GREGORY A. DAVIS Department of Geological Sciences, University of Southern California, Los Angeles, California 90007 B. C. BURCHFIEL Department of Geology, Rice University, Houston, Texas 77001 Garlock Fault: An Intracontinental Transform Structure, Southern California ABSTRACT Sierra Nevada. Westward shifting of the north- ern block of the Garlock has probably contrib- The northeast- to east-striking Garlock fault uted to the westward bending or deflection of of southern California is a major strike-slip the San Andreas fault where the two faults fault with a left-lateral displacement of at least meet. 48 to 64 km. It is also an important physio- Many earlier workers have considered that graphic boundary since it separates along its the left-lateral Garlock fault is conjugate to length the Tehachapi-Sierra Nevada and Basin the right-lateral San Andreas fault in a regional and Range provinces of pronounced topogra- strain pattern of north-south shortening and phy to the north from the Mojave Desert east-west extension, the latter expressed in part block of more subdued topography to the as an eastward displacement of the Mojave south. Previous authors have considered the block away from the junction of the San 260-km-long fault to be terminated at its Andreas and Garlock faults. In contrast, we western and eastern ends by the northwest- regard the origin of the Garlock fault as being striking San Andreas and Death Valley fault directly related to the extensional origin of the zones, respectively. Basin and Range province in areas north of the We interpret the Garlock fault as an intra- Garlock. Recent models for development of continental transform structure which sepa- that province related to intracontinental rates a northern crustal block distended by spreading east of an east-dipping subduction late Cenozoic basin and range faulting from a zone along the Cenozoic margin of western southern, Mojave block much less aifected by North America may best account for the dilational tectonics. Earlier ideas that the Gar- differential east-west extension which has oc- lock fault terminates eastward at the Death curred in the crustal blocks to the north and Valley fault zone appear to us to be in error, south of the Garlock fault. although right-lateral offsetting of the Garlock Other possible examples of intracontinental along that zone by about 8 km is necessary. transform faults in the southwestern Cordillera Displacement along the Garlock fault must with geometries similar to that of the Garlock increase westward from its eastern terminus, a fault include the left-lateral Santa Cruz-Sierra point of zero offset now buried beneath alluvial Madre fault zone along the southern margin of deposits in Kingston Wash to the east of the the western Transverse Ranges, and the right- Death Valley fault zone. Much of the dis- lateral Las Vegas shear zone and Agua Blanca placement on the Garlock fault due to east- fault of Baja California. west components of basin and range faulting appears to have been derived from block fault- INTRODUCTION ing in the area between Death Valley and the The Garlock fault zone of southern Cali- Nopah Range. Westward displacement of the fornia strikes northeastward to eastward and crustal block north of the Garlock by exten- has been recognized as a major structural ele- sional tectonics within it totals 48 to 60 km ment of this region for nearly 50 years. It is in the Spangler Hills-Slate Range area and equally important as a physiographic boundary, probably continues to increase westward at for it separates along its length the Tehachapi- least as far as the eastern frontal fault of the Sierra Nevada and Basin and Range provinces Geological Society of America Bulletin, v. 84, p. 1407-1422, 5 figs., April 1973 1407 Downloaded from https://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/4/1407/3418372/i0016-7606-84-4-1407.pdf?casa_token=jv0gNjCIMW0AAAAA:UsGY5Ien0hbR0j-xzOfYLKmZBj1Ffs3jvP6Wu4-2aW17Ro16_h1drYrjalKmW7u8Gv5rMlNs by California Geological Survey, 19774 on 20 July 2019 Figure 1. Displacement criteria, Garlock fault, southern California. Displaced features, for example AA', BB', are explained and referenced in text. Base map from Geologic Map of California, scale, 1/250,000 :Los Angeles sheet (Jennings and Strand, 1969), Bakersfield sheet (Smith, 1964), and Trona sheet (Jennings and others, 1962). Inset: Location map of Garlock, Big Pine, and San Andreas faults in southern California. Downloaded from https://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/4/1407/3418372/i0016-7606-84-4-1407.pdf?casa_token=jv0gNjCIMW0AAAAA:UsGY5Ien0hbR0j-xzOfYLKmZBj1Ffs3jvP6Wu4-2aW17Ro16_h1drYrjalKmW7u8Gv5rMlNs by California Geological Survey, 19774 on 20 July 2019 GARLOCK FAULT, SOUTHERN CALIFORNIA 1409 of pronounced topography to the north from ing major regional significance, although the the Mojave Desert block of more subdued important paper by Hill and Dibblee (1953) topography to the south. All previous