HR: 1340h
AN: T13C-0488    [Abstracts]
TI: Tilting History of the Northern Snake Range Decollement From Footwall Geothermobarometry
AU: * Cooper, F J
EM: fcooper@usc.edu
AF: USC Dept of Earth Sciences, 3651 Trousdale Parkway, Los Angeles, CA 90089-0740 United States
AU: Platt, J P
EM: jplatt@usc.edu
AF: USC Dept of Earth Sciences, 3651 Trousdale Parkway, Los Angeles, CA 90089-0740 United States
AB: The Northern Snake Range in eastern Nevada is a classic Cordilleran metamorphic core complex with a low-angle domiform detachment surface (the Northern Snake Range Decollement - NSRD) separating unmetamorphosed Paleozoic carbonates above from medium-grade Precambrian to Cambrian metasediments below. The origin and evolution of the subhorizontal NSRD is controversial, with a key debate centering on the dip of the NSRD when it was active. 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 NSRD represents a subhorizontal zone of brittle-ductile decoupling) to a low-angle normal fault with <60 km of displacement. We address this problem by establishing the tilting history of the NSRD through detailed thermobarometric analyses of pre-NSRD metamorphic phase assemblages and mineral chemistry of the footwall rocks across the range. These should reveal any gradient in depth of burial in the direction of NSRD motion, and thus its tilting history. Previous thermobarometric work is limited to a single study on the eastern side of the range, where a calculated T of 610°C and P of 810 MPa (equivalent to a burial depth of ~30 km) suggest that the NSRD has brought rocks from considerably greater depths than would be true for an exhumed brittle-ductile transition. We are building on this previous work by expanding the area of study to the whole Northern Snake Range, where mineral assemblages within the footwall rocks are appropriate for barometers such as GASP, GRAIL, GRIPS and GMBP, while thermometry will mostly be established from garnet-biotite Fe-Mg exchange equilibria. Preliminary work has identified suitable rocks with equilibrium assemblages and textures in small domains from most of the main drainages in the Snake Range. Assuming equilibrium, pilot samples from the E side give pressures in agreement with previous estimates, while T estimates from both sides of the range indicate an increase from garnet cores to rims of ~430 to 570°C in the E and ~400 to 500°C in the W. Further work will investigate this T change through prograde history maps of the garnets in P-T space.
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3625 Petrography, microstructures, and textures
DE: 3651 Thermobarometry
DE: 3660 Metamorphic petrology
DE: 8109 Continental tectonics: extensional (0905)
SC: Tectonophysics [T]
MN: Fall Meeting 2005