LEGG, MARK R., ACTA Inc., Torrance, CA, and CRAIG NICHOLSON, Institute for Crustal Studies, University of California, Santa Barbara, CA

ABSTRACT: Geologic Structure and Tectonic Evolution of the San Diego Trough and Vicinity, California Continental Borderland

The San Diego trough lies in the central part of the inner California continental borderland across the United States-Mexico border. The geologic structure of this region is dominated by northwest-trending fault zones that have a late Cenozoic history of Miocene-age extension followed by oblique right slip. Currently, regional strain appears to be partitioned into discrete zones dominated by right-lateral shear separating areas undergoing either local crustal extension or contraction. The major inner borderland shear zones include the San Clemente, Catalina, San Diego Trough, Coronado Bank, and offshore Newport-Inglewood faults. Numerous other north- and northwest-trending faults have been recognized in the region, but have yet to be mapped in detail, especially in areas of Fortymile nd Thirtymile Bank. The overall structural style of this region consists of sets of northwest-trending tilted crustal blocks, forming horsts and half grabens that define local basins, ridges, and islands. This structure evolved mostly during the middle to late Miocene along normal-separation faults that have been flexurally rotated into low-angle detachments characteristic of large-scale extension and crustal unloading. This extensional structure was subsequently dissected, overprinted and, in places, re-activated and folded by oblique right slip associated with the evolving Pacific-North America (PAC-NOAM) transform plate boundary. Components of oblique-slip varied in both space and time from transtension to transpression as a result of changing plate-motion vectors and local fault geom try. Proximity of the East Pacific Rise during the waning stages of Farallon plate subduction, coupled with elements of inner borderland transtension, also yielded widespread rift-related volcanism during the middle to late Miocene. As the San Andreas dextral transform became the locus of the PAC-NOAM plate motion, the San Diego trough region continued to experience dextral shear, although at a significantly slower rate.

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