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