Date of Award

May 2021

Degree Type

Thesis

Degree Name

Master of Science

Department

Geosciences

First Advisor

Dyanna Czeck

Committee Members

Lindsay McHenry, Barry Cameron

Abstract

Thirty-four quartz-rich samples were collected across the Willard thrust fault in Utah to explore the effects that fluid has on faulting. In particular, the relationships between fluid, deformation mechanisms, crystallographic preferred orientation (CPO) fabric development, and kinematic vorticity were investigated. A petrographic analysis of all 34 samples determined the relative contributions of brittle fracturing, crystal plasticity, and diffusional processes along with their possible relation to fluid interaction. Crystal plasticity dominates throughout most of the field area except where significant contributions from diffusive deformation occurs in rocks with significant mica concentrations and fine grain sizes, especially those located near the fault. The degree of fluid interaction and each deformation mechanism was also compared to each sample’s mineralogy, grain size, and fault proximity. Generally, increased fluid interaction enhanced either brittle mechanisms (in coarse grained, quartz rich samples) or diffusive deformation (in fine grained samples). Fault proximity also influenced fluid intensity and deformation mechanisms. The closer the samples were to the fault, the more fluid interaction and diffusion were present, consistent with earlier work that suggested the fault acted as a channel for syndeformational fluids. Electron backscatter diffraction (EBSD) was used to further investigate the relationship between fluid intensity and dominant deformation mechanisms, and to also explore relationships between these factors and vorticity, fabric strength, and quartz slip system activation. Sixteen samples were selected for this analysis. Overall, the samples produced weak CPO fabrics, suggesting deformation by crystal plasticity with contributions from diffusion. Because of the weak patterns, a quantitative vorticity analysis was not completed, but a qualitative analysis showed that both pure and simple shear components were present during deformation. Quartz CPO was strongest in samples close to the fault with high quartz concentrations. It also had a weaker, positive correlation to fluid interaction and grain size. Generally, samples closer to the fault displayed pole figure geometries with active slip systems associated with higher temperatures, suggesting that hydrothermal heating may have occurred as fluids were channeled along the fault. Crystal plasticity was the primary mechanism that allowed rocks to deform on both the footwall and hanging wall. The highest degree of crystal plasticity and diffusive deformation occurred near the fault, as evidenced by the strongest CPO fabrics and microstructures related to diffusional deformation. It is likely that the concentrated diffusional deformation occurred due to the channelized fluid low along the fault, which may have been continuous, cyclical, or sporadic. Overall, deformation mechanism was controlled by a variety of factors with the presence of fluid being of primary importance. Proximity to the fault (not unrelated to the presence of fluid) and quartz concentrations also played a significant role, and grain size exerted control to a lesser degree.

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