A NEW ANGLE ON THE TECTONIC EVOLUTION OF THE RIDGE BASIN, A STRIKE-SLIP BASIN IN SOUTHERN CALIFORNIA

The Ridge basin, located in southern California between the San Andrea s and San Gabriel faults, contains as much as 14 km of Mio-Pliocene ma rine and nonmarine sedimentary fill. These strata are exposed as a nor thwest-dipping homoclinal sequence that becomes increasingly younger t o the northwest. Pliocene to Holocene deformation, uplift, and dissect ion of this basin provide an opportunity to study stratal geometry and sedimentary facies in what is commonly cited as a classic ''strike-sl ip'' basin. Newly acquired seismic data from the northern Ridge basin provide new evidence for the subsurface basin geometry. These data lea d us to conclude that the principal northern strand of the San Gabriel fault is a listric, east-southeast-dipping, oblique-slip fault rather than a subvertical, strike-slip fault. The principal San-Gabriel faul t appears to become subhorizontal at the southeastern end of the seism ic line at a present-day subsurface depth of approximately 4 km. This interpretation is consistent with preliminary two-dimensional gravity models across the basin and with the homoclinal structure of basin fil l. Subsidence within the Ridge basin is explained as the result of nor thwest-southeast extension locally accommodated along the listric, rig ht-lateral San Gabriel and associated late Miocene strike-slip faults. As a consequence of oblique normal slip on the gently dipping San Gab riel fault, sedimentary strata were continuously tilted and moved sout heastward in the hanging wall away from the depocenter. This mechanism of basin formation allowed 14 km of sediment to accumulate without de ep burial. The geometry and displacement history of the San Gabriel fa ult controlled sediment accommodation and resultant stratal geometries within the basin. Slip along the San Gabriel fault and associated sub sidence of the Ridge basin correspond temporally with regional tectoni c events in southern California, reflecting both changes in the relati ve motion of Pacific and North American plates and the evolution of st rain partitioning across this plate boundary.