GEOPHYSICAL AND SEDIMENTOLOGIC EVIDENCE FOR PLIO-PLEISTOCENE DEFORMATION IN THE OFFSHORE SAN ANDREAS FAULT ZONE BETWEEN GUALALA AND SAN FRANCISCO, NORTHERN CALIFORNIA

Sedimentary sequences in the offshore region of northern California between Gualala and San Francisco record the transition from a convergent to a transform plate boundary during the Miocene Epoch. Since the transition, the geologic record shows varying tectono-stratigraphic conditions that included episodes of contraction and subsidence. We analyzed two sets of seismic reflection data that have recently become available—multichannel (deep-penetration) seismic reflection data, with associated offshore exploratory well logs, that were collected for petroleum companies; mini-sparker (high-resolution) seismic reflection data collected within the 3-miles limit by the U.S. Geological Survey.

North of the Golden Gate entrance to San Francisco Bay, converging strands of the San Andreas fault (SAF) system have produced a Holocene graben east of the SAF, and a contractional region west of the SAF. Marine terraces on the south end of the Point Reyes Peninsula have been uplifted ~1 mm/yr; this rate accelerated as the contractional zone migrated north and began to impinge on the peninsula. Farther north, near the Point Reyes 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. Our analyses suggest that the PRF was most active at this location 5–0.5 Ma.

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.