BASIN MIGRATION AND SYNOROGENIC DEPOSITION IN THE SILVER PEAK, EXTENSIONAL COMPLEX, CENTRAL WALKER LANE, NEVADA

The Silver Peak Extensional Complex of west-central Nevada acted as a structural stepover kinematically linking faults of the central Walker Lane and Furnace Creek Fault system. WNW-directed displacement along the shallowly dipping Silver Peak detachment separated a highly extended, brittlely deformed, upper-plate assemblage from amphibolite facies metamorphic tectonites in the lower plate. Upper-plate rocks consist of weakly metamorphosed Proterozoic and lower Paleozoic strata unconformablely overlain by late Tertiary volcanic and volcaniclastic rocks. The Tertiary rocks preserve a history of mid-Miocene to Pliocene synorogenic deposition and record a changing pattern of isolated fault-bound basins in the disarticulated upper plate of the extensional complex. Basin development was controlled by two orientations of faults; NNE-striking, listric normal faults, oriented perpendicular to upper-plate motion, and WNW-striking transcurrent faults that paralleled upper-plate displacement. The NNE faults soled into the underlying detachment and with progressive displacement, resulted in stratal tilt and fan-geometry characteristic of normal growth. Along strike, basins are offset and linked by the WNW-striking transcurrent faults that separate coeval units exhibiting different thickness and degree of tilt. The total thickness of synorogenic deposits, which are composed of four lithostratigraphic units, varies from 750 to 550 m but individual units vary from zero to 350 m over distances of less than 3 km. Thick volcanic and volcaniclastic successions are localized near major listric normal faults and thin toward the tilted margins of fault-bound basins. Locally, synorogenic deposits seal older basin-bounding faults and are cut by younger generations of structures. The two sets of faults acted together as small-scale stepovers within the actively extending upper plate and facilitated migration of basin deposition in space and time. In the Pliocene, displacement on the basal detachment ceased and all extensional complex structures were cut by NNE-striking normal faults with down-to-west displacement. The younger system of faults cut Quaternary alluvial deposits and are seismically active.