Coastal Marine Terraces of Northern California: A Datum for Estimating Pleistocene Slip Rate across the Northern San Andreas Fault

A well-developed flight of marine terraces intersects the San Andreas Fault (SAF) near Fort Ross in Sonoma County and again farther northwest near Point Arena in Mendocino County, California. We have analyzed large-scale stereo aerial photographs as well as 10-meter (horizontal resolution) and 1.8-meter (horizontal resolution) LiDAR derived DEMs to map the marine terrace sequence and active traces of the SAF from Fort Ross to approximately 40 km north of Point Arena. Near Fort Ross a series of right-laterally offset streams is incised into the marine terrace sequence; reconstructions of offset streams and age estimates of the marine terraces based on correlation to global sea-level high-stands yields a slip rate estimate of 17-20 mm/yr, similar to the Holocene slip rate determined from the offset of nearby Mill Gulch. Near Point Arena, approximately 70 km to the northwest of Fort Ross, the SAF again intersects the flight of coastal marine terraces. At this location, we correlate the paleo-shorelines of two marine terraces across the SAF. The paleo-shoreline associated with the marine terrace that we correlate with stage 5a is offset approximately 1.5km. The paleo-shoreline associated with the marine terrace we correlate with stage 5e is offset approximately 2.5km. Coupled with age estimates of the marine terraces, these offsets provide a best estimate of the slip rate of 19-20 mm/yr. This slip rate is comparable to Holocene slip rates determined nearby, and are similar to, but somewhat lower than SAF slip rates estimated in Marin County, 60 km to the southeast. Our mapping and age correlations also provide estimates of regional coastal uplift rates, as well as a basis for determining regional, long-term, off-fault deformation patterns. ->