PLAGIOCLASE DEFORMATION RESULTING IN LOCALIZED STRAIN IN A LOWER CRUSTAL SHEAR ZONE, FIORDLAND, NEW ZEALAND

We perform microstructural and EBSD analysis on naturally-deformed plagioclase to identify strain localization processes in a lower crustal shear zone that formed at the root of a Mesozoic continental arc system exposed in Fiordland New Zealand. Three samples correspond to different depths and different strain intensities. Sample 068C is a granulite from a low-strain zone within the Resolution Orthogneiss and represents the deepest level (≥50 km). Samples 0210D and 0212D are from a retrogressed (amphibolite facies) part of the Pembroke granulite at structurally higher levels (~40 km) and correspond to high and low strain zones, respectively.

Our microstructural observations show distinct differences in plagioclase microstructural development for the low- and high-strain shear zone samples. The plagioclase in the low-strain sample of the Pembroke Granulite (0212D) exhibits annealed textures, but relict grains of medium-coarse grain size show undulose extinction, minor subgrain development, and deformation twinning. The high strain Pembroke sample (0210D) has slightly elongate relict grains with deformation twinning, small recrystallized grain sizes (< 10 µm), and core-and-mantle structure consistent with subgrain rotation (SGR) recrystallization. Sample 068C contains a range of grain sizes where many grains have straight to lobate-shaped grain boundaries consistent with grain boundary migration (GBM) recrystallization.

The data indicate that low-strain samples deformed by a single deformation mechanism, whereas multiple deformation mechanisms operated in the high strain sample. We interpret the microstructures in 0212D (high strain) to indicate deformation by dislocation creep followed by annealing. Sample 0210D (low strain) underwent dislocation creep accommodated by SGR, followed by GBS in the recrystallized grain fraction. We interpret 068C to have deformed by dislocation creep via GBM at higher temperature, lower stress conditions relative to the Pembroke samples. The results suggest that the dominant deformation mechanism(s) vary with crustal depth, with dislocation creep promoting localization via dynamic recrystallization at high temperature and lower stress conditions, and GBS becoming dominant at lower temperature, higher stress conditions.