SINGLE AND MULTIGRAIN LUMINESCENCE DATING OF SEDIMENTS RELATED TO THE GREENVILLE FAULT, EASTERN SAN FRANCISCO BAY AREA, CALIFORNIA

Near urban areas and extending ca. 60 km along the eastern margin the Livermore Valley, the Greenville fault is the easternmost right-lateral strike-slip fault of the San Andreas system in the greater San Francisco Bay area. The 1980 Livermore earthquake sequence (main shock M_L 5.9) on the Greenville fault demonstrated the potential of this fault to generate significant earthquakes. However, there is no record of recency or Holocene rates of activity on the Greenville fault. This fault exhibits clear geomorphic evidence of late Quaternary faulting, yet the timing of prehistoric earthquakes (and thus inferred slip rates) on the fault and the relation of the fault to other geologic structures are effectively unknown. In trenches parallel and normal to the fault through alluvial fan deposits at the Laughlin Road site almost no charcoal or other suitable carbon was found for AMS ¹⁴C dating. AMS ¹⁴C dating of pedogenic carbonate was conducted, providing, at best, minimum age estimates for some sedimentary units. Therefore, we applied photon-stimulated-luminescence (PSL) sediment-dating procedures to both polymineral fine silt and quartz sand fractions of 6 samples. In addition to regular multigrain single-aliquot-regenerative-dose (SAR) analysis, we applied pulsed-diode multigrain SAR to 5 samples and single-grain-quartz (SGQ) PSL dating to 3 samples. The polymineral fine-silt multi-aliquot age estimates are generally inaccurate, but the SAR ages from quartz sand grains are in stratigraphic sequence. These SAR ages range from 93.6±1.2 yrs (before 2007) within the topmost unit in trench 3A to 13.45±0.79 ka in the base of the lowermost channel-fill unit in this trench. The results have important implications for both the utility of the various PSL methods for dating paleoseismological sediments and for the deduction of accurate slip rates along the Greenville fault.