The investigation of the progressive microstructural development and related deformation mechanisms in shear zones is important for our understanding on strain localization, which occurs throughout the lithosphere and is important for determining lithospheric strength and geodynamic processes in general. Shear zone localization generally occurs on a planar rheological heterogeneity, either a compositional boundary or a precursor fracture, which may also channel and promote fluid-rock interaction, metamorphism and deformation.
Microstructures and textures represent the link between natural and experimental rock deformation, allowing for an application and extrapolation of laboratory data to natural shear zones and rock rheology. Changes in mechanical behaviour and rock strength are usually documented by the deformation microstructure. These changes can be due to transitions in deformation mechanisms, fluid-rock-interaction and metamorphic reactions, and result either in deformation softening or hardening potentially leading to strain localization vs. spreading. Metamorphic reactions allow for the determination of PT conditions and for an estimate on the presence or absence of water. However, connecting the microstructural and textural data to metamorphic conditions and timing of deformation remains mostly qualitative. Likewise, linking the absolute timing of deformation to PT paths is commonly hampered by: 1) difficulty in correlating thermobarometric and chronologic data from the same rock sample to its deformation, 2) relatively large uncertainties associated with conventional thermobarometry, 3) lack of suitable metamorphic mineral assemblages in mylonites. Ti-in-quartz thermobarometer (TitaniQ) may offer a quantitative estimate of deformation temperatures in quartz mylonites. Combining structural and microstructural investigations, metamorphic petrology, trace element geothermometry, thermodynamic modelling, and dating deformation is therefore essential for our understanding of the formation and maintenance of shear zones in the lithosphere.

This session presents case and generic studies of rock deformation under range of metamorphic conditions in nature, experiments and theory. It aims at spanning a bridge from micromechanics to multi-scale field-based investigations and to lithosphere rheology. Broad areas of discussion will include (a) strain localization and rheology, (b) metamorphic conditions of ductile deformation, and (c) deformation mechanisms, and (d) interaction between fracturing, metamorphism and ductile deformation, with special emphasis on the continental lower crust.