MARINE TERRACE AND OFFSHORE SEISMIC EVIDENCE FOR SPATIALLY AND TEMPORALLY VARYING UPLIFT RATES ADJACENT TO THE SAN ANDREAS FAULT NORTH OF SAN FRANCISCO, CALIFORNIA

The San Andreas fault (SAF) system near San Francisco comprises a complex array of sub-parallel faults ~70 km wide. The SAF strand active in the 1906 earthquake is a young fault that probably became active since 2 Ma. West of San Francisco the SAF lies offshore and is part of a ~15-km-wide distributed zone of faults that accommodate transform plate motion and have vertical components producing localized areas of subsidence and uplift. To the north, an on-land segment, where the largest amount of horizontal offset was measured after the 1906 earthquake, occupies a narrow zone that separates the Point Reyes Peninsula from mainland California. Marine terraces beveling the flanks of Inverness Ridge, west of the SAF zone on the peninsula, were studied to quantify vertical deformation; results were compared to structures imaged on offshore seismic reflection data.

Point Reyes Peninsula is a syncline with plutonic basement rocks on the east and west limbs (Inverness Ridge and Point Reyes headland), and an overlying sequence of Tertiary marine sedimentary rocks. Marine terraces formed along this emergent coastline during Quaternary sea-level high stands. Measurements on the lowest terrace, formed during the stage 5a high stand ca. 80 ka, suggest uplift rates ranging from ~0.2 mm/yr to ~1 mm/yr (south end of Inverness Ridge near Bolinas). Data from higher (i.e., older) terraces suggest an accelerating rate of uplift near Bolinas since ~300 ka. Strands of the San Gregorio fault zone (SGFZ) are imaged on seismic profiles in the offshore south of the peninsula; they show reverse offset and appear to be active. In 1999, an earthquake near Bolinas resulted from reverse motion on a fault that was probably part of the SGFZ. The Point Reyes fault is another fault with reserve offset that bends around the Point Reyes headland and may connect to faults of the SGFZ. It has uplifted the headland where one terrace suggests recent emergence from the sea. Zones of normal faulting and subsidence are found several km from areas of thrust faulting and uplift, reflecting complex fault geometry and heterogeneous basement rock types. Zones change rapidly from uplift to subsidence (or vice versa), as fault strands and geometries continually evolve.