HIGH RADIATION DAMAGE APATITE AND ZIRCON (U-TH)/HE DATA, AND CONSTRAINTS ON THE LOW-TEMPERATURE THERMAL HISTORY OF THE BLACK HILLS UPLIFT, SOUTH DAKOTA

The Black Hills of Wyoming and South Dakota are the northeasternmost major basement-cored Laramide uplift associated with Late Cretaceous to Early Paleogene shallow-angle subduction of the Farallon plate. We present 19 new apatite and 8 new zircon (U-Th)/He ages (AHe and ZHe) from 5 samples to investigate the low-temperature cooling history of the crystalline basement core of the Black Hills uplift. AHe ages range from ~764 Ma to ~76 Ma, and ZHe ages range from ~902 Ma to ~160 Ma, and are almost exclusively pre-Laramide. Our data are also complicated by “inverted” AHe and ZHe ages and AHe ages older than previous apatite fission track ages. Approximately half of the apatites have effective uranium (eU) concentrations >150 ppm, and up to ~960 ppm. The very high eU concentrations results in high levels of alpha-recoil radiation damage that range from ~2×10¹⁶ to 3×10¹⁸ α/g using a dose accumulation time of 800 Myrs. All alpha doses are above the alpha dose threshold of ~1.9–2.5×10¹⁶ α/g where a decrease in AHe ages has been previously observed. These results support the interpretation that He diffusivity in apatite increases at high alpha doses due to the formation and interconnection of alpha damage (percolation), resulting in younger AHe ages. Percolation of alpha damage in apatite has important implications for sufficiently old or high eU samples that resided at shallow depths for much of their history (e.g. cratons). These data support previous interpretations that He diffusivity in highly damaged apatites is different than what current models predict. Although more work is needed to understand He diffusivity in highly damaged apatite, and how it influences AHe ages and the resulting thermal and tectonic interpretations, we interpret that the pattern of (U-Th)/He data and thermal models is consistent with ~2.5 ± 0.4 km and ~1.5 ± 0.3 km of exhumation in the central and eastern basement exposures in the Black Hills. The difference in exhumation likely results from W-vergent back thrusts above an E-vergent master thrust fault controlling exhumation of the Black Hills. The preservation of older AHe ages and thermal models, together with stratigraphic constraints, also allow for a <0.5 km during the Ancestral Rocky Mountain Orogeny, followed by exhumation from within the apatite partial retention zone during the Laramide Orogeny.