Oak Ridge Fault, Ventura fold belt, and the Sisar decollement, Ventura Basin, California

The rootless Ventura Avenue, San Miguelito, and Rincon anticlines (Ventura fold belt) in Pliocene-Pleistocene turbidites are fault-propagation folds related to south-dipping reverse faults rising from a decollement in Miocene shale. A northeast-trending line connecting the west end of Oak Ridge and the east end of the Sisar Fault separates an eastern domain where late Quaternary displacement is taken up entirely on the Oak Ridge Fault and a western domain where displacement is transferred to the Sisar decollement and its overlying rootless folds. This implies that (1) the Oak Ridge Fault near the coast presents as much seismic risk as it does farther east, despite negligible near-surface late Quaternary movements; (2) ground-rupture hazard is high for the Sisar Fault set in the upper Ojai Valley; and (3) the decollement itself could produce an earthquake analogous to the 1987 Whittier Narrows event in Los Angeles.--Modified journal abstract.