HR: 1340h
AN: T33C-1397    [Abstracts]
TI: Late Pleistocene to Holocene extension along the southern Sierra Nevada frontal fault zone, California
AU: * Le, K
EM: kimberlyle@fullerton.edu
AF: Central Washington University, 400 E. University Way, Ellensburg, Wa 98926
AU: Lee, J
EM: jeff@geology.cwu.edu
AF: Central Washington University, 400 E. University Way, Ellensburg, Wa 98926
AU: Owen, L
EM: lewis.owen@uc.edu
AF: University of Cincinnati, P.O. Box 0013, Cincinnati, OH 45221
AU: Finkel, R
EM: finkel1@llnl.gov
AF: Lawence Livermore National Laboratory, 7000 East Ave, Livermore, Ca 94550
AB: Quaternary vertical slip rate estimates along the southern Sierra Nevada frontal fault zone in California are not well constrained due to the absence of absolute ages on offset Quaternary geologic markers. This study presents new geologic and geochronologic evidence for late Pleistocene to Holocene extension across the southern Sierra Nevada frontal fault zone from Oak Creek to just south of Lubken Creek by incorporating geologic field mapping, tectonic geomorphology and beryllium-10 cosmogenic radionuclide geochronology. The southern escarpment of the Sierra Nevada exposes numerous NNW-striking, east-facing, en echelon normal fault scarps that offset three distinct Quaternary surfaces. The oldest alluvial fan surface, Qf1, is generally smooth and overlain with scarce, typically strongly weathered granitic boulders that are well-embedded into the alluvial surface. Qf2 surfaces are incised into Qf1 surfaces and characterized by ridge and ravine topographic patterns, and moderately weathered granite boulders. Qf2 surfaces are, in turn, incised by Qf3 surfaces, which are distinguished by a bar and swale surface morphology and large abundant unweathered granite boulders. The youngest fan surfaces are Qf4, which are defined by active or recently abandoned channels. Beryllium-10 cosmogenic radionuclide surface exposure dating of twenty-nine granitic boulders from these surfaces provide the age of deposition and abandonment: 140 ñ 34 ka for Qf1, 64 ñ 16 ka for Qf2, 23 ñ 7 ka for Qf3a, 5 ñ 1, and 4 ñ 1 ka for Qf4. These ages are consistent, within error, of published ages for surfaces elsewhere in the region. Topographic profiles measured across normal fault scarps that cut and offset these surfaces yielded minimum vertical surface offsets of 40.8 ñ 8.2 m for Qf1 surfaces, 21.9 ñ 4.4 m for Qf2 surfaces, 10.2 ñ 2.0 m for Qf3a, and 6.4 ñ 1.3 m for Qf3c surfaces. Qf4 surfaces did not show evidence of offset. These data suggest late Pleistocene vertical slip rates of 0.3 to 0.4 ñ 0.1 mm/yr since ~140 ka to ~23 ka and 1.4 mm/yr since ~5 ka. If we assume a fault dip of 60ø, then the late Pleistocene and Holocene horizontal extension rate across this part of the Sierra Nevada frontal fault zone is 0.2 ñ 0.1 and 0.8 ñ 0.2 mm/yr. Our slip rate estimates are the same as or somewhat slower than slip rate estimates elsewhere along the Sierra Nevada frontal fault zone and are comparable to late Pleistocene slip rate estimates across the Basin and Range Province.
DE: 8107 Continental neotectonics
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting