APPLICATIONS OF ZIRCON (U-TH)/HE THERMOCHRONOLOGY OF THE ELKHORN MOUNTAIN VOLCANICS TO DETERMINE LOCAL VARIATIONS IN COOLING HISTORIES IN SOUTHWEST MONTANA

The Tobacco Root Mountains in southwest Montana, part of the western Cordillera, is an area with a complex deformational history consisting of thin-skinned Sevier-style thrusts, thick-skinned Laramide-style thrusts, and extensional deformation. While a majority of the Rocky Mountains displays a progression from Sevier-style to Laramide-style to extensional deformations, the structures in the Tobacco Root Mountains display Sevier-style deformations that truncate Laramide-style deformation and are, therefore, younger than the thick-skinned thrusts. Normal faults cross-cut both styles of thrust faults and is the youngest large-scale deformation in the area.

The Cretaceous Elkhorn Mountain Volcanics (EMV), a unit comprised of volcaniclastic conglomerates, volcaniclastic sandstones, basaltic flows, and rhyolitic tuffs, crops out as the youngest unit in the footwall of large-scale (~1.5-2 km throw) Sevier-style thrusts and normal faults. To better understand the surface and structural evolutions of the area, I collected samples of the EMV from two locations for zircon (U-Th)/He analyses. One suite of samples is from the footwall of the Jefferson Canyon Thrust Fault and the second suite of samples is from the footwall of the normal fault that truncates the Mayflower Mine Thrust Fault in Bone Basin. I suggest that although both locations experienced similar amounts of reburial from their respective thrust faults, the dominant cause of exhumation differs: exhumation is more directly a result of erosion in the Jefferson Canyon area while normal faulting is the main form of exhumation in Bone Basin. Here I present preliminary thermochronologic results on the influence of differing exhumation styles on the cooling history of the EMV and the implications for the local surface and structural evolutions of the Tobacco Root Mountains.