HR: 16:30h
AN: V34C-03    [Abstracts]
TI: Adding t to garnet P-T paths using the Lu-Hf method: Age constraints on thrusting and exhumation events in the hinterland of the Sevier orogen
AU: * Cruz-Uribe, A M
EM: Alicia.M.Cruz-Uribe@alum.dartmouth.org
AF: Northern Arizona University, PO Box 4099, Flagstaff, AZ 86011, United States
AU: Hoisch, T D
EM: Thomas.Hoisch@nau.edu
AF: Northern Arizona University, PO Box 4099, Flagstaff, AZ 86011, United States
AU: Wells, M L
EM: mlwells@unlv.nevada.edu
AF: University of Nevada Las Vegas, 4505 S. Maryland Parkway, Las Vegas, NV 89154-4010, United States
AU: Vervoort, J D
EM: vervoort@wsu.edu
AF: Washington State University, 1228 Webster Physical Sciences Bldg, Pullman, WA 99164, United States
AB: In orogenic belts, garnets in metamorphic rocks grow in response to changes in pressure and temperature (P-T) conditions that result from burial and exhumation. Thermodynamic modeling may be used to extract P-T paths from garnet growth zoning provided temperature thresholds for cation diffusion are not exceeded and the garnets grew in equilibrium. Garnet dating by the Lu-Hf method provides a powerful method to date garnet growth and thus date tectonic events. Here we apply these techniques to constrain the tectonic history of the hinterland of the Sevier orogen. Previous work from the Albion Range, Idaho, has determined a P-T path from the schist of Mahogany Peaks consisting of an isothermal pressure increase, followed by a pressure decrease during a temperature increase, followed by second isothermal pressure increase. We interpret the isothermal pressure increases to be the result of tectonic burial by thrusting along the Basin-Elba fault and the exhumation event to be the result of combined extensional and erosional exhumation. To place age constraints on these events we have determined a garnet Lu-Hf age for the same sample on which we determined a P-T path: 150 ± 12 Ma (2σ; MSWD = 9.1) based on 3 garnet fractions and a whole rock. The same unit 50 km away in the western Raft River Mountains in Utah yielded an identical but much more precise age of 149.1 ± 1.2 Ma (2σ; MSWD = 1.1), also based on three garnet fractions and a whole rock. Both determinations constrain a Late Jurassic age for these events over a broad area. This is considerably older than the 105 ± 12 (2σ) Ma age recently determined for the oldest pervasive fabric (D1) in the core complex by dating strain fringes using the Ar-Ar laserprobe method, which indicates that development of the fabric occurred much later than major thrusting, consistent with a recent interpretation of this fabric developing by orogen-parallel gravitationally induced extension. The Lu-Hf age constraints provide the first clear evidence for major Late Jurassic thrust burial in the hinterland of the Sevier belt. This helps resolve important questions including the timing of hinterland to foreland thrust progression, and the presence and location of hinterland thrust loads necessary to explain otherwise enigmatic Late Jurassic subsidence histories in foreland sediments.
DE: 1040 Radiogenic isotope geochemistry
DE: 1115 Radioisotope geochronology
DE: 3652 Pressure-temperature-time paths
DE: 8102 Continental contractional orogenic belts and inversion tectonics
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting