Shallow-crustal metamorphism during Late Cretaceous anatexis in the Sevier hinterland plateau: Peak temperature conditions from the Grant Range, eastern Nevada, U.S.A.

Abstract

Documenting spatio-temporal relationships between the thermal and deformation histories of orogenic systems can elucidate their evolution. In the Sevier hinterland plateau in eastern Nevada, an episode of Late Cretaceous magmatism and metamorphism affected mid- and upper-crustal levels, concurrent with late-stage shortening in the Sevier thrust belt. Here, we present quantitative peak temperature data from the Grant Range, a site of localized, Late Cretaceous granitic magmatism and greenschist facies metamorphism. Twenty-two samples of Cambrian to Pennsylvanian metasedimentary and sedimentary rocks were analyzed, utilizing Raman spectroscopy on carbonaceous material, vitrinite reflectance, and Rock-Eval pyrolysis thermometry. A published reconstruction of Cenozoic extension indicates that the samples span pre-extensional depths of 2.5–9 km. Peak temperatures systematically increase with depth, from ∼100 to 300 °C between 2.5 and 4.5 km, ∼400 to 500 °C between 5 and 8 km, and ∼550 °C at 9 km. The data define a metamorphic field gradient of ∼60 °C/km, and are corroborated by quartz recrystallization microstructure and published conodont alteration indices. Metamorphism in the Grant Range is correlated with contemporary, upper-crustal metamorphism and magmatism documented farther east in Nevada, where metamorphic field gradients as high as ∼50 °C/km are estimated. These data have implications for localized but significant thermal weakening of the plateau crust, including attaining temperatures for quartz plasticity at depths of ∼5–6 km, and the potential for partial melting possibly as shallow as ∼12–15 km. Thermal weakening may have contributed to a slowing of shortening rates documented in the Sevier thrust belt at this latitude at ca. 88 Ma, by locally inducing mid- and lower-crustal ductile thickening in the hinterland.