LATE PLEISTOCENE EXTENSION ALONG THE EASTERN SIERRA NEVADA, CALIFORNIA

Quaternary slip rates along the Sierra Nevada frontal fault (SNFF) system in California are not well known. This study utilizes geologic field mapping, tectonic geomorphology and beryllium-10 cosmogenic radionuclide (CRN) geochronology to calculate vertical slip rates along a section of the SNFF system from Onion Valley to just south of the Whitney Portal. Numerous NNW-striking, east-facing fault scarps cut distinct Quaternary surfaces. The oldest alluvial fan surface, Qf1, is generally smooth and overlain with scarce, highly weathered granitic boulders that are well embedded into the alluvial surface. Qf2 surfaces are characterized by unconsolidated sands with minor surface dissection and moderately weathered granite boulders. Qf3 surfaces are distinguish by large abundant fresh granite boulders, hummocks, and bar and swale morphology. Qf4 surfaces define of active or recently abandoned channels. Beryllium-10 CRN surface exposure dating of 29 granite boulders from these surfaces provide the age of deposition and abandonment: 105 ± 15 ka for Qf1, 63 ± 18 ka for Qf2, 23 ± 6 ka for Qf3, and 3.7 ± 0.3 ka for Qf4. These ages are consistent within error of published ages for surfaces elsewhere in the region (Bierman, et al. 1995). Topographic profiles measured across normal fault scarps that cut and offset these surfaces yielded vertical surface offset of a minimum of 80 ± 5.0 m for Qf1 surfaces, 30 ± 1.0 m for Qf2 surfaces, and 14 ± 1.0 m for Qf3 surfaces. Qf4 surfaces did not show evidence of offset. These data suggest late Pleistocene vertical slip rates of 0.8 mm/yr since the abandonment of Qf1 and ~0.5-0.6 mm/yr since abandonment of Qf2 and Qf3 surfaces. If we assume a fault dip of 60°, then the late Pleistocene horizontal extension rate across this part of the SNFF system is 0.3- 0.5 mm/yr. Our slip rate estimates are similar to slip rate estimates elsewhere along the SNFF system.