Low-Temperature Thermochronology of Laramide Ranges in Montana and Wyoming Provides Information on Exhumation and Tectonics Associated with Flat-Slab Subduction

Timing of exhumation of Laramide basement uplifts can be used as a proxy for tectonic processes associated with thick-skinned deformation resulting from flat-slab subduction. Despite its significance, the timing and pattern of Laramide deformation remains poorly constrained in Montana. Thermochronological data from Wyoming indicate exhumation of Laramide ranges during the late Cretaceous and Paleogene. Whereas a few data exist for the Bearthooth Range in Montana; the exhumation history of most of the Montana ranges remains unexplored preventing testing of current tectonic models. We report apatite fission track thermochronologic (AFT) data from modern river sands derived from Laramide ranges, bedrock basement samples, and synorogenic conglomerate clasts to determine the regional exhumation history of the Beartooth, Gravelly, Tobacco Root, Ruby, the Highland Mountains, and the Wind River Range. AFT permits reconstruction of thermal histories and rates of erosion of the upper few kilometers of the crust. In particular detrital AFT of river sands provides information on regional exhumation of the drainage area.

AFT detrital ages derived from the southern end of the Beartooth Range are dominated by a 60-80 Ma signal, consistent with ages reported for bedrock basement samples in the Beartooth Range. A Cenozoic synorogenic conglomerate clast was obtained from the Highland Mountains, AFT results show a 69.56 +/- 5.45 Ma cooling age. In the Wind River Range, Wyoming AFT data from a Cenozoic synorogenic conglomerate clast from the Wind River Formation indicates a 59.32 +/- 4.83 Ma cooling age. This age is consistent with AFT ages from Gannett Peak indicating rapid cooling at ~60 Ma and ~50 Ma (Fan and Carrapa, 2014). Overall, samples from the easternmost ranges, the Beartooth and Bighorn, clearly preserve a Cretaceous signal; samples from Wind River Range and the rest of southwest Montana mainly record a Cenozoic signal. This suggests deeper and younger exhumation to the west than to the east. These results combined with thermal modeling provide additional constraints on the tectono-thermal history of Laramide ranges. In addition, these results allow for a temporal-spatial comparison between cooling and exhumation in the Montana and Wyoming Laramide regions and help test current models of the Laramide Orogeny.