Date of Award

December 2022

Degree Type

Thesis

Degree Name

Master of Science

Department

Geosciences

First Advisor

Dyanna Czeck

Committee Members

Dyanna Czeck, Barry Cameron, Zachary Michels

Abstract

The primary goal of this project is to document deformation mechanisms in quartz-rich rocks across a 100 m wide ductile shear zone to evaluate whether strain localization at the brittle-ductile transition is accompanied by variations in active or dominant deformation mechanisms. A secondary goal of this project is to evaluate whether the kinematic framework varies across a shear zone with a major rheological boundary. The Southern Iberian Shear Zone (SISZ), a major terrane bounding shear zone within the Iberian Massif, is an ideal location to study these questions because it is a regional scale shear zone currently exposed at the level of the brittle-ductile transition that has well-documented strain and kinematics. Previous studies focused largely on deformation in the Beja-Acebuches metabasite (BAM) rocks on the northern side of the shear zone; whereas this study focuses on the Pulo do Lobo (PdL) metasedimentary rocks on the south side of the shear zone. Using microstructural analysis and EBSD data on 24 oriented samples, new information was learned about the PdL unit that is in contact with the SISZ. Microstructural analyses revealed that brittle, diffusional, and dislocation deformation mechanisms were active in quartz deformation. Electron Backscatter Diffraction (EBSD) analysis of quartz provided a dataset that had helped uncover information about the kinematics of the shear zone, the slip systems that were active, and whether diffusional processes or dislocation processes dominated during ductile deformation. The EBSD data was used to generate crystallographic preferred orientation (CPO) plots in quartz. Weak CPO patterns indicated that, although dislocation processes contributed to the deformation, diffusional processes dominated. CPO patterns and misorientation axis plots were also used to infer that prism is the dominant slip systems within quartz. Previous studies demonstrated that prism slip dominates at approximately 500°C, which gives a constraint on temperature during deformation. This temperature estimate is broadly consistent with the upper greenschist facies metamorphic assemblage in the PdL near the SISZ, but likely inconsistent with lower grade slates >150 m away from the SISZ which have the same dominant slip systems. Further studies are needed to unravel that apparent discrepancy.A crystallographic vorticity axis (CVA) analysis, which uses the internal distortion of grains, was used to further understand the kinematics of the SISZ. The CPO and CVA analyses were used to interpret the kinematics of the SISZ within the PdL. Results were consistent with a combination of flattening and simple shear. The orientation of the vorticity vector that is associated with the noncoaxial part of the strain regime varies at the meter, or potentially even submeter scale. The simple shear varies between left-lateral, oblique, and thrust orientations. This is in contrast to the consistent left-lateral simple shear component in the BAM.

Download

Share

COinS