about 1.35 m (or roughly half the displacement of a point within the subseismogenic zone with respect to some point beyond the plate boundary). Differential movement of this magnitude could easily increase ih to values large enough to produce failure within the decoupled zone. Accordingly, if the southern California uplift is, in fact, attributable to slip along the horizontal San Andreas fault, cyclic repetition of uplift at about 50-year intervals may be much more expectable than would have otherwise seemed possible. Given the occurrence of slip along the horizontally decoupled zone (fig. 69B), the displacement may be braked through the restoration of equilibrium between and the horizontal shearing resistance (that is, through a reduction of Tfe), the strength of the brittle seismogenic layer (which would tend to inhibit breakthrough at the bends along the vertical fault), and, conceivably, the intersection between the northward-dipping frontal128 THE EVOLUTION OF THE SOUTHERN CALIFORNIA UPLIFT, 1955 THROUGH 1976 NORTH AMERICAN PLATE MORELS PACIFIC PLATE FIGURE 69.—Schematic representation of layered lithosphere astride the plate boundary in southern California. A, Preuplift. B, Postuplift. All movement is with respect to A-A’, whose position and configuration are fixed. The shear stress along the east side of the vertical part of the San Andreas fault is indicated by T; that along the seismogenic layer on thethe origin op the sottthitdxt 190 ntj SOUTHERN CALIFORNIA UPLIFT 129 NORTH AMERICAN PLATE subseismogenic layer of the horizontal part of the San Andreas is indicated by Th. The uplift is assumed to be the product of slip along the decoupling surface (or layer) between the seismogenic and subseismogenic layers. Relative displacements of upper layer indicated by dimensionless arrows. See text for details.130 THE EVOLUTION OF THE SOUTHERN CALIFORNIA UPLIFT, 1955 THROUGH 1976 fault system and the decoupled zone—or through some combination of all of these. Because the maximum uplift associated with this postulated decoupling seems to have been about the same for each of the two historic uplift cycles, there may be some threshold load above which collapse may occur through flow within the low-viscosity layer (Castle, 1978). Moreover, the prevention of complete collapse could result from strain hardening set up within the low-viscosity layer during the uplift period. We hasten to add, however, that we see no reason why a general collapse should necessarily preclude further slip through the decoupled zone and, hence, the occurrence of localized uplift during or immediately following this general collapse. The explanation we present here may be unrealistically idealized, and we probably would be well advised to simply dismiss the evolutionary complexities of the uplift as the products of unassessed variations and perturbations within the decoupled zone. However, several of the more evident second-order features may be significant to our understanding of the overall process. For example, Thatcher (1976, p. 693) observed that the southern California uplift nucleated immediately west of the southwestern end of the 1952 Kern County aftershock zone. If the slip event that produced the 1952 earthquake is viewed as underthrusting of the wedge defined by the White Wolf fault and the base of the seismogenic zone, it would tend to increase the gradient in ih westward from the rupture zone and load the horizontal fault in the area north of Ventura. Thus, there is some basis for arguing that the uplift should have nucleated in this area. Similarly, the early-20th-century uplift propagated well into the Peninsular Ranges province, whereas the southern boundary of the recent uplift is closely coincident with the active front of the Transverse Ranges. Thus, it seems to us that there is a significant likelihood that the recent uplift may have terminated on the south through slip along the frontal fault system and, hence, further loaded this potentially active series of faults. CONCLUSION We have attempted in this report to outline the evolution of the southern California