Orogens of Big Sky Country: Reconstructing the Deep‐Time Tectonothermal History of the Beartooth Mountains, Montana and Wyoming, USA
Chance Ronemus, Devon Orme, William R. Guenthner, Stephen E. Cox, Christopher Kussmaul
Abstract
Abstract 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 40 Ar/ 39 Ar, 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 40 Ar/ 39 Ar 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 ∼1.90 and ∼1.80 Ga, likely related to Big Sky orogeny thermal effects; (b) Reheating between ∼1.80 Ga and ∼1.35 Ga consistent with Mesoproterozoic burial; (c) Cooling to ≤100°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.