Masters Thesis

Integrating Microkinematic Crystallographic Vorticity Axis (CVA) Analysis and Petrochronology to Investigate the Temporal Development of Fabric within a Transpressional Shear Zone, New Zealand

We investigate strain accommodation deep within the crust of a continental arc by performing electron backscatter diffraction (EBSD) and crystallographic vorticity axis (CVA) analyses on the transcrustal George Sound Shear Zone (GSSZ) in Fiordland, New Zealand. We combine microkinematic analyses with petrochronology to evaluate temporal changes in fabric development in the GSSZ due to Cretaceous development of the shear zone along an older, crustal-scale discontinuity marked by a linear distribution of Carboniferous granites. Samples were collected from middle-crustal exposures of the GSSZ, where it separates the Cretaceous hornblende diorite of the Misty Pluton (117-114 Ma) from the Carboniferous Large Granite (316-308 Ma) in Central Fiordland. We present results from samples taken along traverses across the GSSZ at Mary Peaks and Myth Tarn. These samples include low- and high-strain rocks from the GSSZ along the margin of the Misty Pluton, as well as host rock granite from the Large Granite. Petrographic light microscopy and EBSD mapping of thin sections revealed microstructural fabrics and crystallographic preferred orientations (CPOs) within our samples. At Mary Peaks, microstructures and CPO patterns of quartz and plagioclase indicate deformation at upper-amphibolite facies conditions (600-700°C) with titanite thermometry yielding temperatures of ~ 710°C. At Myth Tarn, analyses indicated a wider range in thermal conditions during metamorphism (400-700°C) and microstructures provided evidence of annealing. Bulk CVA plots show a variety of deformation geometries which vary by field site and rock composition. Quartz-rich Mary Peaks samples have a variety of CVA patterns while hornblende-rich Myth Tarn samples show a very consistent deformation geometry. CVA data of single-phase plots for quartz, plagioclase, hornblende, titanite, and biotite reveal how strain was partitioned into different mineral phases with varying rheologies. When used in coordination with the petrochronology of metamorphic titanite, our single-phase CVA analysis provided evidence for how the kinematic deformation geometry of the GSSZ evolved over time and found preserved fabric from a pre-existing shear zone in the Large Granite pluton. From these results we interpret that the geometry of kinematic deformation evolved along-strike through different crustal sections of the GSSZ, where strain developed from triclinic transpression to monoclinic pure-shear dominated transpression over time. This microkinematic study can be used as evidence for how shear zones naturally evolve over time and may narrow along rheological boundaries at the regional scale, where strain accommodation in quartz played a significant role in localization.

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