Moving under, around, and through the Miocene Reconstruction of Southern California

Abstract

Geological Survey Menlo Park. CA 94025 Modern southern California is frag- mented by faults that juxtapose blocks with contrasting topographies and dif- fering geologic histories. Many of the tectonic events that have shaped south- ern California were initiated during the Miocene, as subduction along the an- cient trench margin off southern Califor- nia was replaced by transform (strike- slip) faulting, such as that along the San Andreas fault. Miocene reconstruction of southern California was the subject of a Penrose Conference held in Oxnard, California, May 30-June 4. Sixty-four geologists, geophysicists, and paleontologists from academia, industry, and the govern- ment were brought together by A. Eugene Fritsche (Cahfornia State Uni- versity, Northridge) to discuss and de- bate the Miocene geologic history of southern California, south of 36N. A key event used in reconstructing southern California's geologic history was the anival of the ancestral East Pacific Rise at the trench margin off western North America. The model of Atwater (1970) suggests that this event occurred in the late Oligocene (28 Ma) at a latitude near or slightly south of Los Angeles (34N). A progressive shift of the convergent plate regime to one of right-lateral shear then followed, as the spreading center was split into two seg- ments, one of which migrated to the north, the other to the south. During the last 12 years various modifications of the Atwater (1970) model have been invoked by researchers such as Crowell (1974), Blake et al. (1978), Dickinson and Snyder (1979), and Crouch (1979) to explain basin formation, the cessation of volcanism, and the initiation of strike- slip faulting in different regions of cen- tral and southern California. As if the Miocene tectonic history of southern California were not compli- cated enough, the Transverse Ranges, a large segment of southern California trending east from Santa Barbara through northern Los Angeles to the Mojave Desert, apparently have been rotated clockwise by as much as 90 since the early middle Miocene, as a result of dextral shear between the North American and Pacific plates (Luyendyk et al., 1980, 1985). Furthermore, it seems that major strike-slip faulting in southern California has moved progressively inshore with time. The San Gabriel fault, west of the San Andreas fault, first became a major strike-slip surface in the late middle Miocene (11-12 Ma), while transform faulting was not initiated on the modern-