Master of Science
This thesis analyzes a suite of mylonites from the Grenville Front shear zone exposed southeast of Sudbury, Ontario. Lattice preferred orientations, titanium-in-quartz thermometer, and dynamically recrystallized grain size piezometer measurements were applied to obtain the deformation mechanisms, deformation temperatures (T), and differential stresses (σ), respectively. Results show that these mylonites were formed in the shear zone during the terminal stage of the Grenville Orogeny. The dominant deformation mechanism is by regime 2 dislocation creep. The deformation temperature is between 425-567 °C, and the differential stress is between 56-133 MPa. These results are discussed in the context of wet quartzite flow laws. This study shows that a recently calibrated flow law that considers the pressure dependence of the activation enthalpy is applicable to the Grenville Front shear zone. The thesis also improves our understanding of the Grenville Front deformation zone.
Summary for Lay Audience
The Earth’s crust has a significant impact on our daily life. Studying the crust provides us with a better understanding of tectonic history. Quartz plays a key role in determining the crustal strength as it is abundant. As a result, scientists study how quartz reacts to different temperatures and pressures in experiments and make flow laws that can describe the deformation behavior of quartzite. However, these flow laws do not always produce the same results. This is primarily due to how fluid changes the properties of rocks. Also, we cannot simulate the strain rate, which occurs naturally, in the lab. This is the limitation of working with models. The question is, how do we know the flow laws obtained via experiments can apply to natural quartzite. The solution is analyzing the deformation conditions of quartz in natural shear zones and comparing them with experimentally deformed quartz. If the microstructures are similar, that means similar deformation mechanisms are taking place. This thesis focuses on the deformation conditions of naturally deformed quartz-rich mylonites from the Grenville Front. Deformation temperatures were obtained from measuring the amount of titanium in quartz because titanium concentration has a temperature dependence when Ti is substituted for silicon in quartz. Deformation mechanisms were interpreted by the pattern of quartz c-axes orientation. Dynamically recrystallized grain sizes were measured because they reveal the paleo-stress. The temperatures, stress, and deformation mechanisms were compiled and plotted on a flow law profile to see how well they fit into current flow laws. I propose that the deformation condition of our samples from the Grenville Front can fit with the current flow law. According to the overall observation, I interpret that these mylonites were brought to the surface by a process called exhumation. This thesis enhances our understanding of the Grenville Front deformation and flow law studies.
Chang, Xueke, "Deformation Conditions of Quartz-rich Mylonites of the Grenville Front Tectonic Zone and Application to the Crustal Strength" (2022). Electronic Thesis and Dissertation Repository. 8626.