BOHANNON, R. G., and E. L. GEIST, U.S. Geological Survey, Menlo Park, CA, and CHRIS SORLIEN, Institute of Crustal Studies, University of California, Santa Barbara, CA

ABSTRACT: Miocene Extensional Tectonism on California Continental Borderland Between San Clemente and Patton Escarpment

The California continental borderland south of the northern Channel Islands comprises four tectonostratigraphic terranes. The offshore segment of the Santa Ana terrane near Oceanside has coherent, high-amplitude reflectors developed in a thick Cretaceous to Miocene sedimentary section that dips westward above basement. Several large listric normal faults with intermediate to low eastward dips juxtapose this section with schistose rocks of the adjacent Catalina terrane to the southwest. These faults are overlapped by the Neogene strata in the Gulf of Santa Catalina. Reflective horizons are thin and discontinuous across the Catalina terrane where Miocene sedimentary and volcanic rocks lie directly on Catalina Schist. Southwest of San Clemente Island, the Nicolas terrane can be easily di tinguished from the Catalina by a thick and gently deformed reflective sequence of Cretaceous to late Miocene strata in the southern San Nicolas basin and Cortes Bank. The boundary between the Nicolas and Catalina terranes is a west-dipping normal fault that lies along the southeast side of the San Nicolas basin. In the Patton terrane, reflective strata are confined to small Neogene basins that develop above low-grade metamorphic rocks, which are acoustically chaotic.

Models that explain the juxtaposition of these terranes are commonly based on large offsets on strike-slip faults, but Neogene offsets of greater than several hundred kilometers are needed to explain the occurrence of the Catalina terrane inboard of the Nicolas terrane. We prefer a model of tectonic unloading of the Catalina terrane on large oblique-slip and detachment faults, whose displacement resulted in a west-southwest migrating hinge of uplift and denundation at the boundary between the Nicolas and Catalina terranes. This model requires a doubling of the width of the borderland tectonic province during the Neogene. Extension, oblique slip, and volcanism on the borderland postdated the initial contact of the Farallon-Pacific spreading center with the North American plate edge. Th primary driving force for the deformation came from the divergence between the North American and Pacific plates, which was accommodated by deformation in the weak continental rocks after the spreading ridge was destroyed.

AAPG Search and Discovery Article #90992©1993 AAPG Pacific Section Meeting, Long Beach, California, May 5-7, 1993.