EVIDENCE FOR SINISTRAL OFFSET ALONG A SHEAR ZONE EXPOSED IN SUPRACRUSTAL PROTEROZOIC ROCKS NEAR THE EASTERN MARGIN OF THE BLACK HILLS, SOUTH DAKOTA

The suturing of the Wyoming and Superior provinces (ca. 1780-1715) is recorded in the Black Hills of SD where the easternmost portion of the Archean Wyoming Province is exposed. Across the Black Hills, the dominant deformational style associated with this event is regional-scale folding of Proterozoic supracrustal rocks. This research recognized a change in deformational style from folding to shearing near the eastern margin of the uplift where a 0.5–1 km-wide, NNW-striking shear zone has been identified. West of this shear zone the supracrustal rocks are predominantly medium-grained meta-greywacke with minor amounts of interbedded meta-pelite. The meta-greywacke contains well-preserved graded beds commonly aligned parallel to a cm-scale spaced cleavage and schistosity in the pelitic portions. The NW-striking S-fabric in the supracrustal rocks are defined by the alignment of continuous biotite. Biotite, garnet and staurolite porphyroblasts are restricted to the pelitic layers. Garnet-grade supracrustals exposed near Rockerville, SD record a different deformational history. As these rocks are approached from the west, bedding is refolded parallel to a penetrative mylonitic foliation, graded beds are no longer preserved and the rocks have developed an S-C or S-C' fabric. The strain in the region appears to be partitioned around mega-lithons of thickly bedded meta-greywacke. Locally the shear fabric is refolded into small-scale isoclinal folds and outcrop-scale asymmetric folds with near-vertical hinge lines. Supracrustal rocks with in the shear zone have undergone significant grain-size reduction and preserve a more complicated structural and metamorphic history, which includes three distinct S-fabrics. The first and second are defined by NW and NNW striking alignment of continuous biotite, respectively. The third fabric, defined by weakly aligned discontinuous biotite porphyroblasts, strikes NNE and cross cuts both continuous biotite S-fabrics. Peak metamorphism in the shear zone is recorded by post-deformational, euhedral garnet porphyroblasts. Preliminary thin-section work supports the interpretation for a left lateral component of shear which precedes the peak metamorphism and is consistent with and perhaps part of the shear zone described in the Little Elk Creek terrane to the NW.