LETTIS, W. R.
William Lettis & Associates, Inc., Walnut Creek, CA

Abstract: The San Gregorio/Hosgri Fault System, California - An Evaluation of the Style and Rate of Quaternary Deformation

The San Gregorio/Hosgri fault system extends for a distance of 410 km along the coast of central California coming onshore at three, and possibly four, locations: Seal Cove, Ai~o Nuevo, San Simeon, and possibly Point Sur. The fault system consists of at least three distinct faults: from north to south, the San Gregorio fault, the San Simeon fault, and the Hosgri fault. The San Gregorio and San Simeon faults clearly offset a sequence of marine terraces at Seal Cove, Ano Nuevo and Point San Simeon. The offset marine terraces and an offset archaeological site at Seal Cove demonstrate right-lateral strike-slip displacement at a preferred rate of 6 to 8 mm/yr along the San Gregorio fault and 1 to 3 mm/yr along the San Simeon fault. Paleoseismic trenches also show that the faults have had multiple surface faulting events during the Holocene. To the south, the San Simeon fault forms a 3- to 5-km wide right en echelon stepover with the northern Hosgri fault zone. Geophysical and stratigraphic data define a distinct subsiding basin between the two faults in the stepover region. We interpret the basin to be a pull-apart structure accommodation transfer of right slip between the San Simeon fault and northern Hosgri fault. The decreasing slip rate, from 6 to 8 mm/yr on the San Gregorio fault to 1 to 3 mm/yr on the San Simeon and Hosgri fault, shows that slip rate decays southward along the fault system. We attribute this decay to north-northeast-directed crustal shortening along the eastern margin of the fault system in the southern Coast Ranges and Santa Maria valley area. The decay in slip and crustal shortening appear to be the result of progressive transfer of slip across a left-restraining stepover between the northern San Gregorio/Hosgri fault system and the southern Rinconada fault or perhaps the Carrizo Plains segment of the San Andreas fault. In addition, we recognize a large restraining bend along the San Gregorio/Hosgri fault system centered on the San Simeon fault near Piedras Blancas. The restraining bend has produced localized crustal shortening along the fault zone associated with the offshore Piedras Blancas antiform and uplift of the northern Santa Lucia Range.

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