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