SHALLOW-CRUSTAL METAMORPHISM DURING LATE CRETACEOUS ANATEXIS IN THE NEVADAPLANO: INSIGHTS FROM A METAMORPHIC FIELD GRADIENT THROUGH THE UPPER CRUST, GRANT RANGE, EASTERN NEVADA, U.S.A

Documenting spatio-temporal relationships between the thermal and deformation histories of orogenic systems can elucidate their evolution. In the Sevier hinterland plateau (Nevadaplano) in eastern Nevada, an episode of Late Cretaceous magmatism and metamorphism affected upper-crustal levels, concurrent with shortening in the Sevier thrust belt. To quantify the thermal conditions during this event, we collected peak temperature data from the Grant Range, a site of localized, Late Cretaceous (~84 Ma) granitic magmatism and greenschist facies metamorphism in eastern Nevada. 22 samples of Cambrian-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 2-9 km. Peak temperatures increase with depth, from 100-300°C between 2-5 km, 400-500°C between 5-8 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 (70-90 Ma), upper-crustal metamorphism and magmatism documented further east, in the Egan, Schell Creek, Kern, and Deep Creek Ranges, where metamorphic field gradients as high as 50°C/km have been estimated. This event has been interpreted as the shallow thermal expression of an episode of lower-crustal anatexis. The rise of anatectic melts to their emplacement as upper-crustal granitic plutons provided an efficient heat transportation mechanism. 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 as shallow as 12-15 km. Thermal weakening may have contributed to a slowing of shortening rates in the Sevier thrust belt at this latitude at ~88 Ma, by locally inducing ductile thickening in the hinterland. This could help explain the disparate observations of minimal upper-crustal shortening across much of eastern Nevada and crustal thicknesses of 50-60 km interpreted to have been attained by the end of orogenesis.