Abstract

An exceptional exposure of the San Gregorio Fault provides the opportunity for detailed observations of structural fabrics within an active fault zone. Where it is exposed in the intertidal zone in Moss Beach, California, the San Gregorio Fault juxtaposes different sedimentary lithologies within the Pliocene Purisima Fm. An approximately 10 meter-wide zone of clay-rich foliated gouge marks the fault. The damage zone is approximately 100 meters wide, and the distribution of deformation is heterogeneous across the fault zone. Structural fabrics in the northeast fault block include breccias and both microscopic and outcrop-scale shear zones; these record the effects of cataclasis on porous sandstones and conglomerates. Deformation in the mudstones of the southwest fault block is accommodated by an incipient scaly foliation as well as by numerous fractures and faults. Microstructural analyses indicate that the San Gregorio accommodates dextral strike-slip offset as well as a component of west-side up reverse motion. Evidence for the role of fluids in this fault zone includes field relations and geochemical data. Anomolous hydrocarbon content within the foliated fault gouge indicate that the fault is a migration conduit. Fluctuations in fluid pressure within this fault zone may help elucidate the mechanics and seismogenic potential of the San Gregorio Fault.