NORTHERN SAN ANDREAS FAULT SLIP RATES ON THE SANTA CRUZ MOUNTAIN SECTION: 10BE DATING OF AN OFFSET ALLUVIAL FAN COMPLEX, SANBORN COUNTY PARK, SARATOGA, CA

To assess seismic hazard and fault behavior along the southern peninsula in the San Francisco Bay Area on the Santa Cruz Mountain section of the San Andreas Fault, we combine field observations and high-resolution topography data with ¹⁰Be exposure dating on offset landforms to estimate geologic fault slip rates. Our mapping at Sanborn County Park near Saratoga reveals a progression of alluvial fans and debris flows offset from their upstream sources by dextral slip on the San Andreas Fault. These upstream sources are 3 drainages, Todd Creek, Service Road Creek and Aubry Creek. Coarse alluvial deposits from each of these creeks contain large Tertiary sandstone boulders of varying size and abundance, derived from the Vaqueros Formation, that allow us to constrain the provenance of offset alluvial deposits to their upstream sources. Final results of detailed geomorphic mapping and ¹⁰Be surface exposure geochronology (N=27) indicate the presence of four distinct geomorphic surfaces, ranging in age from Qf4 at 1-3 ka to Qf1 at 15-18 ka, with offsets of 60 m to 600 m, respectively, which we use to reconstruct four separate stages of offset at this site. If our reconstructions and ages are correct, these new data suggest an average slip rate of at least 24 mm/yr on the Santa Cruz Mountains section of the San Andreas Fault, which is higher than the previously estimated rate of 14-17 mm/yr. A higher slip rate in this section suggests the San Andreas Fault is accommodating the majority of plate boundary motion at this latitude, and therefore may present a higher seismic hazard than previously expected.