Abstract

Metapelitic Eocambrian strata from eastern exposures of the footwall of the northern Snake Range decollement experienced Mesozoic amphibolite facies metamorphism and greenschist facies overprint accompanying extreme ductile thinning. 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. Textural relations and consistency of rim analyses among multiple domains in each sample indicate equilibrium. Rim pressure-temperature estimates were obtained using the garnet-biotite Fe-Mg exchange thermometer and the garnet-muscovite-plagioclase-biotite and garnet-aluminosilicate-plagioclase barometers. The results suggest final equilibration at 610 ± 50 °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 with unmetamorphosed upper Paleozoic and Tertiary strata. From west to east, exposures of Eocambrian rocks in the two ranges collectively show increasing intensity of crystal-plastic deformation, metamorphic recrystallization, and metamorphic grade from subgreenschist to amphibolite facies, and a monotonic decrease in 40Ar/39Ar muscovite ages from Mesozoic age to about 23 Ma. In light of these relations, the pressure-temperature data suggest that Eocambrian strata were inclined eastward during Mesozoic metamorphism with ∼15–20 km of structural relief. We interpret the eastward tilting and burial to be the result of west-directed thrusting, as expressed by folding to the north in the Deep Creek Range. Unroofing of the deeply buried strata may have occurred partly in Cretaceous or early Cenozoic time, with final unroofing in Oligocene and Miocene time along the northern Snake Range decollement. These results exclude the hypothesis that the northern Snake 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.

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