uplift as clearly as the data permit, recognizing that our reconstruction almost certainly errs in detail. That is, we must contend not only with the possibility that some of the measurements may be systematically contaminated owing to the occasional use of a bad rod or local runs of unequal-refraction er- ror, but also with the much more difficult problem associated with intrasurvey movement. Nonetheless, and in spite of persisting uncertainties surrounding the changing configuration of the uplift, the coherence of the reconstruction argues convincingly that the general pattern of vertical displacements described here is indeed real and not simply an artifact of the measurement system. This conviction is strengthened, moreover, by the 1978 general releveling of southern California which shows that nearly the entire uplift sustained a general collapse (Burford and Gilmore, 1984) remarkably similar to the collapse that followed its early-20th-century counterpart. To suggest that the measurements that defined the recent uplift and its partial collapse were characterized by cyclically distributed distortion of the same form and the same order as were those that defined the earlier episode, asks too much of coincidence. Moreover, the consistency of the pattern of uplift with the late Quaternary tectonic history of southern California strongly supports the existence of this feature, although obviously not its spasmodic growth and subsequent collapse. The presumably cyclic phenomenon that produced the southern California uplift may be unique to this area and directly related to the very recent and certainly complex, if not convoluted, development of the San Andreas system in southern California. Regardless, those whose efforts are directed toward an improved perception of not only the recent tectonic history of southern California, but the orogenic process in general, can hardly ignore the significance of the episodic and spasmodic deformation that seems to have characterized the evolution of this feature. Finally, how the uplift may be related to the evolving seismicity of southern California is no better understood than is the origin of the uplift itself. Specifically, the question that ultimately will be addressed and hopefully answered is whether this regionally developed uplift (as contrasted, for example, with the localized uplift that preceded the 1971 San Fernando earthquake) is directly or indirectly precursive to a large-magnitude earthquake. We are aware of no firm evidence indicating a one-to-one relation between the growth of the uplift and the temporally associated seismicity. Nonetheless, if our preferred explanation for the origin of the uplift retains any technically redeeming merit, the mere existence of the uplift suggests that the occurrence of a large-magnitude earthquake is especially enhanced in the region of either of the two major bends in the San Andreas faultREFERENCES CITED 131 or in the central section of the frontal fault system. For example, the horizontal slip that is postulated to have produced the uplift would tend to decrease the basal tractional stress through the central part of the uplift, yet could at the same time have armed the frontal fault system along its southern margin. Similarly, if we shift our reference to some point along the San Andreas (rather than to one outboard or west of the boundary zone and well within the Pacific plate—fig. 69), horizontal slip along the decoupled surface would tend to increase t,, in the area west of the San Andreas and north of Ventura (or, and perhaps less likely, east of the San Andreas north of the Salton Trough). In either case, significant indications of the imminence of a large-magnitude shock in southern California may prove quite different than our earlier experience might have suggested. 