Fission track (FT) thermochronology studies on igneous and metamorphic rocks from the Ruby Mountains-East Humboldt Range metamorphic core complex provide important constraints on the timing and nature of major mid-Tertiary extension in northeast Nevada. Rocks from within the Ruby-East Humboldt detachment (brittle-ductile normal-sense shear) zone were analyzed; the dominant lithology studied was variably mylonitic mafic orthogneiss. Nonmylonitic amphibolite from the top of the structurally lower migmatitic core and porphyritic biotite granodiorite from the Oligocene (circa 36 Ma) Harrison Pass pluton were also studied. FT ages on apatite, zircon, and sphene (except one 18.4-Ma apatite from the basal portion of the detachment zone) are concordant and range in age from 26.5 Ma to 23.6 Ma; all ages overlap at 1 σ between 25.4 Ma and 23.4 Ma. These data suggest that rocks of the upper and middle portions of the detachment zone cooled rapidly from temperatures above ∼285°C (sphene closure temperature) to below ∼70°C (lower temperature limit of track stability in apatite) near the beginning of the Miocene (minimum cooling rate of 40°C/Ma). The length distribution of confined fission tracks in apatites from these exhumed middle crustal rocks is strikingly similar to those of rocks known to have cooled rapidly (Fish Canyon ash flow tuff). The lower part of the detachment zone as well as the underlying migmatitic core also cooled through sphene and zircon closure temperatures during this time interval, but apparently only cooled to a temperature within the zone of partial annealing of apatite (∼70°–130°C). Residence of these rocks in this zone led to a partial retention of fission tracks, thus resulting in a reduced age. Confined track length distributions from apatite provide independent evidence of very rapid cooling but also indicate that the rocks of the lower detachment zone experienced a more protracted cooling history (remained hotter longer) than structurally higher levels of the zone. FT data firmly establish the lower limit on the timing of mylonitization during detachment faulting in this area as 23.4 Ma. Rapid cooling of the region is considered to reflect large-scale tectonic denudation (intracrustal thinning), the vertical complement to crustal extension. Rocks originating in the middle crust (10–15 km) were quickly brought near the surface along the Ruby-East Humboldt detachment fault (brittle-ductile simple shear zone) and juxtaposed against brittlely extended rocks deformed under upper crustal conditions.
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