STRUCTURAL AND KINEMATIC ANALYSIS OF THE NEMO SHEAR ZONE, DOCUMENTS THE FINAL SUTURING OF WYOMING AND SUPERIOR PROVINCES IN THE BLACK HILLS, SD

Several NW striking, shear-related structures along the northeastern margin of the Black Hills record the final suturing of the Wyoming and Superior Provinces, local D₃, at ca. 1750-1715 Ma. West of Nemo, SD, the 1 km wide, NW-striking Nemo shear zone, ductily deforms the Boxelder Creek Quartzite (Xbc) and Blue Draw Metagabbro (Xbd), two units within the >2.4 Ga Nemo Group. Along strike to the SE, the shear zone parallels and overprints the unconformity between the >2.4 Ga Benchmark iron formation, which overlies the Xbc, and the younger Estes Creek Conglomerate. This study documents shear fabrics and accompanying deformation associated with the Nemo shear zone where it cuts the Xbc and Xbd west of Nemo, and determines its kinematics and relationship to other D₃structures in the northeastern Black Hills.

This research mapped a four square mile area NW of Nemo, SD at 1:8000, focusing on the strongly deformed portions of the Xbc and Xbd. The Xbc is an approximately two kilometer thick heterogeneous package of metamorphosed sandstone with local pebble to cobble conglomerate components. The Xbd is an ~1 km thick, coarse-grained, metagabbro sill that intruded the Xbc. Within the field area, stratigraphy in both units strike approximately NE and are overprinted by a strong NW-striking shear fabric. Pebble to cobble sized felsic clasts in the Xbc are deformed asymmetrically, and viewed in vertical and horizontal planes support east-side up and left lateral strike-slip movement, respectively. Microstructures in the Xbc (i.e., sigma grains and calcite glide planes), support this interpretation. Outcrop-scale drag-folding of shear fabrics in the Xbd form upright S-folds with axis steeply plunging to the NW. Thin section analysis of samples from hinge areas identified a NW-striking, axial-planar crenulation of the shear fabric along with grains stretched parallel to the hinge lines.

Deformation in the Xbc and Xbd within the NW-striking Nemo shear zone identified during this research support east-side-up, left-lateral strike slip, identical to the deformation along the unconformityto the SE . The orientation, kinematics, and post-unconformity timing for the Nemo shear zone supports the interpretation that this is associated with other D₃ structures , and records final suturing of the Wyomong and Superior Provinces.