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O., 1969, Surface deformation associated with oil and gas field operations in the United States, in Land subsidence: UNESCO, International Association of Scientific Hydrology Publication 88, v. 1, p. 55-66. Yerkes, R. F., Green, H. G., Tinsley, J. C., Ill, and Lajoie, K. R., 1980, Seismotectonic setting of Santa Barbara Channel area, southern California: U.S. Geological Survey Open-File Report 80-299, 24 p. Yerkes, R. F., McCulloh, T. H., Schoellhamer, J. E., and Vedder, J. G., 1965, Geology of the Los Angeles Basin—an introduction: U.S. Geological Survey Professional Paper 420-A, p. A1-A57. Yerkes, R. F., and Wentworth, C. M., 1965, Structure, Quaternary history, and general geology of the Corral Canyon area, Los Angeles County, California: U.S. Geological Survey report to U.S. Atomic Energy Commission, 215 p. 2 0 26 ? 6 GPO 587-044/10017DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1342 PLATE 1 vt 3+2 flat (Ll(°fl(o) Avila Beach / \ o\-^ Baker - Mojave Needles Palmdale'; Santa Barbara Ventura Los Andeles CHANNEL Pedro San Diegd MEXICO ☆ INTERIOR-GEOLOGICAL SURVEY. RESTON. VA-1984-G82632 1 14‘ SCALE 1:1000000 0 20 40 60 80 100 KILOMETERS 1 _______________I__________________I__________________I___________________I__________________I BATHYMETRIC CONTOUR INTERVAL 100 FATHOMS HACHURES INDICATE CLOSED LOWS AREA OF MAP Modified from Jahns (1954, p. 11) and Yerkes and Wentworth (1965, p. 16) MAJOR TOPOGRAPHIC FEATURES AND NATURALLY DEFINED PHYSIOGRAPHIC AND TECTONIC PROVINCES OF SOUTHERN CALIFORNIAPROFESSIONAL PAPER 1342 PLATE 2 l3+<3 , , DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY 35' 34e 33° POINT CONCEPTION CORRELATION OF MAP UNITS Quaternary 1 Tertiary and / Upper Cretaceous Cretaceous and older CENOZOIC AND MESOZOIC MESOZOIC AND OLDER DESCRIPTION OF MAP UNITS Unconsolidated to poorly consolidated sedimentary deposits (Pleistocene and Elolocene; may include Pliocene deposits locally) Unmetamorphosed generally well-indurated sedimentary and volcanic rocks (Tertiary and Upper Cretaceous; may include Pleistocene deposits locally) Undifferentiated crystalline rocks (Cretaceous and older; may include Tertiary rocks locally) . Contact ....Fault along which historical displacement is known or inferred to have occurred—Arrows show relative horizontal movemen. Dashed where approximately located; dotted where historic^ displacement uncertain .Thrust fault along which historical displacement is known to have occurred—Sawteeth on upper plate Fault along which Quaternary displacement is known or inferred to have occurred—Arrows show relative horizontal movement. Dashed where approximately located; dotted where concealed; queried where uncertain _»__4..Thrust fault along which Quaternary displacement is known or inferred to have occurred—Sawteeth on upper plate. Dashed where approximately located; dotted where concealed SCALE 1:750000 20 _L 40 60 _L 80 _L 100 KILOMETERS ☆ INTERIOR—GEOLOGICAL SURVEY, RESTON, VA—1984—G84634 Modified from Jennings (1973) AREA OF MAP GENERALIZED GEOLOGIC MAP OF SOUTHERN CALIFORNIA, SHOWING MAJOR FAULTS IDENTIFIED WITH QUATERNARY DISPLACEMENTDEPARTMENT OF THE INTERIOR U.S. GEOLGOCIAL SURVEY P6 V- flatc 3Lai l(e>y PROFESSIONAL PAPER 1342 PLATE 3 114° 33° - EXPLANATION q 7_ LINE OF EQUAL DECLINE OF GROUND-WATER LEVEL 33 DO FOR INDICATED PERIOD—Contour intervals 5, 10, and 15 meters. Dashed where inferred from pumping depression shown on map of ground-water levels or from ground-water basin configuration and known areas of ground-water withdrawal and recharge 1957-65 HEAD DECLINE IN CONFINED AND DEEP SEMICONFINED ... _j5___ AQUIFERS FOR INDICATED PERIOD—Contour intervals 15 meters. Shown only in southern San Joaquin Valley where distinguishable over broad areas AREA OF SIGNIFICANT GROUND-WATER WITHDRAWAL 4S/2W-3P1 WATER WELL FOR WHICH A REPRESENTATIVE HYDROGRAPH HAS BEEN PUBLISHED OR CAN BE CONSTRUCTED FROM TABULATED WATER LEVELS GIVEN BY CALIFORNIA DEPARTMENT OF WATER RESOURCES (1932-62; Bulletins 39 and 130) — Standard USGS well name based on township and range, section number, alphabetical Vs section reference, and drilling chronology. Number in parentheses is the maximum recorded water-level decline in meters OUTLINE OF OILFIELD OR GASFIELD NOTE: See plate 2 for explanation of geologic symbols 10. California Department of Water Resources (1960; 1963; 1967a, pi. 