A CASE FOR A PALEOPROTEROZOIC TRANSPRESSIONAL BOUNDARY, BLACK HILLS, SOUTH DAKOTA: A HATCHLING'S VIEW

The Precambrian core of the Black Hills (BHs) is the key to understanding the timing and tectonics of the 1.85 – 1.76 Ga assembly of Laurentia. The metamorphic core has many similarities to clastic wedges in younger collisional orogenic belts, including the Appalachians. Detailed structural mapping combined with detrital zircon (DZ) ages were used to constrain the timing of deposition and tectonism during the final stages of the Wyoming-Superior collision and coeval arc collision. Previous workers have posited that the BHs represent an intrinsic portion of the Wyoming Provence. Detailed mapping in the BHs has revealed a series of fault-bounded blocks (FBBs) that can be separated based on a key structural discontinuity along the NNW trending Keystone-Empire Mine fault (KEM). Rocks to the NE of the KEM have as many as five distinct fabrics. Rocks south of the KEM can be divided into two separate FBBs: one that contains rocks overprinted by a strong fabric related to the intrusion of the Harney Peak granite and one that shows little to no deformation related to the intrusion. Rocks not strongly overprinted display tight recumbent folds of original bedding surfaces with a weak axial plane foliation. Little evidence for correlative S2 fabric has been observed SE of the KEM in the study area. Many of the metasedimentary units within the eastern and central uplift have been grouped into lithostratigraphic units that transect the KEM. DZ samples from the study area were collected from the FBBs to help with lithostratigraphic correlation. All samples have distinct Trans-Hudson age populations with peaks of ~1870 Ma. However, there are distinct differences between older and younger populations that indicate that the FBBs received input from multiple sources of various ages, suggesting that previously correlated lithostratigraphic units of the FBBs had different provenance sources and, hence, may have different tectonic histories. DZ populations have distinct ages (~2020, 2348, 2540, and 2860 Ma) that vary from sample to sample but cluster in peaks. Together, the structures and ages are consistent with the KEM being a transpressional boundary. FBBs NE of the KEM are interpreted to have moved N as a result of the earliest Yavapai-age arc collision prior to the terminal collision between the Wyoming and Superior provinces.