Orogens of Big Sky Country: Reconstructing the Deep‐Time Tectonothermal History of the Beartooth Mountains, Montana and Wyoming, USA

Archean rocks exposed in the Beartooth Mountains, Montana and Wyoming, have experienced a complex >2.5 Gyr thermal history related to the long-term geodynamic evolution of Laurentia. We constrain this history using "deep-time" thermochronology, reporting zircon U-Pb, biotite Ar-40/ Ar-39, and zircon and apatite [U-Th(-Sm)]/He results from three transects across the basement-core of the range. Our central transect yielded a zircon U-Pb concordia age of 2,805.6 +/- 6.4 Ma. Biotite 40Ar/39Ar plateau ages from western samples are <= 1,775 +/- 27 Ma, while those from samples further east are >= 2,263 +/- 76 Ma. Zircon (U-Th)/He dates span 686.4 +/- 11.9 to 13.5 +/- 0.3 Ma and show a negative relationship with effective uranium-a proxy for radiation damage. Apatite (U-Th)/He dates are 109.2 +/- 23.9 to 43.6 +/- 1.9 Ma and correlate with sample elevation. Multi-chronometer Bayesian time-temperature inversions suggest: (a) Cooling between similar to 1.90 and similar to 1.80 Ga, likely related to Big Sky orogeny thermal effects; (b) Reheating between similar to 1.80 Ga and similar to 1.35 Ga consistent with Mesoproterozoic burial; (c) Cooling to =100 degrees C between Mesoproterozoic and early Paleozoic time, likely reflecting continental erosion; (d) Variable Paleozoic-Jurassic cooling, possibly related to Paleozoic tectonism and/or low eustatic sea level; (e) Rapid Cretaceous-Paleocene cooling, preceding accepted proxies for flat-slab subduction; (f) Eocene-Miocene reheating consistent with reburial by Cenozoic volcanics and/or sediments; (g) Post-20 Ma cooling consistent with Neogene development of topographic relief. Our results emphasize the utility of multi-chronometer thermochronology in recovering complex, non-monotonic multi-billion-year thermal histories. Plain Language Summary The Beartooth Mountains in southern Montana and northern Wyoming consist of some of the oldest rocks in North America. These rocks have been exhumed to Earth's surface and reburied numerous times. However, sedimentary rocks recording many of these events have been eroded, erasing much of the primary record of the geologic history of this region. To recover this history, we use minerals within existing rocks that act as heat-sensitive clocks. These "clocks" record a cooling date, reflecting the last time the rock was within a specific temperature range. We measure cooling dates for several different types of minerals and model the history of temperature fluctuations they record, corresponding to burial and erosion events. We interpret the results of these models by comparing them with the geologic history recorded in nearby preserved sedimentary rocks. We find that the Beartooth Mountains were involved in mountain-building events not previously recognized and cooling associated with building of the modern Rocky Mountains began earlier, and is more complex, than is generally recognized. The ability to recover such long histories of temperature fluctuations is important, suggesting we may be able to learn about events far in Earth's past even when the rock record is missing.