ABSTRACT Recently-drilled wells in the Chaffee’ Canyon oil field, Ventura County, California, reveal that the Wiley Canyon producing anticline formed in the Pleistocene prior to much of the displacement on the Oak Ridge fault. The east and west plunge in part predates deposition of the Vaqueros Formation of early Miocene age. The anticline changes trend from S70°W in the west to S80°E in the east. The dip on the south strand of the Oak Ridge fault increases eastward across the field from 70-75° to 83-85°; farther east, the fault plane is overturned and dips north. The Torrey fault can be traced northwest under a landslide east of Wiley Canyon anticline, but not farther northwest. Production in the Chaffee Canyon field is primarily from marine sands more than 145 m below the top of the Eocene Llajas Formation. Many oil fields producing from Oligocene and older reservoirs south of the Oak Ridge fault have a proto-structure that existed during the Miocene, when geothermal gradients were high. At first glance, the Chaffee Canyon oil field appears to violate this rule because the Miocene section is much thicker there than at Torrey Canyon field to the east and Shiells Canyon field to the west. However, the evidence for a pre-Vaqueros broad fold seen on an east-west longitudinal cross-section suggests that a low-relief anticline did exist at Chaffee Canyon, possibly with a trend oblique to the younger Wiley Canyon anticline.

ABSTRACT 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.