Abstract: Neogene Basin Formation and Hydrocarbon Accumulation Related to San Andreas Fault System and Plate Tectonics in Western California

M. C. Blake, R. H. Campbell, T. H. Nilsen

More than 90 percent of the known petroleum accumulations in California is in Neogene strata deposited in deep localized basins that originated during a major mid-Tertiary reorganization of the regional structure of western California. Although some of the basins appear to have had Paleogene precursors, all contain features that mark a mid-Tertiary shift from Paleogene depositional environments having relatively broad, regional distributions, to Neogene distributions dominated by deep local basins. Many of the structural traps in which the hydrocarbons have accumulated do not appear to be related directly to the geometry of the Neogene basins as much as to the present active fault systems, and may reflect a further reorganization of latest Tertiary age. The evolution of each of the Neogene basins is complex, and in some respects the kinematics of each is unique; nonetheless, all can be considered as resultants of an overall right-shear system associated with a sliding margin between the Pacific and North American plates.

The west edge of the North American continent first became a sliding margin only about 29 m.y. ago (late Oligocene or early Miocene), after the subduction of the trailing (west) edge of part of the Farallon plate. By about 12 m.y. ago (late Miocene) right slip between the Pacific and North American plates was shifting inland from the west edge of the continent to right-slip faults in the continental margin such as the San Andreas. Continued interplate right slip along the jagged boundary formed by the northern San Andreas fault, the Transverse Ranges structures, and the southern Borderland faults (possibly including the Patton escarpment) resulted in left slip within the Transverse Ranges and at their southern boundary. The Neogene basins of western California formed during the interv l in which the boundary between the Pacific and North American plates shifted from the edge of the continent to the San Andreas fault, apparently because the piecemeal shift to different surfaces of weakness caused resultant local extensional and compressional stresses within the broad zone of right-lateral shear. The younger structural traps appear to be the results of local compression developed from interplate right slip along the present San Andreas and related faults.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA