PASSIVE FOLDING MODEL FOR DEFORMATION OF NEMO GROUP DURING FINAL SUTURING OF THE WYOMING AND SUPERIOR PROVINCE, NE BLACK HILLS, SOUTH DAKOTA

The Nemo group is a suite of earliest Proterozoic supracrustal rocks located in the NE Black Hills, South Dakota. The two major units of the Nemo Group, the Boxelder Creek Quartzite and the Blue Draw Metagabbro are folded into steeply plunging anticlines and synclines that are cut by the NW-striking, mylonitic Nemo shear zone, which formed during final suturing of the Wyoming and Superior Provinces (local D3). This research investigates the fold mechanism, active vs. passive, and interprets the temporal relationship between the folding and the Nemo Shear zone.

The Boxelder Creek Quartzite (Xbcq) is a buff-weathering, pink to grey, coarse-grained quartzite. The beds are typically graded with silty tops and pebble bottoms. A strong NW-striking planar fabric present in the Xbcq is interpreted as an S-C fabric with left-lateral strike-slip shear sense, identical to the shear sense interpreted for the Nemo shear zone. The strike of the shear fabric remains the same regardless of strike of bedding, and within the Nemo shear zone the bedding is oriented parallel with the NW-striking shear fabric. The Blue Draw Metagabbro intruded the Xbcq and is interpreted as a sill due to its near parallel contact with bedding in the Xbcq in most areas. Outside the shear zone two planar fabrics, one typically trending NW, form diamond-shaped outcrop patterns. This pattern remains to be interpreted. Within the shear zone the metagabbro is a fine-gained, chlorite, tremolite-+/- talc-+/- serpentine phyllonite with a NW-striking shear fabric.

The deformation in the Nemo Group seen in the map pattern of the metagabbro and the bedding in the quartzite can be modeled with passive folding during left-lateral east-side up shearing. The Nemo shear zone also has the same orientation and shear sense making interpretation of this folding synchronous with shear zone development reasonable. Previous WSU research interpreted the Nemo shear zone as part of the D3 event, marking the final suturing of the Wyoming and Superior provinces. Therefore we propose a passive folding model affecting all the Nemo Group rocks during D3 with partitioned strain into the Nemo shear zone during the final Wyoming-Superior suturing event.