ABSTRACT: Transpressional Deformation in the Sacramento Valley and Coast Ranges of California: Decoupled Strike-Slip and Thrust Wedging above a Master Detachment
PHIPPS, STEPHEN PAUL, University of Pennsylvania, Philadelphia, PA, JEFFREY UNRUH, William Lettis and Associates, Lafayette, CA, and University of California, Davis, CA, VINCENT RAMIREZ, Pecten International Company, Houston, TX, and ELDRIDGE MOORES, University of California, Davis, CA
The California Coast Ranges and Central Valley form a single transpressional "orogenic float,- with strike-slip and thrust wedging mainly decoupled from each other, but linked through a deep master decollement.
In the Coast Ranges, imbricate thrusting has shuffled Mesozoic and Cenozoic rocks into a complex structural stack, which is also transected by strike-slip faults by the San Andreas system. In the northern Coast Ranges, thrusts and related ramp anticlines strike north-south to north-northwest-south-southeast and generally verge westward. In the Sacramento Valley, asymmetric ramp anticlines trend generally north-northwest-south-southeast, plunge south-southeast, and are cut locally by west-vergent thrusts. The amplitude of these structures diminishes eastward.
Surface and subsurface data lead us to interpret the Sacramento Valley and Coast Range structures as a unified system in which strike-slip and thrust motions are largely decoupled. West-vergent thrusts are backthrusts form a master, east-vergent blind detachment that underlies the entire Coast Ranges and the western Sacramento Valley. The easternmost backthrusts in the Sacramento Valley rise from near the tip of the detachment, and together with it form a thrust wedge. Geophysical, radiometric, structural, and stratigraphic data suggest that this and similar wedges may have been active since at least the Paleocene. At present, the wedge is propagating eastward into the Sacramento Valley, forming rising anticlines above its ramps and backthrusts.
Strike-slip faults of the San Andreas system also are detached at the level of the master decollement, which therefore is an oblique-slip fault with strike-slip displacement increasing westward. Strike-slip movement is mainly decoupled from thrust movement, as shown by the general absence of high topography along the most active strike-slip strands.