PROTEROZOIC SHEARING AND FOLDING IN THE ARCHEAN LITTLE ELK TERRANE, BLACK HILLS, SOUTH DAKOTA

The Black Hills uplift in South Dakota contains the Little Elk Terrane (LET), one of two exposures of Archean crusts from the easternmost margin of the Wyoming Province. Rocks exposed in the LET include: 1) Archean coarse-grained, muscovite-biotite Little Elk Granite (LEG) that grades rapidly to 2) an Archean biotite-rich, K-spar augen gneiss (BFG) with similar felsic composition, and 3) a narrow conformable exposure of Proterozoic supracrustal rocks. The LEG has a NW-striking spaced foliation and locally includes similarly oriented cm-scale shear bands. Approaching the BFG from the east, the abundance of biotite increases and forms a penetrative NW-striking shear fabric with local exposures of a NE-trending mylonitic foliation. Asymmetric porphyroblasts are present in all three lithologies across the LET, thus requiring the left-lateral strike-slip coupled with east-up dip-slip movement to be Proterozoic in age. Quartz and feldspar grains have undergone grain-size reduction both between the biotite foliation bands and within the coarse feldspar grains.

Prior to this research, the NE and NW fabrics in the BFG were interpreted as two separate events: the NE developing in the Archean during balloon emplacement of the LEG into the BFG, and the NW developing as part of a younger regional deformation event. Detailed mapping where the shear fabric in the BFG strikes NE identifies these areas as outcrop-scale fold hinges with NW-striking limbs parallel to cross-cutting NW-striking mylonitic fabric. Fold axes for these hinges determined using π-analyses plunge steeply to the SE, similar to measured hinge lines and lineations from across the LET.

We propose that both the NE and NW shear fabrics formed during a single event where the early-formed mylonitic foliation was folded into isoclinal, SE-plunging folds oriented parallel to the shear plane. As shearing continued, the NW-striking mylonitic fabric continued to developed and cut across the NE-striking fabric exposed in the fold hinges. The proposed model, which couples folding with a shearing event, is comparable to that interpreted for a similarly oriented shear-fold couple in the Proterozoic supracrustal rocks to the south near Rockerville, SD. If these two areas are correlated, then these rocks may record a significant tectonic event along the eastern boundary of the Wyoming Province.