authors first gave the Garlock its present level of recog- have considered the 260-km-long left-lateral nition. Hill and Dibblee suggested that the fault to be terminated at its western and eastern left-lateral Garlock and Big Pine faults had ends by the northwest-striking San Andreas once been continuous before being disrupted and Death Valley fault zones, respectively along the San Andreas, and that displacements (Fig. 1). This fault geometry, although some- along these two faults have produced the west- what unusual, posed no serious problems until ward bending of the San Andreas near its inter- Smith (1962) offered evidence for a 64-km section with them (Fig. 1). They interpreted left-lateral displacement along the Garlock the Garlock and Big Pine faults as left-lateral fault based on an offset late Mesozoic dike shears conjugate in a regional strain pattern to swarm. The absence of an equivalent total off- right-lateral faults of the San Andreas system. set of the Garlock's bounding fault zones—the Others, for example, Kupfer (1968), have also San Andreas and Death Valley zones—posed a considered the Garlock and San Andreas faults serious geometric problem, especially in light as conjugate shears and have attempted re- of Smith's estimate of displacement along the gional strain and stress analyses for southern Garlock as being equal to one-quarter of its California on this basis. entire length. The idea that the Garlock and San Andreas Although the San Andreas fault zone swings faults are regionally related has led some geolo- westward near its junction with the Garlock gists (for example, Hill and Dibblee, 1953; fault, it is the Garlock-Death Valley fault zone Bucher, 1955; Hewett, 1955, PI. 1; D. L. junction that presents the most serious geo- Anderson, 1971) to conclude that the wedge- metric problems. Here, offset features suggest- shaped Mojave Desert structural block, bound- ing up to 64 km of left-lateral displacement ed on the west by the northwest-striking, along the Garlock fault are found on the south right-lateral San Andreas fault and by. the side of the Garlock within 16 km of its junction northeast-striking, left-lateral Garlock fault with the right-lateral Death Valley fault zone, (Fig. 1), must be undergoing eastward dis- thus producing radically different breadths of placement away from the intersection of the terrane on the two sides of the fault between two faults. This conclusion raises serious struc- these offset features and the Death Valley fault tural problems at the previously inferred zone. Smith recognized the geometric prob- eastern end of the Garlock fault where the lems posed by the inferred geometry of the presumably "active" Mojave block is bounded Garlock and its bounding faults, but he con- by the Garlock fault to the north and the cluded that "the evidence that the two dike Death Valley fault zone to the east. These swarms document 40 mi of lateral displacement problems are discussed in detail below. is stronger than the evidence that the tectonic pattern at the ends of the fault precludes it" DISPLACEMENTS ACROSS THE (Smith, 1962, p. 103). We agree, and offer in GARLOCK FAULT this paper an explanation for the geometry and Smith's study (1962) of the northwest- origin of the Garlock fault which is compatible trending dike swarms in Mesozoic granitic with regional geologic relations in the southern terranes north and south of the Garlock fault Death Valley-eastern Mojave Desert region. zone led him to conclude that approximately 64 km of offset has occurred along the fault EARLY STUDIES since the presumed late Mesozoic emplacement The Garlock fault was discovered and named of the dikes (Fig. 1, AA'). This displacement, by Hess (1910) during studies in the Randsburg although equivalent to one-quarter of the area of the north-central Mojave Desert. Hulin length of the known fault trace, has been (1925) mapped the fault in his study of the strongly supported by subsequent field studies. Randsburg quadrangle; he recognized dis- Smith and others (1968) described a previ- placement of Quaternary and older rock units ously unrecognized thrust fault in the southern along it, and concluded that total displace- Slate Range to the north of the Garlock fault ments of left-lateral nature amounted to ap- (Fig. 1, B). Here, cataclasized Mesozoic granitic proximately 8 km. Hulin (1925) and Noble rocks and gneisses of Precambrian age overlie (1926) both regarded the Garlock fault as hav- Mesozoic metavolcanic and granitic rocks along Downloaded from https://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/4/1407/3418372/i0016-7606-84-4-1407.pdf?casa_token=jv0gNjCIMW0AAAAA:UsGY5Ien0hbR0j-xzOfYLKmZBj1Ffs3jvP6Wu4-2aW17Ro16_h1drYrjalKmW7u8Gv5rMlNs by California Geological Survey, 19774 on 20 July 2019 1410 DAY: 3 AND BURCHFIEL the west-dipping (30° to 40°) Layton Well be offset some 65 krr.. from each other (Jahns thrust. In reconnaissance studies south ol the and others, 1971; 196ii, oral commun.).