Description
Deformation microstructures from mylonites in the Plattengneis Shear Zone (PGSZ) are newly described for complex garnet-mica domains, giving further insight towards the mechanical behaviours of highly-anisotropic natural rocks during deformation. In the Eastern Alps of Austria, the PGSZ comprises a highly deformed, mechanically-anisotropic rock sequence with an atypical macroscopic fabric that, despite prominent N-S ductile stretching, is absent of the typical deformation structures expected for anisotropic materials. Here, the microstructures of Plattengneis mylonites are investigated in search of where the high finite strains experienced by the PGSZ potentially localised and the deformation processes involved. Our focus moves forward from previously studied quartz-feldspar microfabrics and onwards to mechanically-complex, polyphase ‘garnet-mica’ domains. These sites are anticipated to have a locally higher mechanical anisotropy than the quartz-feldspar multilayers and provide favourable mechanical conditions for concentrating strain into highly-localised deformation structures.Using optical microscopy and scanning electron microscopy (SEM) techniques, we identified for the first time micro-scale shear bands, monoclinic lenses and boudinage structures in the microfabric, almost exclusively associated with garnet-mica domains. At the domain-scale, stretched lenses and elongated layers host central clusters of garnet- and kyanite, with biotite and mica in the matrix and surrounding foliation. These structures are concurrent with the orthorhombic kinematics of the PGSZ. Probing deeper, backscattered electron (SEM-BSE) imaging of the internal architectures of these structures revealed relatively low strain deformation structures where complex clusters of garnet and kyanite are present, sheltering mica and biotite grains that develop no clear shape preferred orientations (SPO). Higher strain fabrics are still present, with an SPO strongly defined in mica and biotite at domain margins where there is an interface with the main foliation or quartz-feldspar multilayer.
Here, strain in the PGSZ is shown to be highly-localised at the micro-scale, favouring garnet-mica domains for development of deformation structures. Strain concentrated in domain margins and mechanically weaker zones enriched with mica and biotite, whereas internal regions populated with garnet experienced relatively smaller strains. Occurrence of garnet (and potentially kyanite) contribute a strengthening effect, locally inducing a high mechanical anisotropy and redirecting strain into marginal mica-rich zones. We conclude that deformation structures in the PSGZ are not entirely absent, but occur as concentrated microstructures in and around garnet-mica domains, and within these regions, nucleation and distribution of the concentrated deformation was majorly influenced by clusters of mechanically strong garnet.
| Period | 7 Jan 2026 → 8 Jan 2026 |
|---|---|
| Event title | Tectonics Studies Group TSG 2026: TSG Dublin 2026 |
| Event type | Conference |
| Location | Dublin, IrelandShow on map |
| Degree of Recognition | International |