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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 related to south-dipping reverse faults rising from a decollement in underlying Miocene shale. To the east, Sulphur Mountain anticlinorium comprises the upper plate of a south-dipping set of thrusts (Sisar, Big Canyon, Lion) that merge downward into a decollement in mudstone of the Rincon Formation, named here the Sisar decollement. Shortening of the Miocene and younger sequence is ~ 3 km greater than that of the underlying competent Vaqueros, Sespe, and marine Eocene sequence in the Ventura fold belt and ~ 7 km greater farther east at Sulphur Mountain. Cross section balancing requires that this difference be taken up by the Paleogene sequence at the Oak Ridge fault to the south. Convergence is NE to NNE based on earthquake focal mechanisms and piercing-point offset of the South Mountain seaknoll by the Oak Ridge fault. A NE-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 shallow post-0.2 Ma movement, (2) ground-rupture hazard is high for the Sisar fault set in the upper Ojai Valley, and (3) the decollement could fail on a low-angle thrust at depth and produce an earthquake analogous to the 1987 Whittier Narrows event in Los Angeles.
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