HR: 10:40h
AN: T32C-02 INVITED     [Abstracts]
TI: Characterization of Late Quaternary Deformation of the Mt. Diablo Fold-And-Thrust Belt, Eastern San Francisco Bay Area, California
AU: * Sawyer, T L
EM: tom@piedmontgeosciences.com
AF: Piedmont GeoSciences, Inc., 10235 Blackhawk Dr., Reno, NV 89506 United States
AU: Unruh, J R
EM: unruh@lettis.com
AF: William Lettis & Associates, Inc., 1777 Botelho Ave., Suite 262, Walnut Creek, CA 94596 United States
AB: Geomorphic and structural geologic studies provide evidence for the loci, style, and rates of late Quaternary uplift, and possibly insights into the paleoseismic behavior of the Mt. Diablo fold-and-thrust belt (MDFTB). The MDFTB is comprised of map-scale fold-thrust fault structures oriented obliquely to and lying largely within a restraining left-stepover between the dextral Greenville and Concord faults of the eastern San Andreas fault system. Reverse slip on the blind, southwest-verging Mt. Diablo thrust fault, the largest structure in the belt, has uplifted and folded the Great Valley Sequence about the Franciscan-cored Mt. Diablo anticline, which culminates in the 1,173 m peak of Mt. Diablo. Detailed longitudinal profiles of well-preserved stream terraces that cross the structure at a high angle place the loci of active surface deformation above the tip of the thrust fault, which underlies the Tassajara anticline fault-propagation fold. Profiles along Alamo and Tassajara creeks, separated by 4 km strike distance, similarly record ~4 m of localized uplift of terraces radiocarbon dated between 3 and 1 ka. These data suggest tectonic quiescence between 3 and 1 ka followed by considerable tectonic uplift. Estimated source dimensions of the Mt. Diablo thrust fault and historical earthquake data suggest uplift resulted from multiple earthquakes on the fault in the past 1 ka. Fold-thrust structures within the deep Neogene sedimentary basin beneath Livermore Valley south of Mt. Diablo have a more westerly vergence direction and exhibit lesser structural and geomorphic relief. Based on construction of cross sections through the basin, we interpret a NE-dipping imbricate system of thrust faults, the "Vallecitos thrust fault system", that intersects the Greenville fault at seismogenic depths to the NE, and impinges on the Calaveras fault at shallow crustal levels to the SW. Detailed surficial geologic mapping and geomorphic profiles reveal `hanging' geomorphic surfaces, wind gaps, and abandoned stream channels, all of which are consistent with late Quaternary activity of the fold-thrust structures. The distribution and elevation of these features constrain late Quaternary geomorphic growth of the "Springtown anticlines", "Livermore anticline", Verona homocline, and "Vallecitos hills anticline". Subsurface data show late Quaternary growth of a southeast-plunging syncline flanked on the east by the Livermore anticline and on the west by the Verona homocline, the depocenter for reportedly the tenth largest gravel resource in the U.S.
DE: 7223 Seismic hazard assessment and prediction
DE: 7230 Seismicity and seismotectonics
DE: 8005 Folds and folding
DE: 8010 Fractures and faults
DE: 8015 Local crustal structure
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting