Late Cretaceous to late Eocene (ca. 85–37 Ma) pre-extensional cooling of the Nevadaplano (east-central Nevada and west-central Utah, USA): A case study for lithospheric refrigeration during Laramide flat-slab subduction

Investigating the thermal history of orogenic plateaus provides insights into the geodynamic processes that drive lithospheric temperature changes. Here, we integrate time-temperature histories from new (n = 31) and published (n = 227) cooling ages from the Cordilleran Nevadaplano in east-central Nevada and west-central Utah, USA, with published peak metamorphic temperatures and depth histories. The upper 10−14 km of the crust cooled 120−365 °C (average 229 ± 85 °C) at average rates of 2.6−7.6 °C/m.y. between peak metamorphism at 85 ± 8 Ma and the initiation of extension at 37 Ma. This corresponded with a slow decrease in average geothermal gradient from 46 ± 10 °C/km at 85 Ma to 20 ± 11 °C/km at 37 Ma. Erosion levels beneath a subvolcanic unconformity constrain ≤1.7−4.0 km of erosion of this region prior to 37 Ma, which cannot account for such significant cooling. We interpret that pre-extensional cooling was dominantly driven by lithospheric refrigeration of the Nevadaplano during Laramide flat-slab subduction, with a minor contribution from erosion. The 20 ± 11 °C/km average geothermal gradient at 37 Ma demonstrates that the crust was cold and strong at the initiation of extension, which indicates that the decrease in interplate coupling that accompanied post-Laramide slab rollback was likely the dominant mechanism that drove extension, instead of thermal weakening during the Great Basin ignimbrite flareup. The footwalls of detachment fault systems in the Schell Creek and Northern Snake ranges cooled rapidly (typically 8−17 °C/m.y., but up to 46 °C/m.y.) between 37 Ma and 28 Ma and between 37 Ma and 17 Ma, respectively. Several ranges transitioned to slow cooling (1.3−3.5 °C/m.y.) at 13−10 Ma, which could represent the waning of displacement on range-bounding normal faults.