9), Hardt (1971), Koehler (1970), Riley (1956), Lewis (1972) 11. Schaefer (1978), Kunkel (1962; 1963) 12. Los Angeles County Flood Control District (1962, Map no. 20; 1948, Map no. 14), California Department of Water Resources (1968a; 1968c; 1975b; 1975d), Eckis (1934), Mendenhall (1905b; 1905a) 13. California State Water Resources Board (1961, pi. 33), California Department of Water Resources (1975a; 1975c; 1976) 14. California Department of Water Resources (1933; 1975a), California State Water Resources Board (1933; 1956, pi. 19B and C; 1975a) 15. Tyley (1974, fig. 14), California Department of Water Re- sources (1964; 1975a) 16. Bloyd (1971) 17. California Department of Water Resources (1941, pi. 20; 1970b; 1971a; 1973; 1975a) 18. Singer (1970, fig. 7) 19. Lofgren (1975, pis. 2E and 2D) 20. Lofgren and Klausing (1969, fig. 25) 21. California Department of Water Resources (1949; 1967b; 1969; 1975a) 22. Skrivan (1976), California Department of Water Resources (1971b; 1975a) 23. California Department of Water Resources (1956b, pis. 10B, 1 IB; 1971c; 1975a) 24. California Department of Water Resources (1964) 25. Loeltz and others (1975) 26. Miller (1976, fig. 7), LaFreniere and French (1968), Upson and Thomasson (1951), California Department of Water Resources (1975a) 27. Warner (1971), Miller and Rapp (1968), Upson (1951), California Department of Water Resources (1975a) 28. Hughes and Freckleton (1976, p. 284), Warner (1971), Worts (1951), California Department of Water Resources (1975a) 29. Moyle (1976, fig. 10) 30. Lofgren (1976), California Department of Water Resources (1959a, pi. 9; 1975a) 31. California Department of Water Resources (1968b; 1975a) [(Outlines of oilfields and gasfields from California Division of Oil and Gas (1973a; 1974a)] ☆ INTERIOR—GEOLOGICAL SURVEY, REST0N, VA-1984-G84634 AREA OF MAP AREAS OF FLUID EXTRACTION IN SOUTHERN CALIFORNIAPROFESSIONAL PAPER 1342 PLATE 4 V- 13^ Ho) DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY 35' 34° 33° SOURCES OF DATA 1. Castle and Yerkes (1976, p. 20), Grant(1944, p. 130-131), Mayuga and Allen (1969, p. 68-70), Yerkes and Castle (1969, p. 57-58), Grant and Sheppard (1939), Gilluly and Grant (1949), Wentworth, and others (1970). California Division of Oil and Gas (1973, p. 22), National Geodetic Survey lines 82583; L—18364, L-18296, L-18299, L-11398, L-21868, L-21807 2. California Division of Oil and Gas (1974, p. 20-23) 3. California Division of Oil and Gas (1973, p. 20-22), Estabrook (1962), Parkin (1948) 4. Estabrook (1962, fig. 1), National Geodetic Survey lines L-386, L-15577, L-15457 5. Wentworth, and others (1970, pi. 1), Lamar (1972, p. 33), Lamar and Lamar (1973, p. 75) 6. Lofgren (1971a, p. 14) 7. Castle and others (1974, p. 62) 8. Lofgren (1971b), Engineering News-Record (1937), Rezin (1969), City of Pomona Engineering Department (1965), National Geodetic Survey lines L—21615, L-21877, L-23707, L-23908 9. Miller and Singer (1971, p. 13—16) 10. Fett, and others (1967, p. 30), Morton (1977, p. 118, 123), Lofgren (1976) 11. Lewis and Miller (1968, p. 16), National Geodetic Survey lines L-15618, L-23671 12. Yerkes and Castle (1969, p. 57-58), Lofgren (1975, p. 30, pis. 3F, 4H) 13. Poland and others (1975, p. 27-32), Lofgren and Klausing (1969) 14. Buchanan-Banks, and others (1975, p. 123), Castle and others (1977, p. 222) 15. Church and others (1974), National Geodetic Survey lines L-18658, L-23126 16. National Geodetic Survey lines L-17200, L-21369 17. National Geodetic Survey lines L-8531, L-23208 0 10 20 30 40 50 KM 1 ___!