Evidence for Pleistocene slip on a low-angle normal fault, Searles Valley, CA

Tye Numelin, The Pennsylvania State University, Department of Geoscience, University Park, PA, [email protected]

This project will measure Quaternary slip rates of a low-angle normal fault juxtaposing Quaternary alluvial gravels against subjacent Proterozoic gneisses along the base of the Slate Range in Searles Valley, CA.  Searles Valley is a small pull-apart basin bordered by the Garlock fault to the south, the Slate Range to the east, and the Argus Range to the west. The basin contains 1-2 km of mixed fluvial and lacustrine deposits. Shoreline deposits drape Pleistocene alluvial fans, and record the waxing and waning of Lake Searles.  Our mapping reveals a network of high angle normal fault scarps, that offset Pleistocene alluvial fan gravels, and a bedrock rooted low-angle normal fault at the range-front. Previously this feature had been mapped as a thrust fault. Field observations suggest that the high-angle alluvial normal faults sole into and record recent slip on this active low-angle normal fault.  Constraining the nature and timing of faulting in Searles Valley allows us to contribute to two vibrant debates: 1) the debate on whether or not slip occurs on normal faults dipping < 30°.  2) Measuring extension in Searles Valley will expand the catalogue of slip rate data for the Eastern California Shear Zone.  These data are critical to evaluating the spatial and temporal distribution of strain throughout the ECSZ. Within the last decade the proliferation of GPS has allowed scientists to make tremendous strides in quantifying intraplate strain in areas such as the ECSZ.  What remains unclear is what crustal and/or sub-crustal processes contribute to this deformation? In identifying and distinguishing the contributions of these processes to the cumulative strain it is critical to build a robust catalogue of field measurable geologic slip rates.  This project will employ high-precision topographic surveys and geochronologic techniques to accurately measure slip rates in an evolving regional strain field.