Cenozoic structure of the Piru 7 1/2-minute quadrangle, California

The Piru 7!/2-mmute quadrangle in the central Ventura basin contains a thick accumulation of Plio-Pleistocene sedimentary rocks bordered by opposing reverse faults, on the north by the north-dipping San Cayetano fault and on the south by the south-dipping Oak Ridge fault. Deformation in the Piru quadrangle can be separated into three phases: (1) pre-Vaqueros (late Oligocene-early Miocene) tilting in the hanging-wall block of the Oak Ridge fault that is coincident with normal faulting farther south at Big Mountain, (2) pre-Pleistocene reverse faulting of the Torrey fault, and (3) Quaternary deformation. Strands of the San Cayetano and Oak Ridge faults offset late Quaternary surfaces. East of Sespe Creek, the San Cayetano fault has a lobate surface trace, and the dip of the fault shallows to as much as 27 N. Farther east, the fault bifurcates into two strands; the northern, Main strand trends northeast, cuts an alluvial fan 2V2 km ENE of Piru, and dies out; the southern, Piru strand trends east and dies out near the eastern boundary of the quadrangle. The Oak Ridge fault has variable, low dips in its upper 2.5 km to the west and is more steeply dipping to the east where it bifurcates into three strands and dies out south of the eastern terminus of the San Cayetano fault. Near the eastern termini of the San Cayetano and Oak Ridge faults, displacement is taken up on east-trending folds and southdipping reverse faults of the East Ventura fold belt.