_____I_____I____I_____I INDEX MAP SHOWING SOURCES OF DATA AREAS OF DIFFERENTIAL SUBSIDENCE ATTRIBUTED TO FLUID EXTRACTION, SOUTHERN CALIFORNIA AREA OF MAPDEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY Q&- IS % ,, V■ 13^ s r .. \ ptateSWf H”) PROFESSIONAL PAPER 1342 PLATE 5 PRINCIPAL ROUTES AND DATES OF LEVELINGS USED IN THIS REPORT area of MapA/?, IN MILLIMETERS PROFESSIONAL PAPER 1342 PLATE 6 —1400 -North-northeast- 1 East-west X CD < LU CO 3 X o < LU LU 0- OQ 3 3 o < CO O < 3 CL CD ro in 10 ro CM cvj m X Ll z LU 3 CD < CO < o CO OJ ro in cr < < s Ll h- < or z 3 3 o < cn CL 3 CD 10 ro ro ro in m 8° 5 x X X Q. I— LU oo 0.0 o cn 3 O >- o c o CO < £ £ CD CL < CD a o o $ iu o CD 3 O 4— a> CJ> ■g ir c o in in < or < 00 or 2=< 3 00 o Sz < Sen < or LU a. or < a 3 O *— c o c. 3 o "O a> or < or 3 l~ Z LU > < & £ CD O H LU 3 00 CO o > CO o o in CD CD 00 CO 00 O) O) CD o /-> CD CD ST o Is- 1^ CM CM CM CM in ro L/ rO Is- CD o z £ Li_ O CL < Ll Z ~~D ZD B. ALTERNATIVE RECONSTRUCTION OF HEIGHT CHANGES USING PRE-1960 DATUM -5-6/56 ; 1st order; (L-15972)- -2-3/60 ; I st order; (L-17778)- -6-9/48; 1st order ;(L-I264I)- -1-2/42;I st order ;(L-9 4 49)- r 2/39; 1st order;(L-8470) -3-4/60; 1st order;(L-!7847)- -10/70-3/71; 1st otder(3mm);(L-22292)- -6-9/48 ; I st order; (L-l2641)- -2-3/60; I st order; (L-17856)- I20°45' 35°15' |— I20°I5' ---1 3 5° 15* AVILA BEACH * SAN LUIS OBISPO CO EXPLANATION Ho/70-3/71; 1st order(3mm);(L-22292)H LEVELING SURVEY—Showing months and years of survey, order of leveling, rejection limit if applicable, and National Geodetic Survey line number BENCH MARK 0 10 I 20 30 I 40 I 50 KM _J I I9°l 5° —| 34°45‘ SANTA BARBARA CO i. * ^ 4- GAVI0TA —•X.* POINT CONCEPTION V?* *4? 'V* a'P VENTURA CO SANTA 34°I5'I--- I20°45' H3 ☆ INTERIOR-GEOLOGICAL SURVEY. REST0N. VA-1984-G84634 —I 34°I5' 119° 15' TOPOGRAPHY AND HEIGHT CHANGES (Ah) WITH RESPECT TO BENCH MARK TIDAL 8 ALONG INDICATED ROUTES BETWEEN VENTURA AND AVILA BEACH AND GAVIOTA AND LOS OLIVOSAh, IN MILLIMETERS HEIGHT, IN METERS DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY v < cr Ld > CZ> :•= CD z_ o ir"5 C4— 0- O c co1*- o c rr M LL - w o UJ “O CD cr i§ Sen o ro ro N- CD CD in Ll (/) Ll. (/) ^ 3 (73 ro r^- ro 00 00 o o 03 — ^3" — ro a>t- m in *3-cDro ro ro ro Q z < CD CD >- $ X o u> □ a) i— -o c < c a cn < «♦— 3 o CL 13 o M— O O OC 0) c LlJ Q_ < nN Q) oo in cr o a. cr Q O o $ 1C o o co O at c_ X) c: < D cn cr < a. cr UJ N < cr o o CM z o CD OJ ro Q 03 h- OJ o CD CD CJ ro CD OJ ro CD OJ ro CD 0J ro CD OJ ro CD CJ ro CVJ OJ X —> ~z. a CD 360r- 320 - 280 - -i 240 - 200 - in m in m 03 03 o o in in CD CD 2 cr “3 Ld 320 i- 280 240 - 200 - 160 - 120 -i 320 - 280 - 240 - 200 160 120 80 40 -i & r'-*' ii, 07 ro _j a li- L±J Q CM CM I I I I I CO j- sf a LU CMLlJ o _I 1 1 II 1 1 +634(1976) + 741(1974) +638(1974) -6.43(1976) +524(1976) + 631(1974) -703(1976) u -3-5/55; Ist order;(U—15577) -5-6/55; 1st order;(L-15618)- -5/61; 1st order ,(L-18296) -3-5/61,1st order; (L-18299)- 4/61,1st order;(L-!8364) -3-6/64; 1st order ;(L-I9752)--1-4/65; I st order; (L-20169) -12/61-3/62; 1st order;(Los Angeles Co.) -3/65; I st order; (L-20145) -4-5/64; I st order; (L-I978D- -6/68-4/69; I st order; (L-21739) 5-6/68; I st order;(L-2l723) -1-8/68; 1st order; (L-21589) -2-7/71; 1st order(3mm);(L-22429)- 4-8/69; 1st order (3mm);(L-2l962.A) x2/7l;lst order :3mm) ;(L-22427) -7— 11/73;lst order(4mm) ; (L-23614) ----------------------------11/72—3/73;!st order(4mm) ;(L-23679) N3-7/73;lst order(4mm) ;(L-23691) ------------I0-l2/76;lstorder(3mm) ;(Los Angeles Co.)—---------- -8-9/76;lstorder(3mm); (L-24II6) 4-6/65; I st order;(L-20298)~ -11-12/68; 1st order; (U—21782)- 4/27-6/3/71;!st order(3mm); (L-22422h -3-5/62; I st order; (L-18658) A 1/68;I st order;(L-2l605) -l2/72-5/73;2nd order(8mm) ;(L-23I26)- -10/73-2/74 ;l st order(4mm) ; (L-23671)------11/73-2/74,1 st order(3mm); (L-23208) -j -6-7/76;lst order(3mm) ;(Los Angeles Co.)- TOPOGRAPHY AND HEIGHT CHANGES (Ah) WITH RESPECT TO BENCH MARK TIDAL 8 ALONG INDICATED ROUTES BETWEEN LOS ANGELES AND CANTILDEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY - 3/61;2nd order;(Colif Dept. Water Resources) —* - -----6-9/64; 1st order; (L-19778)------------------L 8-9/64; 1st order; (L-I9787H >% c 3 O o 5/53; 1st order; (L-14799) 11/56-1/57; 1st order; (L-16254) 4/59; 1st order; (L—17212)— II-I2(?)