LONG-TERM THERMAL HISTORY OF THE PRECAMBRIAN BASEMENT IN THE US GREAT PLAINS USING ZIRCON (U-TH)/HE THERMOCHRONOLOGY FROM BORE-HOLE SAMPLES

The timing and magnitude of exhumation of North American Precambrian basement surface, directly below Phanerozoic sedimentary cover sequences (i.e., the Great Unconformity), has possible implications for tectonic processes that link surface movements to supercontinent-related mantle dynamics and dramatic changes to Earth’s climate and ocean chemistry. To investigate the Proterozoic cooling (i.e., exhumation) history of the Precambrian crystalline basement within the U.S. Great Plains and part of the Canadian Shield, we collected zircon (U-Th)/He data (n=98 grains) from an ~4200 km long transect that encompasses 9 widely-spaced basement-penetrating bore holes in the states of New Mexico, Texas, Missouri, Kansas, Illinois, Nebraska, Iowa, and South Dakota and the Saskatchewan province. This transect provides the spatial coverage necessary to test various processes, such as snowball Earth glaciations and Rodinia mantle dynamics, that may have created the Great Unconformity erosional surface. Our zircon (U-Th)/He data show dominant negative trends between single aliquot date and effective uranium (eU, proportional to radiation damage), which we further evaluated with thermal history modeling in QTQt, which accounts for the damage-He diffusivity relationship in zircon. Our preliminary modeling results in Texas, New Mexico, South Dakota, and Saskatchewan, show rapid cooling (~40°C/Ma) from ~400°C to 80 ± 20°C is predicted 50 million years after basement crystallization (i.e., 1.4 ± 0.25 Ga). Results for all locations show another phase of reheating and subsequent cooling (with different rates) post-500 Ma. Based on these results, we infer that the magnitude of exhumed crystalline basement may vary between locations and over time through the midcontinent. Further work will incorporate these results into craton-wide data sets to test various hypotheses for the formation of the Great Unconformity surface.