STRUCTURAL COMPLEXITY AND BASIN MARGIN EVOLUTION ABOUT THE CLARK FAULT TERMINATION TIP, SOUTHERN SANTA ROSA MOUNTAINS AND BORREGO BADLANDS, SOUTHERN CALIFORNIA

Dextral displacements on the San Jacinto Fault Zone have formed a structural re-entrant and composite basin defined by the Borrego and Clark valleys west of the Salton Trough. The crystalline basement cored Santa Rosa Mountains (SRM) form the NE re-entrant margin, displaced to the SE by dextral slip on the Clark Fault. The Clark fault trace termination, adjacent to the south tip of the SRM, is accompanied by immense structural complexity in Miocene to Pleistocene non-marine sedimentary “cover” strata outcropping in the southern SRM and the adjacent Borrego Badlands. The en echelon and sigmoidal patterns of fold axial traces to the S and W reflect the dextral “drag” and oblique thrust-slip affecting the structurally decoupled cover successions mapped about the south margin of the SRM. To the N and E into the southern sector of the SRM deformation styles reveal NNW directed thrusting is accommodated by the late Pleistocene local reactivation of the Western Salton Detachment Fault.

The basin margin clastic cover sequences are folded into an open south plunging anticline, thrust northward onto the south flank of the SRM over the shallow E dipping reactivated detachment. The NE striking Truckhaven fault in the core zone of this anticline represents the “tear” between two structural domains: (1) an elevated uniformly dipping east limb characterised by an intricate network of E-W dextral-slip faults with bedding shears; and (2) a west limb domain where structural complexity decreases upward through the stratigraphic pile with numerous local unconformities indicative of coeval deformation, dissection, and deposition.

The Borrego Badlands are dominated by complex asymmetric en echelon folding on numerous shallow-depth, north dipping décollement developed in a Plio-Pleistocene lacustrine facies. Within this succession short to medium (10’s to 1000’s m) wavelength thrust propagated and “nested” folds reflect numerous shallow-depth, vertically stacked, stratigraphically confined décollement. This style of deformation is driven by the shortening and structural rotations induced by the dextral Clark Fault in basement at depth, partially concealed by the deforming cover sequence. Sediment cannibalism as the SRM “plough” beneath the arching cover succession further contributes to complexity along the basin margin.