Abstract

The rootless Ventura Avenue, San Miguelito, and Rincon anti-clines (Ventura fold belt) in Pliocene-Pleistocene turbidites are fault-propagation folds related to south-dipping reverse faults rising from a decollement in Miocene shale. To the east, the Sulphur Mountain anti-clinorium overlies and is cut by the Sisar, Big Canyon, and Lion south-dipping thrusts that merge downward into the Sisar decollement in lower Miocene shale. Shortening of the Miocene and younger sequence is ∼3 km greater than that of underlying competent Paleogene strata 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 northeast to north-northeast on the basis of earthquake focal mechanisms, borehole breakouts, and piercing-point offset of the South Mountain seaknoll by the Oak Ridge fault. 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 movement; (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.

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