HR: 15:25h
AN: T12E-07    [PDF]
TI: Absence of Late Neogene Offset on the Northern Calaveras Fault
AU: * Walker, J P
EM: jpwalker@usgs.gov
AF: United States Geological Survey, 345 Middlefield Rd. MS 975, Menlo Park, CA 94502 United States
AU: Graymer, R W
EM: rgraymer@usgs.gov
AF: United States Geological Survey, 345 Middlefield Rd. MS 975, Menlo Park, CA 94502 United States
AB: The northern segment of the Calaveras Fault (NCF) runs from Calaveras Reservoir north along the western edge of the Livermore and San Ramon Valleys. Previous mapping extends the Calaveras Fault as far north as the town of Danville (Graymer et. al., 1996), and earlier works extended it even farther north. A high-resolution aeromagnetic survey collected by the U.S. Geological Survey shows a linear aeromagnetic high crossing the mapped trace of the Calaveras Fault on the western edge of the San Ramon Valley, near Crow Canyon. This anomaly is associated with Miocene Neroly Sandstone that outcrops west of the fault in the Las Trampas Ridge Anticline, suggesting that the sandstone is continuos without appreciable offset across the mapped fault trace into the subsurface of San Ramon Valley. Therefore, Neogene offset on the NCF must terminate at or south of the latitude of Crow Canyon, and slip previously inferred for the NCF must be assigned to other structures in the region. The amount of slip previously assigned to the NCF varies between 28 km and 80 km. Recent palinspastic reconstructions (Graymer and others, 2002) indicate that most of the right-slip offset is taken up by the Palomares-Miller Creek-Moraga-Pinole fault system to the west. Others (Unruh and Lettis, 1998) suggest that slip is transferred from the NCF by means of multiple folds and small displacement faults west of the Calaveras Fault. The 2003 Working Group on California Earthquake Probabilities suggests that slip on the NCF is transferred across the Mt. Diablo antiform onto the Concord Fault. This transfer of slip would be a right step in a right lateral system, which should produce extensional features at this step. However, only compressional structures exist between the NCF and Concord Fault. The lack of extension at this right step, combined with little or no late Neogene offset on the NCF indicate that the proposed Concord-NCF right step is either a very young tectonic element or does not exist at all.
DE: 0933 Remote sensing
DE: 1744 Tectonophysics
DE: 8110 Continental tectonics--general (0905)
SC: Tectonophysics [T]
MN: 2003 Fall Meeting