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The ductile Nemo Shear Zone (NSZ) in the northeastern Black Hills, SD, cuts across and deforms two distinctly different rock units, the Boxelder Creek Quartzite (BCQ) and Blue Draw Metagabbro (BDM). Because the quartz minerals in the BCQ are relatively weak compared to the minerals in the BDM, this location provides an excellent opportunity to compare how these two very different rock units accommodate deformation, at both outcrop and mineral-grain scales, in this type of stress environment. The NSZ is a 1.5 kilometer wide high-strain zone within the Precambrian core of the Black Hills. The deformation preserved in the rocks along this structure formed during the final suturing of Wyoming and Superior provinces. Past research by WSU student Benjamin Keute (16) has thoroughly studied the deformation in the BCQ. This project focuses on the crystal-plastic deformation in the SOM, and will compare these observations to those of Keute. Detailed mapping during fieldwork in summer 2015 involved documenting mineralogical and textural variation at the outcrop scale across the SOM. In order to document mineralogical variation and analyze the microstructures at the grain scale, samples were collected at locations within (high strain) and outside (low strain) the shear zone for thin section analysis. Within the NSZ, SDM forms narrow, elongate, outcrops striking NW. Micro-scale structures reveal a closely spaced (sub-mm), NW-striking S-C mylonite fabric; a shear fabric formed by two intersecting planar fabrics defined by the alignment of fine-grained, platy, and elongate minerals. The intersection angles between the S and C fabrics measure 15°-50°. Locally the strong S-C fabric is refolded by continued shearing. Outside the NSZ where stress is lower, SOM forms massive, blocky outcrops shaped like rhombic parallelograms. This appearance is a function of coarse-grained minerals and a wider spaced (3-5 mm) S-C fabric. Micro-textural analysis identified large fractured "ghost" grains (2-3mm) of hornblende (Fe-amphibole) replaced by weakly disarticulated, and rotated grains (0.1-0.2mm) of tremolite-actinolite (Mg-amphibole). We interpret brittle fracturing along the 60° /120° cleavage in amphibole as the mechanism for grain-size reduction during development of the S-C fabric. The angles between the S-C fabrics in thin section as well as the rhombic edges on the outcrop scale measure between 50°-90°, suggesting a causational relationship between the two. The angles between S and C fabrics vary systematically from low to high strain zones, with larger angles (50°-90°) in low strain areas outside NSZ decreasing to 15° in the most highly strained rocks within NSZ. This change coincides with grain-size reduction in the amphibole and spacing of the planar fabrics changing from 3-5mm to sub-millimeter as strain increases. Furthermore, the deformation mechanisms in the BDM and SCQ are different with the SDM deforming by brittle fracture in amphibole grains along 60°/120° cleavage planes, whereas quartz in the SCQ deforms crystal plastically by gliding at the atomic level.

Content Notes

Poster, Presentation, Final Report Form

First Advisor

Stephen Allard



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