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