Geophysical evidence for quaternary deformation within the offshore San Andreas Fault System, Northern California

Abstract

Offshore multichannel and mini-sparker seismic reflection profiles collected north of San Francisco indicate varying tectono-stratigraphic conditions since the Late Miocene. Near the Point Reyes Peninsula headland, Miocene units have been vertically offset at least 2 km on the Point Reyes fault (PRF); deformation began during the Late Miocene, probably in response to a change in relative plate motions. North of the peninsula, multiple Quaternary sequences formed above a subsiding Plio-Pleistocene unconformity (PPU) during eustatic sea-level fluctuations. The mechanism for subsidence may be related, in part, to isostatic loading from thick Pleistocene deposits derived from the Russian River. West of the Gualala Block, preserved sequences have been folded, probably starting ~500 ka, by transpressional structures associated with the Gualala fault. South and adjacent to the headland, few Pleistocene sediments are preserved. The PPU and overlying Holocene deposits are undeformed, suggesting that the PRF has become increasingly inactive since the Middle Pleistocene and that its hazard potential in current fault and tsunami hazard models should be reduced.