Development of Strike-Slip Faults Above Irregular Basement in the California Continental Borderland

Mark R. Legg

Major strike-slip faulting developed in the California Continental Borderland as a result of dextral shear between the Pacific and North America plates. Mapping the fault structure from a dense grid of seismic reflection profiles shows that irregular basement geometry, derived from low- and moderate-dipping extensional structure, controls the character and location of youthful, high-angle, right-slip faulting. The San Diego Trough fault follows the axis of an elongate basin with triangular cross-section formed by block tilting during prior extension. This is fits models that predict strike-slip faults develop where the crust is most thinned by extension. In contrast, the Coronado Bank fault zone is more complex, consists of several sub-parallel and anastomosing faults that lie updip i the hanging wall of the major east-dipping detachment fault, and exhibit transtensional character. This character results from local oblique extension across the detachment fault that continued after the major regional Miocene extension ceased. Plio-Pleistocene extension occurred near San Diego when adjacent regions like the Western Transverse Ranges experienced major shortening. Farther north along the Coronado Bank and Palos Verdes Hills right- slip fault zones, structural inversion of former extensional basins is related to mid-Pliocene northeast-directed shortening. Here, in the Gulf of Santa Catalina, the fault zone is extremely complex, alternating from zones of local transpression at restraining bends of varying scale to broad zones of transtension where the faulting is ill-defin d and difficult to identify in seismic profiles. Irregular basement geometry in this highly extended terrane directs the location and character of the youthful high-angle strike-slip faulting.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California