DEFORMATION OF MARINE TERRACES ALONG THE WESTERN EDGE OF THE NORTH AMERICAN PLATE, BODEGA BAY TO THE RUSSIAN RIVER, CA

Dextral slip of the San Andreas Fault (SAF) along the North American-Pacific plate boundary has resulted in long-term tectonic deformation of the northern California coast. Quaternary sea level fluctuations and motion along the SAF have produced a flight of uplifted and well-preserved marine terraces. The magnitude and rate of uplift calculated from these marine terraces varies with distance from the SAF. 105 km north of San Francisco at Bodega Bay, CA the SAF comes onshore at Bodega Head for 4 km and then extends obliquely offshore towards the northwest. Approximately 18 km northwest of Bodega Bay at the Russian River, the SAF lies 5 km offshore. Using a real-time, differential correcting, Global Positioning System (GPS) receiver, marine terrace platforms and inner edges were surveyed along eleven transects perpendicular to the coast. Along coast terrace correlation was determined from field mapping and aerial photograph analysis. Nine laterally, discontinuous marine terraces occur along this 18 km section of coast and range in elevation from 24 to 268 meters amsl.

Regional uplift rates are calculated by correlating surveyed terrace inner-edge elevations to Quaternary sea level highstands. Inner-edge elevations are assigned to sea level highstands on a composite sea level and Quaternary oxygen isotope curve. A technique used by Merritts and Bull, 1989 that assumes constant uplift rates at a transect was adopted for sea level correlation. Ages for terraces used in uplift rate calculations range from OIS 5c (108 ka) to a possible maximum of OIS 19 (670 ka). Uplift rates decrease by a maximum factor of 3 at 130 ka near the Russian River, but become nearly constant through time at Bodega Bay. Uplift rates gradually decrease towards the south as the SAF approaches the modern shoreline. Pre-130 ka calculated uplift rates near the Russian River are approximately 0.6 m/ka while 18 km to the southeast at Bodega Bay uplift rates are 0.1 m/ka. A crustal deformation model (POLY3D-GUI) developed by Pollard et al. at Stanford University is being used to conduct a basic structural and tectonic analysis of the SAF. Preliminary results indicate that uplift rates vary with the position of the SAF relative to the coast.