ABSTRACT The San Miguelito and Rincon oil fields are located along the northern edge of the western Ventura basin. Both fields produce from the approximately 3350 m (11,000 ft) thick sequence of turbiditic Plio-Pleistocene strata contained in the Ventura Avenue anticlinal trend. The anticlinal trend formed against the basin-bounding north-dipping reverse faults of the Red Mountain fault system. Red Mountain faulting, which may have begun as early as late Miocene in the area of the ancient Red Mountain seaknoll, resulted in the formation of an unyielding barrier against which later basinal deformation to the south occurred. The formation of the anticlinal trend has been rapid and complex. The Ventura Avenue and Rincon anticlines are separated at the surface by the San Miguelito anticline. The San Miguelito anticline is a fault-propagation fold restricted to the hanging wall of the south-dipping Padre Juan reverse fault. The Ventura Avenue and Rincon anticlines, in contrast, are flexural-slip folds which are continuous beneath the Padre Juan fault. Geologic evidence from the Ventura area indicates that the anticlinal trend did not begin to form before 600,000 years ago, and possibly not before 200,000 years ago. For the Rincon anticline and San Miguelito anticline-Padre Juan fault pair alone, this results in shortening rates of 4.2 to 12.5 mm/yr (0.16 to 0.49 in/yr) or greater.

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 Ventura basin in the Transverse Ranges of southernCalifornia (Fig. 1) contains evidence for major folding and faultingin Quaternary time, deformation that is still in progress. Muchof the evidence for age and deformation rates comes from theVentura Avenue anticline that began to form about 200 Ka. The locality exposes the axis of the western continuation of this anticlineand a folded reverse fault similar to larger ones known fromsubsurface information.

Abstract The Ventura basin in the Transverse Ranges of southernCalifornia (Fig. 1) contains evidence for major folding and faultingin Quaternary time, deformation that is still in progress. Muchof the evidence for age and deformation rates comes from theVentura Avenue anticline that began to form about 200 Ka. The locality exposes the axis of the western continuation of this anticlineand a folded reverse fault similar to larger ones known fromsubsurface information.