LATE MIOCENE-PLIOCENE BASIN FORMATION DURING DISARTICULATION OF THE SILVER PEAK EXTENSIONAL ALLOCHTHON, 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. Mid-Miocene to Pliocene movement on a low-angle extensional detachment fault resulted in exhumation of amphibolite facies metamorphic tectonites from beneath an upper-plate extensional allochthon containing weakly metamorphosed Paleozoic strata and unconformably overlying Tertiary volcanic and sedimentary rocks. Deformation within the upper plate accompanied northwest-directed movement on the shallowly dipping detachment and resulted in the development of spatially restricted basins. The basins are bound by northeast striking listric normal faults, oriented normal to upper-plate motion, and northwest striking transcurrent faults, oriented parallel to upper-plate displacement. The two sets of faults acted together as small-scale stepovers within the actively extending upper plate and accommodated spatial and temporal migration of the locus of basin development. The faults soled into the underlying detachment and with progressive displacement developed stratal tilt and the fan-geometry characteristic of normal growth. The total thickness of synorogenic deposits, which are composed of five lithostratigraphic units, varies from 800 m to 100 m with individual units varying from zero to 400 m over distances of less than 3 km. The spatial and temporal distribution of synorogenic volcanic, volcaniclastic sedimentary rocks record a changing pattern of fault-bound basin development during progressive disarticulation of the extensional allochthon. Locally, synorogenic deposits seal older basin-bounding faults and are cut by younger generations of structures. Late stage deformation resulted in folds of both upper and lower plate rocks and local inversion of upper-plate basins. In the Pliocene, displacement on the basal detachment ceased and the extensional complex was cut by NNE-striking normal faults with down-to-west displacement. The younger fault system cuts Quaternary alluvial deposits and is seismically active. Upper plate deformation was active from about 13 Ma and ended at around 3 Ma, when the extensional complex was abandoned and deformation migrated to the northwest into the Mina deflection.