Quaternary faulting in Clayton Valley, Nevada: implications for distributed deformation in the Eastern California shear zone-walker lane
Foy, Travis A.
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The eastern California shear zone (ECSZ) and Walker Lane belt represent an important inland component of the Pacific-North America plate boundary. Current geodetic data indicate accumulation of transtensional shear at a rate of ~9.2 ± 0.3 mm/yr across the region, more than double the total geologic rate (<3.5 mm/yr) for faults in the northern ECSZ over the late Pleistocene [Bennett et al., 2003, Kirby et al., 2006, Lee et al., 2009, Frankel et al., 2007]. Unraveling the strain puzzle of the Walker Lane is therefore essential to understanding both how deformation is distributed through the lithosphere along this transtensional part of the Pacific-North America plate boundary and how the plate boundary is evolving through time. The observed mismatch between geodetic and geologic slip rates in the central Walker Lane is characteristic of other active tectonic settings, including the nearby Mojave segment of the ECSZ [Oskin et al., 2008] and the Altyn Tagh fault in China [Cowgill, 2007]. In each case, lack of fault slip data spanning multiple temporal and spatial scales hinders interpretation of fault interactions and their implications for lithospheric dynamics. The discrepancy between geodetic and geologic slip rates in the central Walker Lane indicates that if strain rates have remained constant since the late Pleistocene [e.g. Frankel et al., in press], then the "missing" strain is distributed on structures other than the two major dextral faults at this latitude (Death Valley-Fish Lake Valley fault and White Mountains fault). Otherwise the region could presently be experiencing a strain transient similar to that of the nearby Mojave section of the ECSZ [e.g., Oskin et al., 2008], or the rate of strain accumulation could actually increasing over the late Pleistocene [e.g. Reheis and Sawyer, 1997; Hoeft and Frankel, 2010]. The Silver Peak-Lone Mountain extensional complex (SPLM), to which the Clayton Valley faults belong, is the prime candidate to account for the "missing" strain. The down-to-the-northwest orientation of the SPLM faults makes them the most kinematically suitable structures to accommodate the regional pattern of NW-SE dextral shear. We use differential GPS to measure fault offset and terrestrial cosmogenic nuclide (TCN) geochronology to date offset landforms. Using these tools, we measure extension rates that are time-invariant, ranging from 0.1 ± 0.1 to 0.3 ± 0.1 mm/yr for fault dips of 30° and 60°. These rates are not high enough to account for the discrepancy between geologic and geodetic data in the ECSZ-Walker Lane transition zone. Based on geologic mapping and previously published geophysical data [Davis, 1981; Zampirro, 2005], deformation through Clayton Valley appears to be very widely-distributed. The diffuse nature of deformation leads to geologic slip rates that are underestimated due to the effects of off-fault deformation and unrecognized fault strands. Our results from Clayton Valley suggest that the discrepancy between geodetic and geologic strain rates at the latitude of the northern ECSZ is a result of long-term geologic rates that are underestimated. If the true geologic rates could be calculated, they would likely be significantly higher and therefore in closer agreement with geodetic data, as is the case everywhere else in the ECSZ north of the Garlock fault [Frankel et al., 2007a, in press; Kirby et al., 2008; Lee et al., 2009a].