YOUNG STRIKE-SLIP BASIN ON THE CALAVERAS FAULT IN SAN FELIPE VALLEY, CA

San Felipe Valley (SFV) southeast of San Jose, is a small, isolated, alluvial valley at the center of a 5-km releasing bend in the Calaveras fault, San Andreas fault system. The main fault trace trending northwesterly, enters the valley at its southwest corner, then changes to a more northerly trend across the valley, and exits at its northeast corner back to a northwesterly trend, thus defining the releasing bend. We examine the possibility that the valley contains a strike-slip basin by estimating its alluvial thickness using gravity data from three densely sampled profiles and scattered regional stations, as well as measured bedrock densities for gravity modeling. To calculate alluvial thickness, we use a 3D iterative basin modeling procedure that simultaneously accounts for both the localized gravity low over the alluvium and the large cross-fault gravity gradient that regionally characterizes the Calaveras fault in the Diablo Range. Using residual gravity that emphasizes shallow crustal sources local to SFV and an alluvium-bedrock density contrast of 0.46 g/cm3, the modeled gravity is then inverted to produce alluvial thickness. The results define a 1.3x0.7 km north-trending rectangular basin occupying the western half of SFV with an alluvial thickness of up to 40 meters. The alluvium thins abruptly eastward across the mapped Calaveras trace to less than 6-7 m, consistent with an 8.8-m depth to rock in a water well. Thus, the rectangular basin is explained as a strike-slip basin formed between the mapped Calaveras trace and a near-parallel inferred fault along the west side of SFV. The toe of a large Quaternary landslide lies along the same west side and may be associated with basin formation. The strike-slip basin is surprisingly small and shallow given that 20 km to the southeast of SFV, the Calaveras fault accommodates >150 km of right slip. Thus, the implication is that the basin is young and produced by only modest offset, which contrasts with other San Andreas system faults that produce much longer (>20 km) and deeper (>1 km) strike-slip basins (San Jacinto Graben and Merced Basin) with only tens of kilometers of right slip.