APATITE FISSION-TRACK EVIDENCE FOR LARAMIDE AND POSTLARAMIDE UPLIFT AND ANOMALOUS THERMAL REGIME AT THE BEARTOOTH OVERTHRUST, MONTANA-WYOMING

The crystalline rocks of the Beartooth overthrust in southwest Montana and northwest Wyoming were uplifted and thrust over the northwest mar gin of the Bighorn basin during the Laramide orogeny. The timing and g eometry of that event is well documented in the synorogenic Paleocene and early Eocene sedimentary rocks that were deposited ahead of the ri sing Beartooth basement block and partially overridden as the block ad vanced. The absence of post-Laramide sedimentary rocks on the block an d in the adjacent Bighorn Basin has, however, precluded the reconstruc tion of the post-orogenic tectono-thermal history of this region. We r eport here the results of fission-track analysis of apatite from the t hree components of the Beartooth overthrust with the aim of reconstruc ting the Laramide and post-Laramide tectono-thermal history of the sou theast margin of the Beartooth Mountains. Apatite fission-track ages a nd corresponding horizontal confined track length distributions (HCTLD s) from Precambrian basement rocks constituting the upper plate of the Beartooth overthrust indicate that from 7 to 12 km of uplift of the R ed Lodge corner of the Beartooth block has occurred since early Paleoc ene time. This amount of uplift occurred in two stages, with an interv ening mid-Tertiary period of tectonic quiescence. The latter was a per iod of either (1) Oligocene and/or Miocene deposition or (2) tectonic quiescence. Uplift of 4 to 8 km occurred during the first phase of coo ling, which lasted from early Paleocene time (approximately 61 Ma) to early Eocene time (approximately 52 Ma). During the second phase, whic h began in late Miocene-early Pliocene time and continues to the prese nt day, about 4 km of uplift occurred. Our fission-track data suggest that the thermal regime in rocks above the Beartooth overthrust was re latively stable during Tertiary time. The maximum geothermal gradient permitted by model thermal histories generated from our observed fissi on-track data during post-Laramide time is 17-degrees-C/km. This value is the same as that of the present-day geothermal gradient measured i n the Amoco Beartooth well and suggests that a low geothermal gradient prevailed throughout Tertiary time. Apatite fission-track ages and HC TLDs from Jurassic and Cretaceous sedimentary rocks beneath the Bearto oth overthrust indicate that these rocks have remained 10-20-degrees-C cooler than overlying rocks in the shear zone and the lowermost part of the upper plate since approximately 61 Ma, an observation that is n ot consistent with the stratigraphic position of these rocks. We inter pret this temperature to be the result of a persistent thermal regime in which ground water circulating through the sedimentary rocks of the lower plate both cooled them and insulated them from conductive heat transfer from hotter overlying rocks; this ground-water circulation ma y have been responsible for flushing hydrocarbons out of the rock colu mn explored by the Beartooth well. Shear-zone rocks experienced a high er temperature during Cenozoic time than did rocks of the upper and lo wer plates; this condition was maintained by flow through the shear zo ne of ground water heated to higher temperatures at deeper levels alon g the thrust.