EXHUMATION HISTORY OF THE BLACK HILLS, SOUTH DAKOTA FROM APATITE (U-TH)/HE THERMOCHRONOMETRY: EVIDENCE FOR LATE PALEOZOIC ANCESTRAL ROCKY MOUNTAINS EXHUMATION?

The Black Hills in South Dakota are the northeasternmost thick-skinned basement uplift formed during the Laramide Orogeny, and have the highest topography east of the Rocky Mountains. The crystalline basement is comprised of Archean and Proterozoic metamorphic rocks deformed during the Trans-Hudson Orogeny and intruded by the ~1.7 Ga Harney Peak Granite. The metamorphic basement was exposed at or near the surface during Cambrian-Ordovician deposition of the Deadwood Formation. Following reburial during Paleozoic through Mesozoic, the basement was exposed by the late Paleocene based on the presence of detrital metamorphic minerals in the Fort Union Fm, reburied by Oligocene White River Group and again subaerially exposed by later erosion. Here we report initial results from the apatite (U-Th)/He (AHe) thermochronometry samples collected along a 30 km long E-W transect to determine the cooling history of the Black Hills. Metaturbidite sample BH5 yielded single-grain AHe ages of ~368, 282, 230, and 230 Ma, whereas Harney Peak Granite sample BH11 yielded single-grain AHe ages of ~342, 293, 281, 271, and 255 Ma. Although uplift and topographic development of the present Black Hills topography occurred during the Laramide Orogeny, preliminary AHe data and indicate <2 km of exhumation during Laramide uplift. Additionally, inverse thermal modeling of AHe data, in concert with stratigraphic constraints, suggests that cooling from ~100-200 °C to ~50 °C occurred between ~300-250 Ma that may be associated with exhumation during the Ancestral Rockies Orogeny. Subtle uplift in the Black Hills and the Chadron Arch to the SE has been inferred from absence of early Pennsylvanian Amsden Formation within the lower portion of the overlying Pennsylvanian Minnelusa Formation and the unconformity with the older Mississippian Pahasapa Formation. Further thermal modeling and additional zircon (U-Th)/He data will help constrain the magnitude and timing of Paleozoic cooling in the Black Hills.