Effects of quartz Dauphiné twinning on strain localization in polymineralic rocks in a mid-crustal shear zone, Fiordland, New Zealand

We use microstructural, electron backscatter diffraction (EBSD), crystal vorticity axis (CVA), and misorientation analyses to evaluate the effects of quartz Dauphiné twinning on strain localization processes within naturally deformed quartz and use recrystallized plagioclase for paleopiezometry calculations. The samples come from a mid-crustal shear zone exposed along the north shore of South Fiord in Lake Te Anau that lies along strike to the north of the Grebe Mylonite Zone (GMZ). We present results from two mylonite samples that deformed under amphibolite facies conditions, a garnet-bearing muscovite tonalite and a biotite granodiorite. A dramatic difference in Dauphiné twin development, exists in the quartz within the two mylonites, which we label as the untwinned quartz sample and Dauphiné-twinned quartz sample, respectively. We focus predominantly on the latter as it shows the best development of Dauphiné twins. In both samples, EBSD analysis reveals dynamic recrystallization (DRX) of quartz. Quartz grains have lobate grain boundaries and low intra-grain crystal plasticity indicative of grain boundary migration (GBM) DRX. Lineation measurements indicate an inclined-transpressional shear zone. CVA analysis reveals changes in the deformation, showing a transition from simple shear- to pure shear-dominated deformation, that indicates a switch of the vorticity axis. Muscovite and biotite have similar patterns in both samples suggesting similar behavior of these minerals within the shear zone. Quartz CVA patterns are dramatically different between the samples, indicating that the 3D deformation geometry promotes different behavior of quartz in these otherwise similar samples. Cross-cutting relationships reveal that the Dauphiné-twinned grains are older than the nearly-untwinned grains. We correlate the CVA patterns of the Dauphiné-twinned grains and the nearly-untwinned grains with simple shear-dominated and pure shear-dominated deformation, respectively, and use this information to display the pole figures in the correct reference frame. Pole figures and misorientation analysis reveal patterns that are consistent with the activation of known high-temperature slip systems, particularly in the nearly-untwinned grain population, while both low- and high-temperature slip systems are preserved in the Dauphiné-twinned grains. The activity of low-temperature slip systems in the Dauphiné-twinned grains results in the grains being less deformable for high-temperature slip compared to the nearly-untwinned grains. The stress calculated from the recrystallized plagioclase grains using the Twiss (1977) piezometer provide a value of 53.8-59.7 MPa. We conclude that Dauphiné twinning reduced grain sizes and helped to initially localize strain caused by deformation in an inclined-transpressional shear zone. Dauphiné twins developed during the onset of deformation and preferentially orientated the grains, resulting in strain localization. The nearly-untwinned quartz grains recorded the change in the deformation due to the grains being more deformable and GBM DRX swept grain boundaries through the grain. Dauphiné-twinned grains were less deformable, therefore the older deformation was preserved as GBM DRX maintained the low intra-grain strain. The higher grain orientation spread (GOS) value of the z-twin compared to the r-twin, as well as the higher GOS value of the Dauphiné-twinned grains compared to the nearly-untwinned grains, drove the mobility of lower-strain grains into higher-strain grains. The mobility of grains acted to reduce strain and maintain localization of the shear zone.