STEVENSON, ANDREW J., and STEPHEN L. EITTREIM
United States Geological Survey, Coastal and Marine Geology, Menlo Park, CA

Abstract: Morphology and Structure of the San Gregorio Fault System Across the Continental Shelf South of Point Ano Nuevo

Recently collected single channel high resolution seismic reflection profiles and Sidescan Sonar images across the San Gregorio Fault System reveal a wealth of detail regarding the near surface structure, morphology and recent activity associated with this important tectonic boundary where it crosses the northern Monterey Bay shelf. The data suggest that, in this region, the primary and most recently active trace of the San Gregorio correlates with the Frijoles Fault onshore and that the onshore Coastways Fault does not appear to the South as a continuos fault trace. Deformation of the wave planed bedrock surface formed during the latest Quaternary sea level lowstand attests to active, ongoing vertical deformation, forming anticlinal fold axes adjacent to the fault which stand above the surrounding recent sediment blanket, forming linear bedrock outcrops parallel to the fault. The Sidescan Sonar image provides an outstanding view of the drag folding associated with the fault, and several anomalous bright features which are interpreted as fluid venting sites. Strong contrasts in the backscatter signature of the bedrock units exposed within the surveyed area has facilitated construction of a detailed surficial geology map. The seismic data was collected using a 475 Joule boomer as a sound source, with a central frequency near 1 Khz. Lines were run across the fault zone with a spacing of 150 m (inner shelf) to 300m (outer shelf), and cover the fault from Ano Nuevo Cove (~5 m water depth) to the outer shelf seaward of Santa Cruz (~250 m water depth). A continuos 100 kHz Sidescan Sonar mosaic with a pixel resolution of Am has been constructed for the entire shelf crossed by the Fault.

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