Date of Award

December 2022

Degree Type

Thesis

Degree Name

Master of Science

Department

Geosciences

First Advisor

Dyanna M Czeck

Committee Members

Barry I Cameron, Lindsay J McHenry

Keywords

Black Hills, Kinematics, Lozenge, Partitioning, Shear Zone, Strain

Abstract

The Nemo region of South Dakota’s Black Hills offers an ideal location to study transpressional shear zones because it hosts an exposed Archean lithological boundary between two contrasting rheological units, the Boxelder Creek Quartzite (BCQ), a rift-depositional quartzite, and the Blue Draw Metagabbro (BDM), a metagabbro sill, deformed within a ductile shear zone that represents the beginning of main-phase formation of the North American continent as we know it today. The tectonic setting of the Black Hills is at the eastern edge of the Archean Wyoming province, located near the Trans-Hudson Orogeny suture zone that formed between the Wyoming and Superior Provinces. The methods used in this study to achieve an updated analysis of the Nemo Group Shear Zone (NGSZ) are: (a) measure deformation fabrics (foliation and lineation) of discrete shear zones at the meter to kilometer scale to determine local and regional differences in strain and vorticity; (b) measure the aspect ratios and acute angles of “diamond-shaped” lozenges formed by shear bands observed in the NGSZ metagabbros and quartzites to quantify strain gradients across the NGSZ; (c) compare field data to kinematic models; and (d) supplement mesoscale analysis with microstructural analysis to identify shear sense indicators and record general mineralogical orientations within the Blue Draw Metagabbro and the Boxelder Creek Quartzite. In order to analyze the Nemo Group Shear Zone (NGSZ) kinematic models with varying inputs for shear obliquity (ɸ), extrusion obliquity (ʋ), and kinematic vorticity (Wk), and shear zone boundary strike and dip were compared to field measurement of lineation and foliation orientations and used to constrain transpression models. Modeling results suggest that the NGSZ zone strikes ~340 with steeply inclined boundaries. It accommodated both left-lateral simple shear and shortening accommodated by pure shear with steep to subvertical extrusion. Modeling results determined the fabrics most closely matched simple shear dominated models with high kinematic vorticity numbers (0.8-0.9). Lineation orientations vary significantly throughout the NGSZ, which is likely due to variations in strain magnitude and small changes in the pure shear related extrusion direction. Modeling results determined that extrusion likely deviated up to 20° from vertical in either NW or SE directions. Field measurements indicate strain partitioning within the high-strain, heterogeneous metagabbro is dominated by bands of strong foliation that encircle meter to kilometer scale blocks with less deformation. Deformation character ranges from mylonitic to ultra-mylonitic within zones of high strain, and little to no deformation moving to the east and west into zones of lower strain, away from central shear zone. Strain is more homogenous in the BCQ unit, with closely spaced foliations throughout. The bulk structural data indicates consistency with a transpression model, but the deformation was manifested differently in each lithology. Within the quartzite, the strain was more homogeneous indicated by consistent tightly spaced foliations. Strain geometries indicate plane strain to moderate flattening. Indications of previous deformation fabrics and folds were not generally observed. Within the metagabbro, strain was extremely heterogeneous on the meter scale, and the deformation was at least partially controlled by the influence of prior folds. Fold hinges nucleated blocks of low strain while high strain zones wrapped around these blocks. Strain shapes ranged from mild constriction to plane strain to flattening to extreme flattening, with most locations having either plane strain or flattening. The kinematic models and strain recorded by the lozenges, primarily suggests simultaneous left-lateral shearing and shortening at the NGSZ. This heterogeneity of deformation may be related to the influence of D1, D2, and D3 fabrics that predated or formed syn-deformational in the NGSZ or may be due to heterogeneity in kinematics or degree of strain within the NGSZ.

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