Geologic field evidence for non-lithostatic overpressure recorded in the North American Cordillera hinterland, northeast Nevada

There is a long-standing discrepancy for numerous North American Cordillera metamorphic core complexes between geobarometric pressures recorded in the exhumed rocks and their apparent burial depths based on palinspastic reconstructions from geologic field data. In particular, metamorphic core complexes in eastern Nevada are comprised of well-documented ~12–15 km thick Neoproterozoic–Paleozoic stratigraphy of Laurentia's western passive margin, which allows for critical characterization of field relationships. In this contribution we focus on the Ruby Mountain–East Humboldt Range–Wood Hills–Pequop Mountains (REWP) metamorphic core complex of northeast Nevada to explore reported peak pressure estimates versus geologic field relationships that appear to prohibit deep burial. Relatively high pressure estimates of 6–8 kbar (23–30 km depth, if lithostatic) from the lower section of the Neoproterozoic–Paleozoic passive margin sequence require burial and or repetition of the passive margin sequence by 2–3× stratigraphic depths. Our observations from the least migmatized and/or mylonitized parts of this complex, including field observations, a transect of peak-temperature (T_p) estimates, and critical evaluation of proposed thickening/burial mechanisms cannot account for such deep burial. From Neoproterozoic–Cambrian (Ꞓ) rocks part of a continuous stratigraphic section that transitions ~8 km upsection to unmetamorphosed Permian strata that were not buried, we obtained new quartz-in-garnet barometry via Raman analysis that suggest pressures of ~7 kbar (~26 km). A T_p traverse starting at the same basal Ꞓ rocks reveals a smooth but hot geothermal gradient of ≥40 °C/km that is inconsistent with deep burial. This observation is clearly at odds with thermal gradients implied by high P-T estimates that are all ≤25 °C/km. Remarkably similar discrepancies between pressure estimates and field observations have been discussed for the northern Snake Range metamorphic core complex, ~200 km to the southeast. We argue that a possible reconciliation of long-established field observations versus pressures estimated from a variety of barometry techniques is that the rocks experienced non-lithostatic tectonic overpressure. We illustrate how proposed mechanisms to structurally bury the rocks, as have been invoked to justify published high pressure estimates, are entirely atypical of the Cordillera hinterland and unlike structures interpreted from other analogous orogenic plateau hinterlands. Proposed overpressure mechanisms are relevant in the REWP, including impacts from deviatoric/differential stress considerations, tectonic mode switching, and the autoclave effect driven by dehydration melting. Simple mechanical arguments demonstrate how this overpressure could have been achieved. This study highlights that detailed field and structural restorations of the least strained rocks in an orogen are critical to evaluate the tectonic history of more deformed rocks.