CLARK, JOSEPH C.
IUP, Indiana, PA

Abstract: San Gregorio Fault Zone: Progress and Problems

The San Gregorio fault is the principal fault west of the San Andreas fault in central California that has accommodated part of the dextral slip between the Pacific and North American plates. Since the Geological Society of America symposium in 1977, much onshore and offshore research has focused on the continuity, cumulative offset, slip rates, and activity of this zone.

My onland studies indicate cumulative right slip of 150-160 km since late Miocene time, with initial late Miocene slip rates of 25-30 mm/yr decreasing to 6 mm/yr during post-late Pliocene time. Recent studies by other workers at Año Nuevo and Seal Cove suggest Quaternary slip of 5-11 mm/yr.

A major outstanding problem remains the postulated continuity between the San Gregorio and Hosgri faults and the apparent discrepancy between the greater inferred cumulative offset on the San Gregorio compared to 110 ± 5 km of postulated offset on the Hosgri fault. Ongoing studies by Clark and Rosenberg suggest that some shear is distributed from the San Gregorio zone to en echelon intra-Salinian fault segments including the active Monterey Bay/Navy/Tularcitos and Garrapata Creek faults and the Quaternary-active Bixby Creek fault.

A present need is to relate the distribution, sense of displacement, and level of activity of offshore faults to the recent onshore work at Seal Cove, Año Nuevo, and Big Sur.

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