PLEISTOCENE SLIP RATE ON THE WHITE MOUNTAIN FAULT ZONE

Normal faulting within transtensional settings is often a consequence of linkage between arrays of subparallel strike-slip faults. Understanding the spatio-temporal evolution of such fault arrays requires assessing the degree to which linking faults transfer slip between the strike-slip zones. Previous geodetic and geologic studies in the Eastern California shear zone suggest that a northward decrease in the rate of dextral slip on the Owens Valley-White Mountain fault system is compensated by an increased rate of slip on the Fish Lake Valley fault system, with displacement transferred by normal faults in Deep Springs Valley. A key unknown in this model is the long-term rate of slip on the White Mountain fault zone (WMFZ) – prior estimates based on Holocene scarps had little radiometric age control. We assess the Pleistocene slip rate of the WMFZ using remnant alluvial deposits that contain the Bishop ash (ca. 760 k.a.). A minimum estimate of lateral offset along the WMFZ is obtained from a shutter ridge south of Poleta Canyon. The shutter ridge is composed of alluvial fan material interbedded with a ~10m thick airfall deposit of the Bishop tuff. Poleta Canyon is the southernmost drainage basin along the White Mountains which contains granitic rocks and acts as a point source of granitic clasts supplied to the alluvial fans. Modern and ancient fanglomerates contain ~30-40% granitic material. In contrast, the shutter ridge contains < 1% granitic clasts and must have been derived from source regions farther south. The minimum lateral offset is ~450 – 550m, yielding an lateral slip rate of ~0.6 - 0.8 mm/yr. Farther north, debris-flow fanglomerates interbedded with Bishop ash are preserved in the footwall of the WMFZ and are also observed at depth beneath the valley floor in well data. Estimates of the vertical component of displacement depend on the dip of the paleo-transport surface and range from ~200 – 300m, yielding displacement rates of ~0.3 – 0.4 mm/yr. We estimate a minimum oblique slip rate of ~0.7-0.8 mm/yr toward 320° since ca. 760 k.a. This estimate is higher than previously inferred, and it appears that a significant component of right-lateral slip on the Owens Valley fault zone continues northward along the WMFZ. However, our data still allow for ~1mm/yr of slip transfer across Deep Springs Valley to the Fish Lake Valley fault.