ABSTRACT Two contrasting views of the Oak Ridge fault in the Ventura basin have emerged: a thick-skinned view based on onshore subsurface oil-well data and a thin-skinned view based on interpretation of seismic profiles and dipmeter data from the Santa Barbara Channel. The offshore Oak Ridge structure is interpreted as an active axial surface above the ramp of a blind, south-vergent thrust; growth triangles in the kink band south of this surface contain parallel, north-dipping strata in which the kink band tapers toward the surface. In contrast, dips of growth strata onshore near the coast at Montalvo steepen with depth, evidence of progressive limb rotation and a steeply-dipping, large-displacement Oak Ridge fault. The onshore Oak Ridge fault is northward-vergent. West of Oak Ridge, the post-Saugus displacement is transferred via the ductile Rincon Formation to folding of the Ventura-Rincon anticline, also a north-vergent structure onshore and offshore. An onshore seismic line along a cross section documented by electric logs shows a narrow panel of north dips that could be the onshore termination of an offshore kink band mapped in the Santa Barbara Channel. North dips at greater depth in this seismic line do not correspond to well data and are artifacts of processing. Farther east, north dips are possible only if the kink band turns to the northeast, parallel to the Montalvo fault. The onshore-offshore controversy will only be resolved by comparing seismic profiles in the Santa Barbara Channel with subsurface well profiles in the same transect using well-log correlations as well as dipmeter data.

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.

Abstract The slip rate on the Oak Ridge fault at South Mountain in the densely-populated lower Santa Clara Valley, California is estimated as 6.15 to 12.3 mm/yr since the end of Saugus deposition at 0.4 to 0.2 Ma. Slightly lower slip-rate estimates result from a different projection of the top of the Saugus in the hanging-wall block of the fault. An assumed displacement of 3 m per event gives an average recurrence interval of 250 to 500 yr, with the major uncertainty being the age of the top of the Saugus. Displacement of 2460 m of the top of the Saugus at South Mountain includes piercing-point displacement and distributed displacement from drag folds near the fault; both are the near-surface expression of faulting at potential mainshock depths beneath well control. Slip rates for the nearby San Cayetano and Red Mountain faults are not well constrained because post-Miocene strata are absent in their hanging-wall blocks, but available evidence suggests that their slip rates are about the same as that for the Oak Ridge fault. Earthquake return times measured in hundreds of years have been shown by others for the 1971 San Fernando fault and a small fault near the Red Mountain fault, and comparable return times are suggested for a normal fault in the hanging-wall block of the Oak Ridge fault. A trench on the Harmon alluvial fan near the Ventura County Government Center revealed evidence of cracks filled with sediments from below, suggesting liquefaction during an earthquake. The lower Santa Clara Valley has not had a large, damaging earthquake in 200 years of record-keeping, but one is expected there in the near future. The December 21, 1812 earthquake may have occurred on the offshore continuation of the Oak Ridge or Red Mountain fault.

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.