SYNOROGENIC DEPOSITION IN AN EAST-NORTHEAST TRENDING HALF-GRABEN DURING LATE OLIGOCENE TO MID-MIOCENE NORTH-SOUTH EXTENSION IN WEST-CENTRAL NEVADA

A period of north-south extension during the late Oligocene and mid-Miocene created a system of east-northeast- and west-northwest-trending half-grabens in the northern part of the southern Walker Lane and the central Walker Lane of the western Great Basin. Miller Mountain, in the southern central Walker Lane, exposes a succession of late Oligocene tuffs that range in aggregate thickness from 100 to 390 m. The tuffs were deposited in and around an east-northeast-trending half-graben and record the evolution and cessation of basin activity. The half-graben is 8.5 to 10 km long and 4 to 5 km wide and contains two units of ashflow tuff and tuffaceous sediments dated elsewhere in the region at 25.76 ± 0.12 to 24.09 ± 0.29 Ma. Inside the half-graben, the basin-fill is 300 m thick whereas equivalent tuffs outside the basin are only 50 m thick. The basin is bounded to the south by a system of one to three east-northeast-striking faults that separate basin-fill from Paleozoic metasediments and a Mesozoic pluton. The basin is bounded to the west by a north-northwest-striking fault and to the east by a northeast-striking fault. North-northwest- and northeast-striking transfer faults segment the half-graben and create 1.0 to 1.5 km long dog-leg steps in the south-boundary fault system. Within the basin, individual tuff units form south-dipping stratal-wedges that thicken southward into the east-northeast-striking faults and exhibit along-strike changes in thickness across the transfer faults. Outside the basin, the tuff is relatively flat-lying and shows a complex pattern of omission of parts of the tuff stratigraphy. The half-graben was buried beneath a post-extensional ashflow tuff, dated at 23.75 ± 0.09 Ma, that maintains a uniform thickness of 90 m across the master fault system. Inversion of fault-slip data collected along the basin-bounding faults taken together with fault-slip data from other half-grabens nearby in the region indicate that the basin formed during north-south extension. Extension occurred during the late Oligocene, when activity on this basin ceased, but continued into the mid-Miocene elsewhere in the region.