/6I; 1st order;(L-!8529). -2-3/61; 1st order; (L-18242) -+ \ 4-6/64; 1st order; (L-19752) -1-4/65; 1st order; (L-20169) -2-8/68; 1st order; (L-21589) 2-4/68;lst order; -4-8/69; 1st order (3mm); (L-2I962.A)- H (L-21366) X 4-5/71; 1st order (3mm); (L-22391) 4-6/71; 1st order(3mm);(L-22422) -----------3-8/73; 1st order (4mm); (L-23675)- -7-l2/74;lst order (3mm);(L-23673)—I l4/73; 1st order (4mm); (L-23677) 11 /56; 1st order; (L—16263) 2-3/59; 1st order;(L-I7I66)--- 11/61-1/62; 1st order;(L-l8529)- 2-3/65; 1st order; (L-20130)___ —3-4/72;lstorder(3mm) ; (L-22r54)______ 3-4/53; 1st order; (L-14778) ----4-6/65; 1st order; (L-20279)--- 11/73-2/74; I st order(3mm) ;(L-23208) 0 10 _L 20 30 40 50 KM II9°00' 35° 30‘ r II8°00' 35°30' MONTALVO 30' 15' 118° 00' ☆INTERIOR-GEOLOGICAL SURVEY, RESTON, VA-1984-G84634 EXPLANATION b3-4/72; 1st order (3mm);(L-22754)-^) LEVELING SURVEY—Showing months and years of survey, order of leveling, rejection limit if applicable, and National Geodetic Survey line number unless otherwise noted BENCH MARK TOPOGRAPHY AND HEIGHT CHANGES (Ah) WITH RESPECT TO BENCH MARK TIDAL 8 ALONG INDICATED ROUTES BETWEEN SAUGUS AND GRAPEVINE, CASTAIC AND FAIRMONT, CASTAIC JUNCTION AND MONTALVO, AND MOJAVE AND BAKERSFIELDDEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1342 PLATE 9 C/> Ixl UJ o CD O OJ ro CD UJ cr CD < CD z < CD CD 01 (- s cc S a m < > 0 01 C0<8 North-so uth-t North-south, East-west^ North- south/ |-East-westj jEast-west | Northrsouth j Northeast-^ E ast-west^ Northeast-1 East-west ^ North-south 1 Northwest J" East-west ^ L North- south CD CD —1934 ; I st order; (L—991)— 3-5/61; I st order; (L-18296) -I934;lst order;(L-99l)- ,4/61; 1st order; (L-18364) -3-5/61; 1st order; (L-18296)- 6/68-4/69; 1st order;(L-2l739)- 1-7/68; j_st_ order;(L—2!725)y ^ ^-1-2/69; 1st order; (L-2I72I) 8-9/68; I st order;(L-2!6l2)- -6/70-5/71 ; I st order (3mm); (L-22427)- 11/70-10/71; Istorder; (L-22613) 7-11/73 ; 1st order (4mm); (L-25614)^ 11-12/73; I st order (4mm);(L-2^l7)^-9-IO/73; I st order(4mm);(L-23632) I* 1--------------------3 ^8-9/76; 1st order(3mm);(L-24116) -1-9/74; 1st order(4mm);(L-23703)- - 3-4/69; I st order;( L—21874)- -7/73-1/74; I st order(3mm);(L-23224)- 0 L 10 20 30 40 50 KM EXPLANATION |—1-9/74; 1st order (4mm);(L-23703)—j LEVELING SURVEY—Showing months and years of survey, order of leveling, rejection limit if applicable, and National Geodetic Survey line number unless otherwise noted • S 32 BENCH MARK _________ Z1?71 . ALTERNATIVE INTERPRETATION—See text for description 118° 30 34° 30' |-- l5'b LLANO 117°00' ---1 34°30' H16' ,0'L 34°00 1 118° 30' _L 15' 118° 00' 45' JL 30' TOPOGRAPHY AND HEIGHT CHANGES (Ah) WITH RESPECT TO BENCH MARK TIDAL 8 ALONG INDICATED ROUTES BETWEEN LOS ANGELES AND SAN BERNARDINO AND AZUSA AND LLANO 2400 2000 1600 1200 800 400 0 80 40 0 40 80 120 160 120 80 40 0 40 80 120DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY |«—Northwest ^Northeast East- west North-south 4 Northeast’ North-northeast Northwest LlI CD Z < CC O I l < N Z < cc LlI CL CO LlI N- ro 3 O ac 00 ro > O cc CD 3 O ° z c CC O LU O CO -o X CD —1 O cZ D < X o cn cn CD CD in OCD o ’sf ro NIO CD 1 Q 1 OCD 11 < 1 Ll 1 o o L T> C < c o CO o I— Uo z O z o O-c -3 D> <-2 OCD shin s-min £ roro to oiS to to LL Sen f5^- coz I I I < x LlI CL CO LlI X CD LlILl ro _l a> LlI cd X X OJ OJ U- I O 'O o o l a> 5 O I— CO cc < cn 2 H i ro LU I o I- _l o o (T> ro Q l +2 ■+— O _d a _sz o M— o cr> *♦- O Se **— o c Q LU a> _q L Z) CD cn c o >> c o O o c= < o c. o CO a ro 5 i LU < cc < LU 00 CD CQ >- H O QC < LU 00 9 00 3 o 0) a ■o c 0) 0) X >- LU < > LU QC LU CD ZD m in in 0J in coco B OJ in OJ ro in rocD cdco Ll N -D o 0JZD I -2-6/56; 1st order; (L-15908)- -10-11/61 ; 1st order ;(L-I8544)- [-7/75-1/76; I st order;(San Bernardino Co.)-j -8/68-6/69; I st order ; (L-21868)- —2-7/74; 1st order (3mm);(L-2 3437)- 10-11/61; 1st order;(L-l8544) h -12/68-3/69;! st order;(L-21764)---- 1 — I — 5/56 ; I st order; (L-15902)— -10-11/61; 1st order;(L—18547)- -9-11/74; 1st order (3mm);(L-23919)- 8/68-4/69 ; tst order;(L-2l485)- -7/73-1/74;! st order;(L-23224)- "^-6/ 74; 1st order(3mm); (PV 896-USGS) 0 10 20 30 40 50 KM EXPLANATION (*“2-7/74; 1st order (3mm);(u-23437)—-| LEVELING SURVEY—Showing months and years of survey, order of leveling, rejection limit if applicable, and National Geodetic Survey line number unless otherwise noted II8°00 35° 00' | r ,1961 BENCH MARK ALTERNATIVE INTERPRETATION—See text for