CROUCH, JAMES, J. K. Crouch & Associates, Inc. Santa Barbara, CA

ABSTRACT: Late Mesozoic-Cenozoic Tectonic Evolution of the Southern California Margin

The southern California continental margin has undergone three distinct phases of tectonic evolution in the late Mesozoic to Cenozoic. For about 100 m.y., from at least the earliest Cretaceous to the middle Oligocene, it was part of the southwestern edge of the North American plate and can best be characterized as a convergent, relatively stable, Andean-type margin created by continuous subduction of tens of thousands of kilometers of the oceanic Farallon plate. During this lengthy time span, coeval, subparallel lithotectonic belts developed along the margin that included, from east to west, a magmatic arc (Peninsular Ranges belt), a fore-arc basin (Great Valley belt), and a subduction complex (Franciscan belt).

Beginning about 27 Ma, subduction along the margin was progressively supplanted by complex transform interaction between the North American and Pacific plates. From the latest Oligocene to the latest Miocene, the tectonic development of the southern California margin was characterized by widespread Basin and Range-type extension, rifting, and basaltic-rhyolitic volcanism, as well as major translation and clockwise rotation of large crustal blocks (e.g., the outer borderland and the western Transverse Ranges, respectively). During this roughly 20-m.y. time span, extreme tectonic instability and disruption of the margin prevailed, and parts of the former well-developed Mesozoic-Paleogene lithotectonic belts were ripped apart, uplifted, translated northwestward, rotated, and overprinted y the development and infilling of numerous Neogene basins.

From about the latest Miocene/earliest Pliocene to the Holocene, a quite different, still ongoing, tectonic phase developed. During the earliest part of this brief, 5-6-m.y. period, the southern California margin became attached to the Pacific plate and has subsequently been translated about 300 km northwestward relative to the North American plate. Also, during this most recent tectonic phase, the late Oligocene-early Miocene style of large-scale disruption has waned and been supplanted, as well as overprinted, by marked crustal shortening (normal to the San Andreas), uplift, and broadly distributed right-lateral and left-lateral shear. This latest tectonic phase markedly added to the infilling of the Neogene basins and created the majority of reservoirs and structures that are produ tive along the southern California margin today.

AAPG Search and Discovery Article #90992©1993 AAPG Pacific Section Meeting, Long Beach, California, May 5-7, 1993.