Abstract

The Ridge basin, located in southern California between the San Andreas and San Gabriel faults, contains as much as 14 km of Mio-Pliocene marine and nonmarine sedimentary fill. These strata are exposed as a northwest-dipping homoclinal sequence that becomes increasingly younger to the northwest. Pliocene to Holocene deformation, uplift, and dissection of this basin provide an opportunity to study stratal geometry and sedimentary facies in what is commonly cited as a classic "strike-slip" basin. Newly acquired seismic data from the northern Ridge basin provide new evidence for the subsurface basin geometry. These data lead 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 fault appears to become subhorizontal at the southeastern end of the seismic line at a present-day sub-surface depth of ∼4 km. This interpretation is consistent with preliminary two-dimensional gravity models across the basin and with the homoclinal structure of basin fill. Subsidence within the Ridge basin is explained as the result of northwest- southeast extension locally accommodated along the listric, right-lateral San Gabriel and associated late Miocene strike-slip faults. As a consequence of oblique normal slip on the gently dipping San Gabriel fault, sedimentary strata were continuously tilted and moved southeastward in the hanging wall away from the depocenter. This mechanism of basin formation allowed 14 km of sediment to accumulate without deep burial. The geometry and displacement history of the San Gabriel fault controlled sediment accommodation and resultant stratal geometries within the basin. Slip along the San Gabriel fault and associated subsidence of the Ridge basin correspond temporally with regional tectonic events in southern California, reflecting both changes in the relative motion of Pacific and North American plates and the evolution of strain partitioning across this plate boundary.

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