THERMOCHRONOLOGICAL TESTS OF THE GLACIAL AND TECTONIC ORIGIN HYPOTHESES FOR THE NORTH AMERICAN GREAT UNCONFORMITY

The formation of the Great Unconformity has been attributed to Snowball Earth glaciations between 717–635 Ma and Neoproterozoic tectonism prior to and after the Cryogenian. The unconformity hypotheses are not mutually exclusive and their general applicability is spatially dependent. Hypothesis-test modeling that integrates multiple thermochronometers is the most robust approach to explore a broad range of time and temperature by leveraging combined chronometer sensitivities to resolve more detailed thermal histories in deep time. Published inversions that average chronometer data, utilize a single low-temperature thermochronometer, or impose many interpretive assumptions in models cannot be relied upon to yield entirely accurate thermal histories. Modeling methodology, thermochronometer resolution, and tectonic setting explain discrepancies in the timing of Neoproterozoic rock cooling in North American studies. While orogenesis and rifting are fully expected to produce thermochronologically resolvable exhumation at continental margins, deep exhumation of stable cratonic interiors cannot be plausibly attributed to active tectonics and would instead be mostly driven by tectonically independent surface processes, namely glaciation. Laurentian margin locations in south/western North America such as the Ozark Plateau, Pikes Peak, Grand Canyon, and the Rocky Mountains were undergoing deformation during supercontinent Rodinia assembly and breakup from ca. 1300–900 Ma and 800–600 Ma—yet cooling at all sites is still compatible with Cryogenian glacial erosion. Late Neoproterozoic tectonism did not occur in the cratonic interior, thus providing the best potential hypothesis test. Bayesian inversions of multisystem thermochronometric data from the Canadian Shield and interior Laurentia are consistent with a significant Cryogenian glacial contribution (>3–6 km of unroofing) towards development of the Great Unconformity. Major unconformities are inherently formed and preserved due to the creation of widespread accommodation (and sedimentation) space involving large-scale processes such as glaciation. We suggest that the most important tectonic contributions to the formation of the Great Unconformity may have been those that potentially contributed to the onset of Snowball Earth.