THERMOBAROMETRIC CONSTRAINTS ON THE TECTONIC EVOLUTION OF THE NORTHERN SNAKE RANGE METAMORPHIC CORE COMPLEX, EASTERN NEVADA

The northern Snake Range in eastern Nevada is a classic Cordilleran metamorphic core complex with a subhorizontal detachment surface (the northern Snake Range Décollement – NSRD) that separates unmetamorphosed Paleozoic carbonates above from medium-grade Precambrian to Cambrian metasediments below. The origin and evolution of the NSRD is controversial, with a key debate centering on its initial dip and total displacement. Current estimates of initial dip vary from subhorizontal (based on consistent stratigraphic position of the NSRD) to 40° (based on a progressive westward increase in Ar-Ar and fission track ages), while displacement estimates vary from 10 km (assuming the NSRD represents a subhorizontal zone of brittle-ductile decoupling) to a low-angle normal fault with ≥60 km of slip.

In order to better constrain the initial dip and penetration depth of the NSRD, we have carried out a range-wide thermobarometric study of exhumed footwall rocks, where pre-NSRD, late Cretaceous metapelitic mineral assemblages are appropriate for the GMBP and GASP barometers and the GARB thermometer. This approach is designed to establish the initial burial depth of rocks throughout the footwall, and hence to constrain any possible gradient in burial depths both normal and parallel to the footwall transport direction. Our results confirm previous thermobarometric work indicating PT conditions of 8.1 kbar and 610°C on the eastern side of the range, corresponding to a burial depth of ~30 km at the time of the Late Cretaceous metamorphic peak. Our new data show that this burial depth is constant to the west in the direction of footwall transport. We also observe a northward increase in burial depth, from ~20 km to ~30 km over a fault-normal distance of ~10 km.

The northward increase in metamorphic grade is consistent with the increase in grade observed between the southern and northern Snake Ranges, and with the higher grade observed in the Ruby Mountains further to the north. This gradient is an enigmatic late Cretaceous feature that is not obviously explained by geotectonic models of the region. Further work is planned to determine the geometry of isobaric surfaces in the northern Snake Range, and to constrain any depth-independent temperature gradients.