STRAIN HETEROGENEITY WITHIN THE NEMO GROUP SHEAR ZONE IN SOUTH DAKOTA'S BLACK HILLS SUGGESTS OBLIQUE CONVERGENCE DURING THE PRECAMBRIAN ASSEMBLY OF THE SOUTHERN TRANS-HUDSON OROGENY

The Nemo area of South Dakota hosts an exposed Archean lithological boundary between the Boxelder Creek Quartzite (BCQ), a rift-depositional quartzite, and the Blue Draw Metagabbro (BDM), a metagabbro sill, deformed within a transpressional shear zone that represents early main-phase formation of North America. The tectonic setting of the Black Hills is at the eastern edge of the Archean Wyoming province near the Trans-Hudson Orogeny suture zone. The methods used in the analysis of the Nemo Group Shear Zone (NGSZ) are: measure deformation fabrics to determine local and regional strain and vorticity; measure the geometry of lozenges formed by shear bands to quantify strain gradients; and compare field data to kinematic models. Analysis of the NGSZ utilized kinematic model inputs for shear obliquity, extrusion obliquity, kinematic vorticity, and shear zone boundary strike/dip, which were compared with field data to constrain transpression models. Modeling suggests the NGSZ strikes ~340° with steeply inclined boundaries, accommodating transpression with left-lateral simple shear and shortening accommodated by pure shear with steep/subvertical extrusion. The fabrics matched simple shear dominated models with kinematic vorticity numbers of ~0.8-0.9. Variations in lineation orientations are due to changes in strain magnitude and pure shear related extrusion, which deviated ~20° from vertical in the NW or SE directions.

The data from both lithologies indicate consistency with a transpression model. Strain in the BCQ was homogeneous with plane strain to moderate flattening. Previous deformation fabrics and folds were not generally observed. Strain in the BDM was heterogeneous with deformation at least partially controlled by the influence of prior folds. Fold hinges nucleated blocks of low strain while high strain zones encircled these blocks. Strain shapes range from mild constriction to plane strain to flattening to extreme flattening, mostly accommodating plane strain or flattening. The kinematic models and strain recorded by the lozenges suggests concurrent left-lateral shearing and shortening. BDM structural heterogeneity may be related to the influence of D_1-3 fabrics that predated or formed syn-deformationally in the NGSZ, or may be due to heterogeneity in kinematics or degree of strain.