RAPID MIDDLE MIOCENE UNROOFING OF THE SOUTHERN RUBY MOUNTAINS, NEVADA

Highly attenuated metasediments and granitoids in the northern Ruby Mountains were exhumed from a depth of 20-30 km along a west-dipping shear zone beginning as early as the Eocene, with the main phase of slip thought to be late Oligocene or early Miocene. This fault system can be traced southward for 100 km, where it drops unmetamorphosed Upper Paleozoic rocks down onto the 36 Ma Harrison Pass pluton and its metamorphosed Lower Paleozoic wallrocks. South of Harrison Pass, the range is a relatively undeformed, E-tilted fault block, making it an ideal place to study the timing of exhumation by apatite fission-track and (U-Th)/He dating of samples tied to specific pre-extensional stratigraphic and structural positions in the crust. Eleven samples collected from an E-W transect spanning about 8 km of structural relief across the Harrison Pass pluton yielded generally high-quality but small (20-60 μm radius) apatite with common zircon inclusions. Nine apatite (U-Th)/He analyses yielded a weighted-mean age of 14.6 ± 1.1 Ma (33 grains); an additional 21 grains ranged in age from 20-102 Ma and were rejected on the assumption that they contained U-rich inclusions. Ten apatite fission-track analyses yielded a weighted mean age of 14.8 ± 1.5 Ma, ranging from 12-20 Ma with no discernable geographic trend. A dike that intrudes Ordovician strata that roof the pluton near the eastern side of the range yielded a fission-track age of 24.8 ± 2.9 Ma; we interpret this as the base of the fission-track partial annealing zone (~120ºC), consistent with stratigraphic depth including 3-4 km of (now unexposed) overlying Mississippian-Permian sediments. Uniformly long measured track lengths (>14 μm) from all samples are consistent with rapid cooling. The simplest interpretation of these data is that the southern Ruby Mountains were rapidly exhumed at 14-15 Ma as an intact east-tilted block during slip along an initially steeply-dipping fault on the west side of the range. This fault system now dips gently west and merges to the north with the mylonitic shear zone that bounds the deeply-exhumed core complex farther north, raising the possibility that the latter area also underwent significant unroofing during the middle Miocene.