Structural Character of Hosgri Fault Zone and Adjacent Areas in Offshore Central California

James K. Crouch, Steven B. Bachman

The Hosgri fault zone extends from the east-west Transverse Ranges structures near Point Arguello northward for more than 150 km to the offshore area near San Simeon Point. The fault zone is seismically active and consists chiefly of a continuous series of eastside-up thrust and high-angle reverse faults. East of the fault zone, Miocene Monterey and volcanic rocks, along with underlying pre-Miocene strata, have been tightly folded as a result of low-angle imbricate thrust faulting during post-Miocene time. These highly deformed strata have been uplifted and truncated along the inner shelf. Immediately west of the Hosgri fault zone, similar Monterey and older rocks, which are less folded, conformably underlie Pliocene and younger basinal strata at structural levels that ar generally 1,200 to 2,000 m deeper than correlative strata east of the Hosgri fault zone.

Following its discovery in 1971, the Hosgri fault zone was characterized by subsequent investigators as a northwest-trending fault that was part of the San Andreas system of strike-slip faults, with disagreements on the timing and amount of right-lateral offset along the fault zone. However, modern offshore seismic-reflection data, earthquake focal-mechanism studies, and recently available offshore well information suggest that the Hosgri fault zone is instead a major imbricate thrust zone. Detailed structural analyses along part of the Hosgri fault zone suggest that little, if any, strike-slip offset has occurred along this structural trend since its post-Miocene inception. Nevertheless, the Hosgri fault zone itself can be interpreted to be a product of the larger overall San Andreas transform system in that compression has developed because the San Andreas is not parallel to the Pacific-North American plate motion.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.