Abstract: Mid-Tertiary Crustal Extension in the Offshore California Borderlands as Demonstrated by Major Crustal Tilt Blocks from Industry Seismic Profiles

ROGERS, K., ERIC FROST, E. AUGUSTINE, B. BARSUMIAN, S. BORRON, E. KLIMCZAK, and T. O'CONNOR, Department of Geological Sciences, San Diego State University, CA

Mid-Tertiary detachment faults are well documented east of the Peninsular Ranges Batholith. They have long been suspected to occur west of the batholith as well. Industry seismic profiles provided by the Minerals Management Service (JEBCO Seismic data set) reveal a large-scale crustal tilt block located just offshore of San Diego where it forms the local bathymetric high generally labeled the Coronado Bank. It is believed that northeast- southwest extensional faulting during Mid-Tertiary time formed this structure. Sedimentary strata capping the tilt block are steeply dipping to the west and are juxtaposed against gently-dipping to horizontallystratified, half-graben basin deposits. The westward dip of the tilt block indicates that the detachment fault roots to the east, towards and presumably beneath the Peninsular Ranges Batholith. Migration of the seismic profiles prevents tracing of the detachment system to depth. Parallel seismic profiles show that the tilted crustal block slopes downward to the north, where it is overlain by sedimentary cover. The 3-D geometry of the tilt blocks suggests heterogeneous displacement along strike and therefore differential basin development during regional crustal extension. Numerous strike-slip faults related to Late-Tertiary transform faulting cut obliquely through the detachment basin and younger sedimentary cover.

AAPG Search and Discovery Article #90935©1998 AAPG Pacific Section Meeting, Ventura, California