description -h ‘b 33045' 1-- II8°00' il6°45 1 35°00' I l H3 H- H 34°00’ J I I7°00l 33°45‘ I6°45' ☆ INTERIOR-GEOLOGICAL SURVEY. REST0N. VA-1984-G84634 TOPOGRAPHY AND HEIGHT CHANGES (Ah) WITH RESPECT TO BENCH MARK TIDAL 8 ALONG INDICATED ROUTES BETWEEN ORANGE AND BARSTOW AND COLTON AND LUCERNE VALLEYAh, IN MILLIMETERS ^5 7-8/64;Ist order;(L-l9775) 4 -8-9/64 ;l st order;(L-l9787)- 6-8/64; I st order; (L-19778 2-3/60 ; I st order; (L-17772)- —f-------------- •4-10—11/61;lst order;(L-l8547H -3-4/65 vI st order;(L-20!75)_ Ll 1/68-1/69; 1st order; (L—21681) -1969 ; 2 nd order;(DWR)- -1-8/68; 1 st order;(L-2l589)- -6-7/68;lst order; (L-21488)- 5-6/68 ; I st order -2-5/71 ; 2nd order;(DWR)- -8-10/71;! st and 2nd order;(L-22498)- -1-6/74; I st order(4mm); (L-23687) ond 10/73-2/74; I st order (4 mm);(L-23671)- -2-3/76; 1st order; (Los Angeles Co.)- 12/71 1/72 ,sing|e. and double-run 1st order; (DWR)--7-8/73;lst order (4mm);(L-2369l)----------- —6/76; I st order; (DWR)-- ; (l_—21485)-' 0 L 10 I 20 30 40 50 KM II9°00' 35° 00' |--- 45' V 30' - 118° 15 35°00' ~l EXPLANATION 7-8/73; 1st order (4mm);(L-2369i)^ LEVELING SURVEY—Showing months and years of survey, order of leveling, rejection limit if applicable, and National Geodetic Survey line number unless otherwise noted *x 53 BENCH MARK KERN CO 34°I5'!— 119°00' 45' 15' 118° 00* ☆ INTERIOR-GEOLOGICAL SURVEY. RESTON. VA-1984-G84634 TOPOGRAPHY AND HEIGHT CHANGES (Ah) WITH RESPECT TO BENCH MARK TIDAL 8 ALONG INDICATED ROUTES BETWEEN QUAIL LAKE AND HESPERIA R4I VICTORVILLEDEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1342 PLATE 12 ?(o v. /3f2 ?(DJr- cr m OJ sr a —i 440 400 < o UJ UJ X OJ "3 I $ o I— CO cr < 00 to 360 320 280 280 - 240 240 200 200 160 160 120 120 - 80 80 40 40 -1956- ________________3-4/61; 1st order; (L-18230) ---12/72-4/73; 2nd order;(L-23l26)---------- 11/73- 2/74; 1st order(3mm); ( L-23208)----- _____________l_4/44; I st order;! L- III15) 2-3/74 ; 1st order (3mm) ( L-23227)--- 2-3/44; 1st order;(L-M067)-- 2/44; 1st order ;(L-11069)- ---3-5/74 ; 1st order(3mm);(L-233!5) -1-5/56; 1st order;(L-l5902)-- -10-11/61; I st order ; (L-18547) -8/68-4/69; 1st order; (L-21485)^ — 4/73; 2nd order;(L-23I26)— 0 10 40 50 KM 35°I5' r II8°00‘ 45' ~r ~r 15' II7°00' T" ~r 45' ~r 30' T II6°00' “T 45' T II5°30 —j 35°I5' EXPLANATION I-—3-5/74; 1st order (3mm); (L-23315)—H LEVELING SURVEY—Showing months and years of survey, order of leveling, rejection limit if applicable, and National Geodetic Survey line number BENCH MARK 30' h ]35°00' H 45' H 30' ---1 34° 00' H 45' o'L 33°30 L I I8°I5‘ 30' II7°00' I 30' ---1 33°30 II5°30' ☆ INTERIOR-GEOLOGICAL SURVEY, RESTON. VA-1984 G84634 TOPOGRAPHY AND HEIGHT CHANGES (Ah) WITH RESPECT TO BENCH MARK TIDAL 8 ALONG INDICATED ROUTES BETWEEN MOJAVE AND COTTONWOOD PASS AND BARSTOW AND BRYMANDEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1342 PLATE 13 C o cc ’ UJ to — l- ^ o § 1 *- $ CP C - UJ _l O < •4— > O' c < c o c o o 3 GQ >- - 40 0 o £ 00 CO O Is- Is- o Is- lO J 1 10 o i ro O i m o CM 1^ T CM < X o => o: CO CM CM i^ CM in r— in N in in in in 1^ in X 2 0. Q X CD o ,4/68-6/69; 1st order (4mm);{L-21770) •3-4/69; 1st order (3mm); (L-21776) ^__________|-l-12/69; I st order(4mm);(!_—21975) -7-9/76;lst order (3mm);(Riverside Co.)- •1-2/73; 1st order; (Son Bernardino Co.)— -6-7/76 ; I st order (3mm); ( L-24074)- • 1-3/56; 1st order (4mm);(L-l5875)- -|/68-6/69;lst order (4mm);(L-2l770) 11/68-5/69; 1st order (4mm);(L-2l883)-^ 3/74; 1st order (3mm);(L-23349) '5-9/74; 1st order (4mm); (L-23501)- ttr INTERIOR—GEOLOGICAL SURVEY. RESTON. VA-1984-G84634 117°30 34°I5‘ r T II6°00 i 34°15' n4 ■f i'i— EXPLANATION 1—3/74; I st order(3mm); (L-23349)-'t LEVELING SURVEY—Showing months and years of survey, order of leveling, rejection limit if applicable, and National Geodetic Survey line number unless otherwise noted BENCH MARK .______.