Integrated multisystem thermochronology resolves the deep-time evolution of the Great Unconformity surface in the Upper Midwest, USA

Deep-time low-temperature thermochronology offers the ability to unravel the >1 b.y. thermal histories of Precambrian rocks and to better understand mid- to upper-crustal geologic processes that span temperatures up to ∼350 °C. This approach can provide insight into the timing of the Great Unconformity surface formation in the U.S. Upper Midwest cratonic interior, which is poorly constrained. We present new medium- to low-temperature thermochronometric datasets from Precambrian basement samples across the Upper Midwest, specifically in Wisconsin and the Upper Peninsula of Michigan. Our data include new biotite 40Ar/39Ar, K-feldspar 40Ar/39Ar, zircon and apatite (U-Th-Sm)/He, and apatite fission-track analyses. We present thermal history models from Thermochron.jl inversions that integrate data from all available thermochronometric systems for each sample location in the U.S. Upper Midwest. These models show variable late Mesoproterozoic and Neoproterozoic cooling histories, although the timing of reheating and cooling in several samples broadly coincides across all sample locations. In the late Mesoproterozoic−early Neoproterozoic, our thermal history models suggest cooling pulse(s) (and by inference, exhumation) took place ca. 1.1−1.0 Ga. These coincided with the opening of the 1.1 Ga Midcontinent rift and the development of the 1.09−0.98 Ga Grenville orogen, suggesting that cooling was due to exhumation either associated with development of a broad rift-margin uplift and/or with the development of basement-cored uplift and erosion due to continental-interior fault inversion due to Grenville compression. Subsequent to this event, our models show variable degrees of reheating in the early Neoproterozoic, possibly as a consequence of widespread early Neoproterozoic Grenvillian foreland sedimentation. Removal of these sediments in the late Neoproterozoic caused another cooling event. Taken together, our results demonstrate a complex history of cooling and heating for the basement rocks directly underlying the current Great Unconformity in the U.S. Upper Midwest. Our low-temperature thermochronometers and models also record a significant reheating and cooling event in the Paleozoic−early Mesozoic, which we attribute to both sedimentary burial and unroofing and to late Paleozoic basin-scale hot brine migration that was focused in high-permeability pathways along the Great Unconformity surface. Overall, our work provides further insight into the ways in which cratonic interiors—regions not subject to metamorphism and fabric development—record distinct thermal events associated with tectonism subsequent to cratonization. Our work also characterizes events contributing to the development of the Great Unconformity and emphasizes that an integrated multisystem thermochronometric approach is essential in resolving the dates and rates of cooling and reheating events in deep time.