Masters Thesis

Strain Localization within Arc Crust: Microstructural Investigation of the Grebe Mylonite Zone in Fiordland, New Zealand

We investigate strain localization and deformation within the Grebe Mylonite Zone (GMZ), a mid-crustal, transpressional shear zone in central Fiordland, New Zealand, using electron backscatter diffraction (EBSD) analysis. The EBSD analysis includes the use of microstructural, crystallographic orientation, and Crystallographic Vorticity Axis (CVA) data. The study focuses on two samples: a diorite mylonite, and a monzogranite mylonite. For each sample we define the rheology-controlling mineral, deformation mechanism, style of dynamic recrystallization, and deformation geometry. We find in the diorite mylonite that even though quartz is abundant, its isolated grains allowed the interconnected and rheologically stronger plagioclase to become the dominant rheology-controlling mineral. CVA analysis yields data suggesting plagioclase and quartz. The analysis of quartz in the monzogranite mylonite yields microstructural and crystallographic preferred orientation (CPO) data consistent with two populations of grain sizes and lattice distortion. One population is consistent with Grain Boundary Migration (GBM) dynamic recrystallization (DRX) and the other is consistent with Subgrain Rotation (SGR) DRX. The quartz data show a correlation between the wrench-dominated transpression geometry with larger, GBM DRX grains and pure shear transpression geometry with the smaller, SGR DRX grains. We present two scenarios for the Grebe Mylonite Zone. The first scenario involves plagioclase that deformed under crystal-plastic processes and quartz that experienced GBM DRX to be associated with the wrench-dominated transpressional geometry, whereas the quartz that underwent GBM DRX have a wrench-dominated transpressional geometry and quartz grains that underwent SGR DRX have pure shear-dominated transpressional geometry that were simultaneously partitioned. The second involves a change in the shear zone conditions that may include a decrease in temperature and/or an increase in strain rate. This resulted in crystal-plastic deformation ceasing in plagioclase, and quartz GBM DRX shifting to SGR DRX and their corresponding deformation geometries described above. This allowed shear zone conditions to be accommodated such that work hardening was counteracted by neighboring grains favorably oriented for lower temperature and/or higher strain rate deformation, and these grains preserve the transition from wrench-dominated to pure shear-dominated transpression in quartz. This study provides insight into strain localization and deformation geometry and their connected roles in plagioclase and quartz.

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