BASEMENT-INVOLVED SHEARING ALONG EASTERN AND WESTERN MARGINS OF THE BLACK HILLS, LOCAL OR REGIONAL PALEOPROTEROZOIC EVENT?

The Black Hills of South Dakota expose the Precambrian rocks of the Wyoming province nearest the intersection of two major orogenic belts associated with the Proterozoic assembling of North America, the Central Plains Orogen and the southern Trans-Hudson Orogen. The Proterozoic and Archean rocks exposed there reveal a complicated structural and petrologic history associated with both these events. NW-directed nappe folds (D1), possibly Yavapai accretion, are reworked by NNW-striking upright folds (D2) presumably during Wyoming-Superior suturing. Our research has focused on a younger (D3) but pre-Harney Peak (HP) event in three areas in the Black Hills: 1) the Proterozoic supracrustal rocks near Rockerville, 2) the Archean gneisses in the Little Elk terrane (LET), and 3) the Archean granite and supracrustal rocks in the Bear Mountain terrane (BMT).

Near Rockerville, a NW-striking shear-fold couple is identified folding and crosscutting D2 folds. This event rotated late-syn D2 porphyroblasts (M2), but had ceased prior to down-grade metamorphism associated with Harney Peak intrusion. A similarly oriented shear-fold couple was recently identified in Archean rocks of the LET, with shear deformation continuing into the adjacent Proterozoic supracrustal rocks. Field work along the NW-striking eastern margin of the BMT determined the Archean and Proterozoic rocks are in shear contact with well developed mylonitic fabrics in rocks of both ages. Additionally, a post-D2 but pre-HP timing is interpreted from rotated M2 porphyroblasts in sheared Proterozoic rocks. In all three cases outcrop and microstructure evidence support left-lateral strike-slip coupled with east-up dip-slip transpression.

The similar orientations (NW-striking), kinematics (left-lateral strike-slip coupled with east-up dip-slip transpression), style (shear-fold couple in two locations) and age (post-M2 age in two locations and Proterozoic in the third) combine to support the interpretation for a significant post-D2 but pre-HP, basement involved shearing event partitioned into the eastern and western margins of the Black Hills uplift. Furthermore, post-Yavapai accretion ages in the Hartville Uplift and the eastern Laramie Mountains may indicate ties between deformation reported here for the Black Hills to a broader regional tectonic event.