ABSTRACT: Rifting, Rotation, Detachment Faulting, and Sedimentation: Miocene Evolution of the Southern California Margin

Steven B. Bachman, James K. Crouch

The evolution of the Los Angeles and adjacent offshore Santa Monica and San Pedro basins of southern California began during the earliest Miocene. The basins formed as the result of rifting and subsequent large-scale rotation of segments within a preexisting Mesozoic-Paleogene forearc basin. Clockwise rotation (> 90°) of the outer two-thirds of this fore-arc basin during the early and middle Miocene moved these once north-trending forearc strata into an east-west trend (the modern Transverse Ranges). The eastern margin of the initial rift remains in its original location and is best documented from outcrop and subsurface data in the San Joaquin Hills. What was once the western margin of the rift has been rotated to a position north of the rift, along the southern anta Monica Mountains.

The early Miocene Vaqueros sandstones, which that are entirely shallow-marine and thousands of feet thick, provide evidence for initial subsidence of the rift. Widening of the rift and separation of the Santa Monica Mountains and the San Joaquin Hills in the early and middle Miocene was accompanied by detachment faulting and volcanism along the rift margins. These detachment faults can be documented in the subsurface of the San Joaquin Hills and in outcrop in the Santa Monica Mountains. A unique aspect of this inner borderland rift is the rapid uplift, exposure, erosion, and then subsidence of high-pressure/temperature metamorphic basement blocks (Catalina schist) within the rift itself. These basement rocks were buried 20 to 30 km beneath the ancestral fore arc prior to rifting. They were uplifted, perhaps due to thermal effects, during pervasive early and middle Miocene

volcanism within the rift. Evidence of these dramatic events is provided by the distinctive San Onofre breccia deposits exposed along the margins of the rift. Rapid subsidence of the rift basin and the associated basement blocks began within 2-3 m.y. after uplift and continued through the late middle and late Miocene. This subsidence gave rise to widespread deep-sea fan (Puente) and interfingering organic-rich pelagic (Monterey) sediments that today are the key to the petroleum productivity and future potential of this area.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990