___J961 ALTERNATIVE INTERPRETATION—See text for description II5°45' n45' —I 33°30' II5°45' TOPOGRAPHY AND HEIGHT CHANGES (Ah) BASED CHIEFLY ON COMPARISONS WITH BENCH MARK TIDAL 8 ALONG INDICATED ROUTES BETWEEN COLTON AND MECCA, INDIO AND BENCH MARK IC (MWD), WHITE WATER AND TWENTYNINE PALMS, AND MECCA AND TRUCKHAVENDEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY (f) PROFESSIONAL PAPER 1342 PLATE 14 CO CO g Q o o o h- I- o a o z =3 < Q UJ tr LlI o tr I < o tr LlI Z tr g X CM < X ro ro O OJ > 8 2 1— > X O CM CM CM CM CM CM a CM CM 1-4/44; 1st order;(L-l 1115)-----*|*--------1931; I st order; (L-7407)— -------------------------------------4-5/76; 1st order (3mm); (L-24077) 400 360 320 280 240 200 160 120 80 40 0 40 80 120 0 10 20 30 40 50 KM 30 ‘ T II3045' 34° 45' 1 EXPLANATION H-4-6/76; I st order (3mm); (L-24080)—H LEVELING SURVEY—Showing months and years of survey, order of leveling, rejection limit if applicable, and National Geodetic Survey line number 33°30' I 1 I l | _L _L 116°00' 45' 301 15* II5°00' 45' 30' 114° 00 ☆ INTERIOR-GEOLOGICAL SURVEY. REST0N. VA- 1984- ---* 33°30 II3°45' -G84634 TOPOGRAPHY AND HEIGHT CHANGES (Ah) WITH RESPECT TO BENCH MARK TIDAL 8 ALONG INDICATED ROUTES BETWEEN COTTONWOOD PASS AND PARKER DAM AND FREDA JUNCTION AND AMBOYA/?, IN MILLIMETERS HEIGHT, IN METERS DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY 1200 r— 800 400 -,1200 800 400 -West-northwest- -East-west -West-northwest North-south - - East-no rtheast- - North-south - LxJ > dc o i— o > si" => 3 O >- M— 111 cn 3 CP c k_ Q. ° > CD 3 O a> llI a *4- oZ o c ■O -occ c o •u o $ o r* a>r) ~o V 13 O co nr in 1^- c\J ro GO ID C\J ro > O CM Is- O o CM Is- N £ £ 3 a c. UJ '2=1 O 28 ,£D CL >- O CM— Ps C o o c b_ 3 a *4— >% a> *5 > o CP c o v_ o in O CD CD CD o c o cn CO 8 in 3 O QC . UJ CP c Ll) DO : Is- i ro O CD -10-11/61 ; 1st order (4mm); ( L-18544)- -1-8/1969 ; 1st order (4mm); ( L-21975)- a v■ I3*f^ (p£oJ-C-‘S b North-south - -Northeast- >- UJ < > < o o 3 >- CVJ h~ c a c 3 O 5 o c CL O 0J 5 5 I cn g Lli >- Lli 5 H sh c ‘c c a DQ CD in x CD m CM cn 5; CD ■5 o M— c o "c 3 o one (-.5 LlI« O a5 -I?" LUCO 3 O H— U) if o-J E< « DC - UJ 3 DC 1- < Q 3 >- LU L. CD Q. E 1 DC 0. 3 < 2: < 3 z DC i— o o _l UJ Ld DD U Z U. o CO o ro CM -1944- '^69/73 _ 0 40 80 120 CD 00 CD CD O CD CD CD Q_ CD CM CM CD CM CM M; CD > O ro CM cn TO o or 3 o *4— c a c 3 o E§ LlI £ O CD -1 3 UJC/) X ^ c o CD Q. 3 CO 3 o CD Q. E UJ _i _j > f— _i o < o < o a: uj < 3 o M— CO o CD T5 a < 3 g < CO CM CD cr >- DQ CD O CO CD CD CM CM 116° 15 33°45'|— n 5° 30' 33° 45' 40 80 120 —1160 -10/71- 1/72 ; 1st order(3mm); (L-22 603)- -2-3/74; 1st order (3mm ); ( L-23237)- -12/73-2/74 ; 1st order(3mm); (L-23243)- -2-3/44;lst order(4mm); ( L—110 67)- -1-2/73 ; 1st order (4mm); (San Bernardino Co.)- f 10 _1_ 20 I 30 I 40 50 KM T 114°45 1 33° 30' T H3 IMPERIAL CO T 33°I5'1--- 116° 15' ---1 33°, I 5' II5°30 B. TOPOGRAPHY AND HEIGHT CHANGES WITH RESPECT TO BENCH MARK D 723 ALONG INDICATED ROUTE BETWEEN COTTONWOOD PASS AND FRINK T EXPLANATION ■5/74; 1st order (3mm); (L-23315)—H LEVELING SURVEY—Showing months and years of survey, order of leveling, rejection limit if applicable, and National Geodetic Survey line number unless otherwise noted BENCH MARK -L ■I 34°00 116° 00* H- H3 T IMPERIAL CO A. TOPOGRAPHY AND HEIGHT CHANGES WITH RESPECT TO BENCH MARK V 325 ALONG INDICATED ROUTES BETWEEN VICTORVILLE AND WHITE WATER AND YUCCA VALLEY AND TWENTYNINE PALMS ,L ----1- -----------------------------------------1 32° 30 115°00 11404 5' ☆ INTERIOR-GEOLOGICAL SURVEY. RESTON. VA-1984-G84634 C. TOPOGRAPHY AND HEIGHT CHANGES WITH RESPECT TO BENCH MARK Y 58 ALONG INDICATED ROUTES BETWEEN OCOTILLO AND OGILBY AND EL CENTRO AND FRINK TOPOGRAPHY AND HEIGHT CHANGES (Ah) ALONG LUCERNE VALLEY, COTTONWOOD PASS-FRINK, AND OCOTILLO-OGILBY LINESDEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY I2I°00 36°00‘ VI* v~. ihUJ-•plahc l to PROFESSIONAL PAPER 1342 PLATE 16 II4°00' 36°00 35°00 34°00 33°00 I2I000' I20°00 119°00 33° 00' II8°00 II5°00 sir INTERIOR-GEOLOGICAL SURVEY. RESTON. VA 1984 - G84634 1I4°00' SCALE 1:750000 Seismicity data courtesy of G. S. Fuis (written commun. 1979) 20 _L 40 60 _L 80 100 KILOMETERS DISTRIBUTION OF EARTHQUAKES IN SOUTHERN CALIFORNIA OF MAGNITUDE >4 FOR THE PERIOD JANUARY 1, 1932—DECEMBER 31, 1976, SUPERIMPOSED ON CONTOURS OF CUMULATIVE UPLIFT DURING THE PERIOD 1959-1974.5