SEDLOCK, RICHARD L.
Department of Geology, San Jose State University, San Jose, CA ([email protected])

Abstract: The San Gregorio Fault Zone: Late Cenozoic Dextral Slip and Slip Rate, and Southward Linkage with the Sur, San Simeon, and Hosgri Faults

The amount and rate of late Cenozoic dextral slip on the San Gregorio fault zone (SGfz) are ambiguous. The most popular view is that the SGfz has accumulated about 150 km of dextral slip since 10-15 Ma, with a modern rate of about 7 mm/yr. However, these interpretations are not unique because the pertinent observations also can be adequately explained by smaller slip magnitudes and slower slip rates of only 1-3 mm/yr. One underappreciated but crucial problem is that the SGfz (and its southeastward extensions) lack true piercing points (rather than similar provenance, facies, stratigraphy, etc.) of Miocene and younger age.

Another popular interpretation is that dextral slip on the SGfz is transferred southeastward to the Sur, San Simeon, and Hosgri faults (SSH). However, kinematic linkage of the SGfz and the SSH complicates modern and late Cenozoic tectonic reconstructions because the slip history of the SSH imposes severe conditions on interpretations of the SGfz. Diverse geological, geophysical, and geodetic data indicate a 1-3 mm/yr late Quaternary and modern dextral slip rate on the SSH. This rate is difficult to reconcile with fast slip on the SGfz if the SGfz and SSH are kinematically linked. In contrast, the slow rate on the SSH is very consistent with a slow late Cenozoic dextral slip rate on the SGfz.

AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California