Abstract

Combined structural and geochronologic studies to the west of the Sevier belt at lat 39°-40°N indicate that peak metamorphism of the upper 15 km of crust occurred during the Late Cretaceous (70-90 Ma), overlapping in time with the intrusion of muscovite-bearing granites and pegmatites. Metamorphism was accompanied by penetrative deformation (top-to-the-east layer-parallel shear); both were localized at the currently exposed structural levels, resulting in impressive vertical and lateral strain gradients and Tmax gradients of >50°C/km. Inception of thrust faulting in the Sevier belt may have predated peak metamorphism and intrusion of granites in the hinterland by 10-15 m.y., but it continued during and ended simultaneously with the end of metamorphism at about 70 Ma. The rise of muscovite-bearing granites and hot fluids, generated by metamorphism and crustal anatexis at greater depth, provides an explanation for the observed localized metamorphism and deformation at relatively high structural levels.

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