NEOTECTONIC UPLIFT OF THE BEARTOOTH PLATEAU ASSESSED USING THE (U-TH)/HE DATING METHOD

The progressive movement of the North American plate over Yellowstone hotspot has resulted in major landscape restructuring, extensive faulting and uplift for over 16Ma or more. The Beartooth Plateau, an 80km by 40km wide section of Precambrian basement rock, is located directly at the mouth of the encroaching Yellowstone hotspot. In this study we collected 20 samples over vertical transects along the Plateau for the apatite (U-Th)/He dating method. In addition, we used the existing Fission Track data provided by Omar et al. (1994).

Apatite (U-Th)/He and Fission Track data from the Beartooth Plateau implies several stages of cooling. The first stage is related to the Laramide style orogeny which affected southwest Montana between 60 and 50Ma. This early cooling history, best seen in modeled fission tracks, indicates cooling past the apatite Partial Annealing Zone during that 60-50Ma time period. Information taken from the track lengths and the sample temperatures in the borehole near Red Lodge points to the second and most recent cooling which commenced around 15Ma. Several modern computer modeling techniques allow us to model data more accurately and indicates a high probability of cooling in the Miocene.

Using the highly temperature sensitive thermochronometer apatite (U-Th)/He, we are able to confirm this recent uplift of the plateau. Computer generated models of the He data shows a clear shift in the cooling history around 15Ma which continues on until the present. Sedimentary records from the adjacent Big Horn Basin, GIS modeling of the region and evidence of fault reactivation, povide further evidence that the impinging Yellowstone hotspot has uplifted and cooled the Beartooth Plateau since the Miocene, just as it has done to many other mountain ranges along its destructive track across the continent.