THE EXCEPTIONALLY HOT LATE CRETACEOUS NEVADAPLANO OF THE NORTH AMERICAN CORDILLERA PROMOTED LOWER CRUST MOBILITY AND DECOUPLING DURING THE LARAMIDE

The style, distribution, and extent of intra-plate deformation may be controlled by the thermal state and mechanical strength of the continental lithosphere. In modern orogens, direct constraints on thermal architecture are limited to deep xenoliths and surface heatflow observations, with geophysical measurements providing some indirect proxies. Conversely, variably exhumed components of ancient orogens can provide insight into the thermal and structural architecture of intra-plate deformation. 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. Protracted contractional deformation constructed a relatively thick orogenic plateau referred to as the “Nevadaplano”. Published thermochronology and P-T-t data from mid-crustal rocks exhumed in the Ruby Mountains-East Humboldt Range and Snake Range metamorphic core complexes show that peak metamorphic conditions were attained in the Late Cretaceous to Early Cenozoic (ca. 90-60 Ma), synchronous with shallowing of the Farallon plate and start of Laramide orogeny far within the continental interior. Here, we present an unprecedented view of the thermal state of the hinterland plateau by determining peak temperature conditions experienced by the upper 15-20 km of crust via Raman spectroscopy on carbonaceous material thermometry and detailed structural reconstructions. We extrapolate our results to constrain the bulk thermal state and rheology of the crust. Our results show that the Cordilleran hinterland plateau was extremely hot during Laramide orogenesis (>40 °C/km), which would have promoted lower crust mobility and crust-mantle decoupling. Such conditions may have led to decreased basal traction during Laramide flat-slab subduction, which explain the lack of Laramide-aged structures in the Cordillera hinterland compared to foreland regions to the east. We confirm these interpretations via 2-D thermomechanical simulations. Results from this study can be used to test competing models for Laramide orogenesis.