Mechanical-anisotropy controlled strain-localisation in garnet-mica domains of the Plattengneis Shear Zone (Koralpe, Eastern Alps)

Deformation microstructures in mylonites from the Plattengneis Shear Zone (PGSZ), Eastern Alps, provide new constraints on how mechanically anisotropic mid‑crustal rocks accommodate ductile strain. Although the PGSZ exhibits a strong Eo-Alpine N–S stretching fabric, it lacks many macroscopic structures typically associated with amphibolite facies deformation in anisotropic rocks. To determine where and how the high finite strains were localised, we investigate the microstructures of PGSZ mylonites with a focus on the polyphase ‘garnet–mica’ domains. Within these microstructural sites, locally elevated mechanical anisotropies form ideal conditions for nucleating and concentrating deformation structures. Importantly, this contrasts the relatively weaker mechanical strength contrasts observed in PGSZ quartz-feldspar domains, where localised deformation microstructures are scarce. Optical microscopy, back-scattered electron microscopy (SEM-BSE), and synchrotron microtomography (S‑µCT) were used to characterise both 2D microfabrics and the 3D architectures of garnet clusters. With this data, we present newly-described deformation microstructures in the PGSZ, discuss the importance of their spatial distributions, and consider the possible deformation processes involved.

SEM-BSE imaging uncovered a range of micro-scale shear bands, boudinage, and pinch‑and‑swell structures occurring exclusively within garnet–mica layers. Their restriction to these domains reflects the locally elevated mechanical strength contrast between competent garnet grains and weaker white-mica and biotite. Deformation is channelled into mica-rich areas, nucleating localised shear structures and rarely propagating further into quartz–feldspar domains. Garnet undergoes microcrack–induced fragmentation during producing synkinematic redistribution of garnet grains and fragments within the mica-rich matrix regions. This redistribution generates a range of (dis)aggregate cluster morphologies and biotite-infilled boudinage structures that align with the kinematic flow geometries predicted for the established D₁ + D₂ polyphase deformation history (Hill et al., in review). S‑µCT imaging resolved the 3D geometry of garnet clusters and revealed how fragmentation and redistribution record the cumulative kinematic evolution of the PGSZ. In more detail, the 3D data shows garnet forming complex clusters of both interconnected and disconnected grains elongated in the N-S direction, which are subsequently transposed in the E-W plane, in concordance with the D₁ and D₂ kinematic flow trajectories.

These results demonstrate that deformation in the PGSZ is highly localised within rheologically complex garnet–mica domains, where the elevated mechanical strength contrasts play a central role in the development of micro‑scale shear structures. Restricted development of shear bands exclusively within garnet-mica microstructural sites contributes to the apparent absence of larger-scale macrostructure development in the PGSZ, demonstrating the importance of a multi-scale approach to structural and kinematic analyses of ductile shear zones. Lastly, the (re)distribution of garnet in the PGSZ is proposed to be controlled by synkinematic growth and disaggregation during polyphase deformation, with the redistribution geometries potentially providing as a means of tracing strain histories in mechanically heterogeneous shear zones.