Orogens of Big Sky Country: reconstructing the deep-time tectonothermal history of the Beartooth Mountains, Montana and Wyoming, USA
Ronemus, Chance Baylor
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The southwestern Montana region has experienced a protracted history of orogeny, burial, and erosion recording the development of the western margin of Laurentia, the core of the North American continent. This > 2.5 Gyr record contains clues about the nature of Precambrian tectonism, the development of economic mineral and hydrocarbon reserves, and the long-term geodynamic evolution of Earth. However, aspects of this history remain enigmatic, with events in the geologic record obscured by erosion and thermal overprinting. The manuscript presented herein, bound by introductory and concluding chapters, comprises a deep-time thermochronologic investigation of the Beartooth Mountains. New biotite 40 Ar/39 Ar, and zircon U-Pb and (U-Th)/He data are presented from 14 samples collected from the Montana part of the range. These data indicate that thermal effects of Paleoproterozoic thermotectonism associated with the Big Sky orogeny (ca. 1.8-1.71 Ga) and/or related mantle metasomatism or mafic underplating penetrated into the core of these mountains. Thermal history model results indicate that this region of the craton experienced multi-phase Proterozoic cooling. The first phase of this cooling is generally coeval with the collapse of the Big Sky orogen. A second phase of Proterozoic cooling culminated in the development of the Great Unconformity surface, across which > 2 Gyr is regionally 'missing' from the stratigraphic record. Constraints placing this latter phase between 1.4 Ga and 0.8 Ga preclude mechanisms predicting later Neoproterozoic-Cambrian cooling, such as erosion associated with Snowball Earth glaciation, and support diachronous development of the Great Unconformity surface in Laurentia. Thermal models resolve a Phanerozoic thermal history involving maximum burial temperatures by late Pennsylvanian time and cooling throughout Mesozoic time. This Phanerozoic thermal history, broadly out of sync with nearby basins, underscores the effects of interactions between far-field tectonism and inherited crustal weaknesses in the Beartooth Mountains and reconciles previous interpretations of pre-Late Cretaceous cooling with other evidence only constraining later phases of uplift. Finally, model results suggest Cenozoic reheating--likely due to burial by volcanics--and later cooling to surface temperatures due to erosional removal of these rocks--potentially related to encroachment of the Yellowstone hotspot and/or regional Basin and Range extension.