TIMING OF EXHUMATION OF LARAMIDE RANGES IN MONTANA CONSTRAINED BY DETRITAL AND IN-SITU APATITE FISSION TRACK THERMOCHRONOLOGY

The Laramide Orogeny resulted from flat-slab subduction processes responsible for thick-skinned deformation, exhumation of basement involved uplifts and disruption of the regional foreland basin. The timing and pattern of Laramide deformation is currently debated. 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 a synorogenic conglomerate clast to determine the regional exhumation history of these ranges: the Beartooth, Gravelly, Tobacco Root, Ruby and the Highland Mountains located in southwest Montana. 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.

An AFT detrital sample collected near the southern end of the Beartooth Range is dominated by 60-80 Ma ages, consistent with ages reported for bedrock basement samples in the Beartooth Range. River samples derived from the Beartooth and Bighorn Ranges preserve Cretaceous ages. A basement sample collected from the Gravelly Range indicates a 49.35 +/- 3.48 Ma cooling age. A synorogenic conglomerate clast and a basement sample were obtained from the Highland Mountains; AFT results show a 69.56 +/- 5.45 Ma cooling age for the syn-orogenic conglomerate clast and a 62.00 +/- 4.12 Ma cooling age for the basement sample. Overall, samples from the easternmost ranges, the Beartooth and Bighorn, clearly preserve a Cretaceous signal; samples from 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 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.