Garlock Fault: An Intracontinental Transform Structure, Southern California

The northeast- to east-striking Garlock fault of southern California is a major strike-slip fault with a left-lateral displacement of at least 48 to 64 km. It is also an important physiographic boundary since it separates along its length the Tehachapi–Sierra Nevada and Basin and Range provinces of pronounced topography to the north from the Mojave Desert block of more subdued topography to the south. Previous authors have considered the 260-km-long fault to be terminated at its western and eastern ends by the northwest-striking San Andreas and Death Valley fault zones, respectively. We interpret the Garlock fault as an intracontinental transform structure which separates a northern crustal block distended by late Cenozoic basin and range faulting from a southern, Mojave block much less affected by dilational tectonics. Earlier ideas that the Garlock fault terminates eastward at the Death Valley fault zone appear to us to be in error, although right-lateral offsetting of the Garlock along that zone by about 8 km is necessary. Displacement along the Garlock fault must increase westward from its eastern terminus, a point of zero offset now buried beneath alluvial deposits in Kingston Wash to the east of the Death Valley fault zone. Much of the displacement on the Garlock fault due to east-west components of basin and range faulting appears to have been derived from block faulting in the area between Death Valley and the Nopah Range. Westward displacement of the crustal block north of the Garlock by extensional tectonics within it totals 48 to 60 km in the Spangler Hills–Slate Range area and probably continues to increase westward at least as far as the eastern frontal fault of the Sierra Nevada. Westward shifting of the northern block of the Garlock has probably contributed to the westward bending or deflection of the San Andreas fault where the two faults meet. Many earlier workers have considered that the left-lateral Garlock fault is conjugate to the right-lateral San Andreas fault in a regional strain pattern of north-south shortening and east-west extension, the latter expressed in part as an eastward displacement of the Mojave block away from the junction of the San Andreas and Garlock faults. In contrast, we regard the origin of the Garlock fault as being directly related to the extensional origin of the Basin and Range province in areas north of the Garlock. Recent models for development of that province related to intracontinental spreading east of an east-dipping subduction zone along the Cenozoic margin of western North America may best account for the differential east-west extension which has occurred in the crustal blocks to the north and south of the Garlock fault. Other possible examples of intracontinental transform faults in the southwestern Cordillera with geometries similar to that of the Garlock fault include the left-lateral Santa Cruz–Sierra Madre fault zone along the southern margin of the western Transverse Ranges, and the right-lateral Las Vegas shear zone and Agua Blanca fault of Baja California.