Neotectonics of the Sacramento-San Joaquin Delta area, east-central Coast Ranges, California

This dissertation documents a reactivated, ancient forearc basin located in the wide zone of transform-related shearing in the eastern California Coast Ranges (;Orientations of neotectonic structures indicate that simple wrenching is deforming the crustal blocks. These findings are contrary to the style of deformation known to the South, i.e., blind thrusting and tectonic wedging. Evidence for wrenching 40 km east of the supposed plate boundary at the Hayward fault suggests that the plate boundary is better characterized as a northward-widening shear zone than as a single lithospheric fault connected to crustal faults through a decollement. The mechanism for wrenching and the cause for anomalous deep seismicity in the study area is traction applied to the bases of crustal blocks by a lower crust that distributes the relative motion between the Pacific and North American plates in a horizontal shear zone. Major through-going strike-slip faults may mark areas of concentrated shear in the lower crust.;My conclusions resulted from integration of new and existing surface geology, subsurface well-log data from commercial exploration, seismic reflection profiles (including BASIX), seismic refraction, and seismicity. A technique developed in this study involves tracking Plio-Pleistocene unconformities from the surface into the subsurface to quantify neotectonic offset, folding and tilting. Two historical earthquakes in 1892 (M6 & 6.5) and another in 1889 (M6.3) identify the Sacramento-San Joaquin Delta as a region of potential seismic risk, though geologic uplift rates of 0.1 to 0.5 mm/y suggest such events are rare.