Abstract

The crystalline rocks of the Beartooth and Bighorn Mountains in southwest Montana and northwest Wyoming were uplifted and thrust over the margins 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 basement blocks and partially overridden as those blocks advanced. The absence of post-Laramide sedimentary rocks on the blocks and in the adjacent Bighorn Basin has, however, precluded the reconstruction of the post-orogenic tectonothermal history of this region.

From apatite fission-track analysis of 30 samples from the Beartooth Mountains, 103 samples from the Bighorn Mountains, and 10 samples from sedimentary rocks in the intermontane basins, we conclude the following:

1. Precambrian basement rocks constituting the upper plate of the Beartooth overthrust have risen from 7 to 12 km with respect to the adjacent Bighorn Basin since early Paleocene time. This uplift occurred in two stages: Four to 8 km of uplift occurred during the first phase of cooling, which lasted from ~61 Ma ago (early Paleocene time) to ~52 Ma ago (early Eocene time). During the second phase, which began in late Miocene-early Pliocene time and continues to the present day, ~4 km of uplift occurred.

2. Four distinct events of rapid cooling due to uplift and erosion are documented in the crystalline rocks of the Bighorn Mountains, at ~144 Ma, 111-120 Ma, ~90 Ma, and ~57 Ma. Only the last of these events is temporally consistent with conventional interpretations of the Laramide event.

3. A thermal event of yet-unknown origin affected sedimentary rocks on the floor of the Bighorn Basin ~52 Ma ago. This event is not documented in two samples analyzed from the Powder River Basin. A swarm of mafic dikes was intruded into Paleocene rocks of the Fort Union Formation near Red Lodge 21.2 Ma ago. No other evidence of a thermal event at this time is known from the region.