CONSTRAINTS ON THE MAGNITUDE OF QUATERNARY OFFSET ALONG THE DEXTRAL BENTON SPRINGS FAULT, CENTRAL WALKER LANE, NEVADA

The Walker Lane, an ~700 km long zone of dextral shear that extends southward from northeastern California, through western Nevada, and into eastern California, accommodates ~25% of the relative motion between the Pacific and North American plates. In the central Walker Lane, western Nevada, this dextral shear is distributed across five subparallel NW-striking faults. To characterize the spatial-temporal distribution of dextral shear across these faults, we combine geologic mapping, structural studies, geomorphology, and geochronology to document the Miocene to Quaternary fault slip histories. Herein, we report the results of geologic mapping and geomorphic studies of dextrally offset geomorphic markers developed within late Pleistocene alluvial fan deposits along the central part of the Benton Springs fault. The ~80 km long Benton Springs fault is dominated by dextral slip along the northern two-thirds of the fault, where it generally strikes 325°, with a southward increasing component of normal slip along the southern one-third where the strike transitions towards 355°. Field measurements and terrestrial laser scan-derived DEMs were used to precisely delineate and measure the magnitude of dextrally offset thalwegs, fan edges, channel margins, and ridges and to generate slope and aspect maps which aided in reconstructions. Point counts of clast lithologies on alluvial surfaces correlated across the fault support offset measurements of geomorphic features. Along the central part of the Benton Springs fault, our results indicate that geomorphic markers record dextral offset magnitudes of ~45, 30, 15, 6, and 1.5 m at two localities 7 km apart. Terrestrial cosmogenic nuclide and U-series geochronology are in process on samples and clast carbonate rinds, respectively, collected from pits dug into offset markers. These data combined with these new offset measurements will yield the first set of multiple dextral fault slip rates over 10³ to 10⁴ years along the Benton Springs fault.