Judson Mead Geologic Field Station

Applications of zircon (U-Th)/He thermochronology to determine exhumation histories and constrain the transition from compressional to extensional tectonic regimes in southwest Montana.

The Tobacco Root Mountains, part of the Rocky Mountains, are an area that has been influenced by fold-and-thrust style deformations with both thick-skinned (Laramide-style) and thin-skinned (Sevier-style) deformation, as well as extensional deformation (Basin and Range). In the central and southern Rocky Mountains, Sevier-style deformations are the oldest, followed by Laramide-style and, ultimately, Basin and Range extension. In the Tobacco Root Mountains, though, Sevier-style deformations are younger than Laramide-style deformations.

For my research, I will use low temperature zircon (U-Th)/He thermochronology to interpret the cooling history of the Elkhorn Mountain Volcanics in the footwalls of three faults: the Mayflower Mine Sevier-style thrust fault, the Jefferson Canyon Sevier-style thrust fault, and a Basin and Range normal fault that truncates the western extent of the Mayflower Mine fault. By analyzing the cooling history, I will be able to determine the exhumation rates of the Elkhorn Mountain Volcanics along each fault.

Furthermore, I hope to use the cooling histories as a proxy for the age of faulting; the time of increased cooling rates may help constrain the age of normal faulting as well as the ages of the onset of erosion that are associated with the increase in topography as a result of thrust faulting. This could ultimately constrain the transition time from the compressional tectonic regime of the Sevier Orogeny to the extensional tectonic regime of Basin and Range.