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

Abstract:

To better 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 lidar 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. Initial reconstruction, based on clast-count data on lithology and size from Todd Creek (n=68), Service Road Creek (N=32) and the offset deposits (n=68), suggest ≥140 m of dextral fault movement. Initial ¹⁰Be cosmogenic dating of sandstone boulders on an offset deposit from Service Road Creek yields a maximum date of 8 ka, a date uncorrected for hillslope residence and fluvial transport of inherited ¹⁰Be concentrations. These data suggest a minimum slip rate of at least 17 mm/yr on the Santa Cruz Mountain section of the San Andreas Fault in the peninsula. Ongoing analysis will refine this fault slip rate. Our preliminary data underscore the potential of this site to provide geologic slip rate estimates, and therefore answer a question critical to seismic hazard assessment, in a region where steep terrain, mass wasting, vegetation and urban development have generally made slip rate estimates challenging to obtain.