INTERPRETING VARIABLY-ORIENTED FOLDS WITHIN THE NEMO SHEAR ZONE, NORTHEAST, BLACK HILLS, SOUTH DAKOTA

Along the eastern margin of the Black Hills Nemo Group rocks, including the Boxelder Creek Quartzite and Blue Draw Metagabbro, experienced ductile shearing under left-lateral transpressional during the final suturing of the Wyoming and Superior Provinces. (local D₃). Shearing was partitioned into a ~1-km wide mylonitic high strain zone known as the Nemo shear zone. Within this shear zone the two units preserve shear related folds with varying hingeline orientations ranging from NW to SE-plunging. This project investigates the genesis of the shear related folds within the Nemo shear zone and attempts to interpret the cause for the variation in their orientations.

Field mapping at 1:8000 focused on the shear related folding and fabrics in the Boxelder Creek Quartzite (Xbcq) and Blue Draw Metagabbro (Xbd). In the Nemo area the >2.48 Ga Xbcq is a grey to white, buff-weathering medium-to coarse-grained quartzite. Locally, it contains layers of pebble to cobble-size clasts of feldspar and quartz. Within the Nemo Shear Zone the quartzite is an ultramylonite with thin 1 to 2 mm thick foliations defined by strongly flattened and elongated quartz grains. The ~2.48 Ga Xbd is a green to grey to black weathering medium to coarse-grained gabbro with variations in composition. Within the Nemo shear zone, there are slight variations in texture and mineralogy due to localized strain partitioning. Where shearing is strongest, the Xbd is a light to medium green talc-chlorite phyllonite. Where there is a decrease in intensity of shear the Xbd is a green actinolite–chlorite schist, and where shearing is absent, the Xbd is a grey to black hornblende gabbro.

Field and microstructural observations from previous WSU research interpret left-lateral, east side up transpression. Therefore, NW-plunging folds are perpendicular to transport direction and SE-plunging folds are parallel to transport direction. NW-plunging folds in the Xbcq commonly contain quartz pebbles and cobbles stretched and elongated parallel to the hingeline. In the Xbd, NW-plunging meter scaled-folds are present in the most strongly sheared phyllonite. The SE-plunging meter scaled-folds are more common in coarse-grained components of both Xbcq and Xbd. Initial interpretations suggest that rheological constraints and shear intensity may affect the orientation of fold axes.