POSSIBLE EVIDENCE FOR LARGE DEPARTURES FROM LITHOSTATIC PRESSURE DURING CRETACEOUS METAMORPHISM IN THE NORTHERN SNAKE RANGE METAMORPHIC CORE COMPLEX (MCC), NEVADA

A major point of contention in studies of MCC’s within the western U.S. Cordillera is a disagreement between estimates of peak pressure of metamorphic assemblages in footwall rocks and the depths to/from which such rocks were buried/exhumed as constrained by map relations and palinspastic restorations: the former suggesting depths nearly twice the latter. These discrepancies usually imply that one or the other data set (and/or its interpretation) is in error, i.e.: 1) the critical structures that bury and exhume these rocks from depth simply haven’t been found yet; or 2) the geobarometric methods employed are unreliable indicators of paleo-pressure because of inapplicable thermodynamic assumptions or over-simplifications compared to real systems. New data sets from the northern Snake Range metamorphic core complex, Nevada, were collected to test these ideas but instead only add to the contradictions. For example, Raman spectroscopy of quartz inclusions in garnet and Ti in quartz thermobarometry appear to replicate the high pressures (>7 kbar) previously estimated for these rocks, whereas Ti diffusion profiles from these same quartz inclusions imply timescales for metamorphism may have been too fast to be due to tectonic loading alone. Also, detrital zircon U-Pb geochronology combined with finite strain estimates show that pre-strain thicknesses of the lower plate units exhibiting high pressure assemblages correspond more closely to the thicknesses of equivalent age units in adjacent ranges rather than to the those of the inferred structurally underlying (para) autochthon, which is inconsistent with an origin deep in the crust. In light of these and other varied data sets we suggest an alternative interpretation be considered, which is that 3) short-lived pulses of super-lithostatic conditions (i.e. tectonic overpressure) may have been induced above regions of high heat flow and partial melting in the retroarc region, most pronounced at the base of regional thrust-bound allochthons. In this scenario, tectonic overpressure may have locally generated departures from lithostatic pressures by up to a factor of two. If 3) is correct, MCC’s would not represent exposures of ~ 25-35 km deep crust, nor would they indicate extreme extension implied by the low-angle fault model as is also commonly assumed.