"Phase Mixing in Lower Crustal Shear Zones: An Example from the Maggia Nappe Switzerland"
Michael Cuilik, Thesis 2018
Introduction: Shear zones are an essential feature of the Earth's crust that allow for strain to be accumulated within a confined space. Several studies have proposed that the process of "Phase Mixing" is an important rheological component of shear zones that allow them to localize strain (White, 1977; Kenkmann and Dresen, 2001; Cross and Skemer, 2017; Tasaka et al., 2017). Phase mixing is the process in which groups of single mineral grains begin to disband and form well-mixed, multi-phase mylonitic rocks. However, the mechanisms that accomplish phase mixing remain undetermined. Several studies have looked at this problem in controlled experiments (Cross and Skemer, 2017) and in mantle rocks (Kenkmann and Dresen, 2001; Tasaka et al., 2017). The motivation for our study is to investigate what drives the process of phase mixing in crustal rocks. The ultimate goal is to contribute to our understanding of the role phase mixing plays in controlling the rheology of shear zones in the Earth's crust.
This paper aims to address this question by analyzing microstructural processes that cause the progression of mylonites from foliated and banded monomineralic layers to disaggregated and mixed polymineralic aggregates. Our samples of granodioritic rock collected from the Maggia Nappe in the Swiss Alps provide an ideal test case for quantifying phase mixing in shear zones. The samples exhibit varying levels of a strain gradient from relatively undeformed to well deformed mylonitic foliation. Assuming that phase mixing is in fact the mechanism that drives strain localization in shear zones, these samples at different levels of strain will present an opportunity to pinpoint where and how the process begins to occur and what deformation mechanisms are acting upon the rock at the time of phase mixing. We approached this question by collecting data using electron backscatter diffraction (EBSD) and analyzing the changes between phase boundaries to quantify the level of mixing present at varying levels of strain.
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