HOT CORDILLERAN HINTERLAND PROMOTED LOWER CRUST MOBILITY AND DECOUPLING OF LARAMIDE DEFORMATION

The thermal architecture of an orogenic hinterland directly impacts the style, magnitude, dynamics, and extent of deformation. Temperature is a fundamental factor in determining lithospheric viscosity, which impacts the intra-plate response to plate-boundary conditions. The North American Cordillera at the approximate latitude of northern Nevada experienced Middle Jurassic to early Cenozoic east-directed contractional deformation during eastward subduction of the Farallon plate along the western margin of North America. Cenozoic extensional structures and thermally induced buoyancy forces in the Cordilleran hinterland helped exhume high-grade metamorphic rocks from the mid-crust, juxtaposing them against the relatively cold, brittle upper crust. Metamorphic core complexes and the adjacent mountain ranges provide exceptional crustal exposures of > 15-km thickness, which can be investigated to explore the thermal structure of the upper crust. Here, we present >40 new peak-temperature estimates from Raman spectroscopy on carbonaceous material thermometry (RSCM) from low-strain mountain ranges surrounding the Ruby Mountains – East Humboldt Range metamorphic core complex in northeast Nevada. New and published Ar thermochronology and P-T-t confirm that peak metamorphic conditions and temperatures here were attained in the Late Cretaceous (ca. 80-70 Ma). We combine our thermal and structural data with a regional temperature dataset to place constraints on the geometry, kinematics, and thermal structure of the Cordilleran hinterland in the Late Cretaceous. Our results show temperature-depth relationships show a regionally elevated, continuous thermal gradient of >40 °C/km, which we project to a range of reasonable Moho temperatures to constrain the overall thermal structure of the Cordillera hinterland crust. Our extrapolation requires a hot, low viscosity lower crust in the Late Cretaceous, which promotes lower crust mobility and crust-mantle decoupling. Such conditions may have led to decreased basal traction during flat-slab Laramide subduction, which explain the lack of Laramide-aged structures in the Cordillera hinterland compared to foreland regions to the east.