Deep burial of the footwall of the northern Snake Range decollement, Nevada

Metapelitic Eocambrian Strata from eastern exposures of the footwall of the northern Snake Range decollement experienced Mesozoic amphibolite facies m etamorphism and greenschist facies overprint accompanying extreme ductile t hinning. Samples from the Hampton Creek area where the overprint is weakest contain the assemblage quartz + muscovite + biotite + garnet + plagioclase + opaque oxides + staurolite +/- kyanite +/- tourmaline +/- apatite, Textu ral relations and consistency of rim analyses among multiple domains in eac h sample indicate equilibrium. Rim pressure-temperature estimates were obta ined using the garnet-biotite Pe-Mg exchange thermometer and the garnet-mus covite-plagioclase-biotite and garnet-aluminosilicate-plagioclase barometer s. The results suggest final equilibration at 610 +/- 50 degrees C and 810 +/- 70 MPa, or a depth of 30 +/- 3 km, about a factor of 3 greater than the stratigraphic depth. To the west, in the Schell Creek Range, correlative strata are contiguous w ith unmetamorphosed upper Paleozoic and Tertiary strata, From west to east, exposures of Eocambrian rocks in the two ranges collectively show increasi ng intensity of crystal-plastic deformation, metamorphic recrystallization, and metamorphic grade from subgreenschist to amphibolite facies, and a mon otonic decrease in Ar-40/Ar-39 muscovite ages from Mesozoic age to about 23 Ma. In light of these relations, the pressure-temperature data suggest tha t Eocambrian strata were inclined eastward during Mesozoic metamorphism wit h similar to 15-20 km of structural relief. We interpret the eastward tilti ng and burial to be the result of west-directed thrusting, as expressed by folding td the north in the Deep Creek Range. Unroofing of the deeply burie d sh ata may have occurred partly in Cretaceous or early Cenozoic time, wit h final unroofing in Oligocene and Miocene time along the northern Snake Ra nge decollement, These results exclude the hypothesis that the northern Sna ke Range decollement initiated as a brittle-ductile transition zone within Cambrian strata of a little-deformed miogeoclinal section, and support the hypothesis that it is a major low-angle extensional shear zone.