HR: 0800h
AN: S31A-1027    [Abstracts]
TI: Preliminary Late Pleistocene Slip Rate of the Green Valley Fault at Lopes Ranch Creek, Cordelia, California
AU: * Baldwin, J N
EM: baldwin@lettis.com
AF: William Lettis & Associates, Inc., 1777 Botehlo Drive, Suite 262, Walnut Creek, CA 94596 United States
AU: Koehler, R D
EM: koehleri@unr.nevada.edu
AF: Center for Neotectonic Studies, MS 169, University of Nevada, Reno, NV 89557 United States
AU: Thompson, S C
EM: thompson@lettis.com
AF: William Lettis & Associates, Inc., 1777 Botehlo Drive, Suite 262, Walnut Creek, CA 94596 United States
AB: The Concord-Green Valley fault system is part of the eastern San Andreas fault system, and is composed of two major fault segments from south to north: Concord fault and Green Valley fault (GVF). The GVF is subdivided into a southern segment that extends from the northern shores of Suisun Bay to near Cordelia, and a northern segment that continues north to Wooden Valley, east of Napa, CA. At the Lopes Ranch Creek site, along the southern GVF, an ephemeral creek preserves the cumulative dextral separation of an abandoned north-trending paleochannel located east of the main fault. Trenches excavated at the site expose a sequence of latest Pleistocene to historic alluvial and fluvial deposits overlying weathered bedrock of the Upper Cretaceous to Upper Jurassic Great Valley Sequence. The main active trace of the southern GVF is well constrained based on tectonic geomorphology (e.g., NE-facing scarps, vegetation lineaments, and springs), and trenches that exhibit a 3-meter-wide fault zone containing vertical faults, west-dipping dextral reverse faults and creep-related fractures. Preliminary estimates of cumulative right-lateral displacement of a distinct paleochannel deposit range from 31 to 58 meters. Radiocarbon analyses of charcoal collected from a burn horizon directly above the paleochannel provide a minimum age of 14,080 to 15,380 cal yr B.P. Based on the estimated cumulative displacement and minimum age of the paleochannel deposit, a preliminary long-term slip rate for the southern GVF is 2 to 4 mm/yr. This preliminary slip rate is close to the historical slip and creep rates for the GVF. For instance, a previous slip rate study of an offset late Holocene paleochannel deposit about 0.4 km northwest of the Lopes Ranch Creek site determined a slip rate of 3.8 to 4.8 mm/yr (over the last 300 years), consistent, in part, with the latest Pleistocene slip rate yielded by this study. These geologic slip rates also are close to the 14.7-year average creep rate of 4.4 $\pm$ 0.1 mm/yr for the southern GVF, and the geologic slip rate of the Concord fault (3.4 $\pm$ 0.3 mm/yr). Because of the: (1) limited exposures of the paleochannel deposit directly west of the fault zone, (2) limited radiocarbon ages, and (3) preliminary nature of the investigation at Lopes Ranch Creek, additional studies are necessary to refine the long-term slip rate for the GVF.
DE: 7221 Paleoseismology
DE: 7230 Seismicity and seismotectonics
DE: 7299 General or miscellaneous
SC: Seismology [S]
MN: 2004 AGU Fall Meeting