The Black Hills: a keystone of North American continental evolution for over 2.6 billion years

The Black Hills region provides an excellent field laboratory to study numerous aspects of the continental evolution of North America from the Neoarchean to the Recent. The Neoarchean basement consists of 2.60- 2.56 Ga gneissic granitoids that formed during an early episode of convergent thermotectonism. In the Paleoproterozoic the region experienced rifting at 2.48-1.98 Ga and renewed convergence at 1.75-1.71 Ga associated with early breakup and final reassembly of the Wyoming and Superior cratons, respectively. The product of this cratonic reconfiguration was a broad deformation zone known as the Trans-Hudson Orogen. Thermotectonic activity within the Dakota segment of this zone occurred at ~1820-1680 Ma compared to ~1860-1790 Ma in the Canadian segment. South of the Black Hills region the Yavapai (Central Plains) province was also accreted during the Precambrian. An extensive sequence of Cambrian through Cretaceous marine and continental sediments were laid down before the Black Hills were uplifted during the Laramide orogeny, which exposed the Precambrian crystalline core and its structures that record evidence for the major episodes of compressional deformation and intervening rifting events noted above. Laramide deformation may have actually localized along the Precambrian structures. Alkalic extrusive and intrusive activity occurred during late Laramide stages (~58 Ma) and continued up until ~46 Ma. Aeromagnetic evidence indicates that these mantle-derived magmas intruded along a N70°W trending fault with 50 km of left-lateral offset on the Precambrian surface. The Black Hills also sits astride the transition from the Rocky Mountain region to the northern Great Plains. South of the Black Hills there is evidence from fission track and stream deposition /erosion that the transition has been uplifting during the past 5 Ma. Other geophysical anomalies exist in the region including the North American Central Plains conductivity anomaly within the Trans Hudson Orogen and the South Dakota Gravity Low/High Heat Flow anomaly to the east of the Black Hills. These zones are possibly explained by ore deposition and fluid migration, respectively. Because of the extensive history of the Black Hills region, the exposure of an array of rock types/ages, and the presence of several anomalous geophysical zones, the Black Hills swath has been proposed as an area in which to establish a portion of the "geological framework" of North America through the integration of multiple geological and geophysical techniques during the EarthScope initiative.

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