Source: Geochemistry, Geophysics, Geosystems
The Great Unconformity remains one of the most alluring mysteries in the Earth sciences. Widespread across North America and present in other parts of the world, the knife-sharp contact between Precambrian crystalline rocks and overlying Phanerozoic sediments can represent a billion years (a quarter of Earth’s history) of missing time.
Precisely because of this missing record, the formation age and mechanism of the Great Unconformity have proven elusive. Formation of the Great Unconformity has been linked to all major tectonic, climatic, and biologic events of the Late Proterozoic – Early Paleozoic, including the assembly and break-up of Rodinia, the Snowball Earth glaciation, and the Cambrian explosion of life.
Thermochronology records the cooling of rocks as they are exhumed to the surface of the Earth and thereby provides one of the few direct proxies for erosion events at geological timescales. Applying thermochronology in “deep time” is challenging, however, because small differences in the kinetics that define how daughter products are retained between the different crystals analyzed can lead to strongly varying ages.
Sturrock et al.  leverage these differences, using apatite (U-Th)/He thermochronology and quantitative thermal-history modeling to constrain the thermal and erosional history of a large tract of the Central Canadian Shield. They convincingly show that the Great Unconformity there formed after 650 million years and link its formation to kilometer-scale erosion in response to mantle-plume related uplift.
Previous work, in part by the same group, has shown that erosion leading to formation of the Great Unconformity in other parts of North America (Wyoming, the Colorado Front Range and the Ozarks) is substantially older, between approximately 850 and 700 million years ago. Thus, there may not be one but several Great Unconformities. This study adds to a growing body of work refuting hypotheses that have suggested that the Great Unconformity occurred in a single worldwide event and was linked to major Earth crises, such as the Snowball Earth glaciation.
Citation: Sturrock, C. P., Flowers, R. M., & Macdonald, F. A. . The late Great Unconformity of the central Canadian Shield. Geochemistry, Geophysics, Geosystems, 22, e2020GC009567. https://doi.org/10.1029/2020GC009567
—Peter van der Beek, Editor, Geochemistry, Geophysics, Geosystems
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