Abstract

The crystalline rocks 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 timing and geometry of that event is well documented in the synorogenic Paleocene and early Eocene sedimentary rocks that were deposited ahead of the rising Beartooth basement block and partially over-ridden as the block advanced. The absence of post-Laramide sedimentary rocks on the block and in the adjacent Bighorn Basin has, however, precluded the reconstruction of the post-orogenic tectonothermal history of this region. We report here the results of fission-track analysis of apatite from the three components of the Beartooth over-thrust with the aim of reconstructing the Laramide and post-Laramide tectono-thermal history of the southeast margin of the Beartooth Mountains.

Apatite fission-track ages and corresponding horizontal confined track length distributions (HCTLDs) from Precambrian basement rocks constituting the upper plate of the Beartooth overthrust indicate that from 7 to 12 km of uplift of the Red Lodge corner of the Beartooth block has occurred since early Paleocene time. This amount of uplift occurred in two stages, with an intervening mid-Tertiary period of tectonic quiescence. The latter was a period of either (1) Oligocene and/or Miocene deposition or (2) tectonic quiescence. Uplift of 4 to 8 km occurred during the first phase of cooling, which lasted from early Paleocene time (∼61 Ma) to early Eocene time (∼52 Ma). During the second phase, which began in late Miocene-early Pliocene time and continues to the present day, about 4 km of uplift occurred.

Our fission-track data suggest that the thermal regime in rocks above the Beartooth overthrust was relatively stable during Tertiary time. The maximum geothermal gradient permitted by model thermal histories generated from our observed fission-track data during post-Laramide time is 17 °C/km. This value is the same as that of the present-day geothermal gradient measured in the Amoco Beartooth well and suggests that a low geothermal gradient prevailed throughout Tertiary time.

Apatite fission-track ages and HCTLDs from Jurassic and Cretaceous sedimentary rocks beneath the Beartooth overthrust indicate that these rocks have remained 10-20 °C cooler than overlying rocks in the shear zone and the lowermost part of the upper plate since ∼61 Ma, an observation that is not consistent with the stratigraphic position of these rocks. We interpret this temperature to be the result of a persistent thermal regime in which ground water circulating through the sedimentary rocks of the lower plate cooled them and insulated them from conductive heat transfer from hotter overlying rocks; this ground-water circulation may have been responsible for flushing hydrocarbons out of the rock column explored by the Beartooth well. Shear-zone rocks experienced a higher temperature during Cenozoic time than did rocks of the upper and lower plates; this condition was maintained by flow through the shear zone of ground water heated to higher temperatures at deeper levels along the thrust.

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