TERTIARY UPLIFT AND EROSION HISTORY OF THE BEARTOOTH OVERTHRUST, MONTANA-WYOMING, FROM MODEL TIME-TEMPERATURE PATHS OF APATITE FISSION-TRACK PARAMETERS

The crystalline rocks in the upper plate of the Beartooth overthrust in southwest Montana and northwest Wyoming were uplifted and thrust over the northwest margin of the Bighorn Basin during the Laramide orogeny. The model time-temperature histories of fission-track parameters reported in Omar et.al. (1994) indicate that, since early Tertiary time, vertical movement of the Red Lodge corner of the upper plate of the Beartooth overthrust occurred in two stages. The first stage began in early Paleocene time (~57 ± 3 Ma); the second stage began in early Middle Miocene time (~15 ± 2 Ma). We also used a backstacking apatite FT approach to separate the amount of tectonic uplift (uplift due to tectonic processes) from the amount of uplift that resulted from isostatic rebound (uplift due to removal of overburden by erosion) for each of the uplift phases. Our results indicate that 1.9 km of tectonic uplift and ~4.5 km of isostatic rebound occurred during the first phase, and that all differential movement between the upper and lower plates of the Beartooth fault took place during Laramide time. During the second phase ~2.6 km of tectonic uplift and ~0.8 km of isostatic rebound took place. Uplift in the second phase is only partly compensated for by erosion. This conclusion is consistent with the present-day high elevation and relief of the Beartooth front. Correlation of our results with published accounts of reconstructed Neogene stratigraphic, volcanic and structural events in the western US leads us to propose that this event was regional in nature. If we assume reasonable uplift rates, the second phase of uplift and erosion may have created a regional highland of significant elevation and areal extent by 10-7 Ma that was responsible for regional change in climate, as evidenced by abundant physical and biological evidence in sedimentary basins in the western US.