Shallow Initial Dip for the Northern Snake Range Footwall: Constraints from Geothermobarometry

The northern Snake Range in eastern Nevada is a classic Cordilleran metamorphic core complex with a Tertiary-age subhorizontal detachment surface (the northern Snake Range Décollement – NSRD) that separates unmetamorphosed Paleozoic carbonates above from medium-grade Precambrian to Cambrian metasediments below. In order to constrain the initial dip of the NSRD, we have carried out a range-wide thermobarometric study of exhumed footwall rocks, where metapelitic mineral assemblages are appropriate for the GMBP barometer and the GARB thermometer. Our results show that P-T conditions along a transect across the center of the range are roughly constant at 8.0 ± 0.5 kbar and 612 ± 32°C in the direction of footwall transport, indicating that the presently exposed footwall rocks along this transect defined a subhorizontal surface at c. 30 km depth. The data also define a northward increase in burial depth, normal to the transport direction, from 22.5 ± 1.9 km to 30 ± 1.8 km over a fault-normal distance of c. 12 km.

The northward increase in metamorphic grade is consistent with the increase in grade between the southern and northern Snake Ranges, and may reflect differential exhumation during late Cretaceous or early Tertiary time. The lack of a gradient in burial depth in the direction of footwall transport suggests that the NSRD must have had a subhorizontal orientation when it cut the footwall rocks. We propose a two-stage model for footwall exhumation: (1) Coaxial stretching of the crust in the early Tertiary exhumed northern Snake Range footwall rocks from c. 30 km depth to mid-crustal levels. (2) Footwall rocks were captured in the early Miocene by a moderately-dipping NSRD that soled into a subhorizontal orientation in the middle crust. Final exhumation occurred by rotation about a rolling hinge, which tilted the rocks back into a subhorizontal orientation at the surface.