Episodic weakening and strengthening during synmetamorphic deformation in a deep-crustal shear zone in the Alps

Abstract

Abstract The Greiner shear zone (western Tauern Window) deformed a variety of metasedimentary, metavolcanic, and plutonic lithologies at conditions of c. 525–575 °C and 30–40 km depth. Microstructural relationships point to a succession of weakening and strengthening episodes. Stage I involved softening via a change in deformation mechanism. Grain-size reduction in plagioclase-rich horizons locally produced rocks with an average grain size of <30 μm microstructural features consistent with deformation via grain boundary diffusion creep (GBDC; a fluid-assisted deformation mechanism similar to pressure solution, which may result in superplastic behaviour). At constant stress, GBDC will result in a significant increase in strain rate relative to neighbouring layers. Stage II involved reaction-induced strengthening. Rapid bulk diffusion rates associated with GBDC allowed rapid growth of large (up to 20 cm) hornblende cystals. This growth of large cross-cutting crystals shut down grain-size-sensitive flow mechanisms in the plagioclase matrix and locally ‘locked’ the shear zone, shifting ductile deformation to weaker horizons. Stage III involved reaction-induced softening. Local variations in bulk composition and/or fluid availability caused large hornblende grains in some horizons to be partially replaced by biotite. These biotite-rich layers localized subsequent deformation, whereas adjacent layers with intact hornblende record minimal strain. Deformation and metamorphism together exert control on fluid availability, diffusion rates, and reaction kinetics, and these factors collectively control fabric development and rheology. The effects of interaction between small-scale deformational and metamorphic processes are difficult to predict, but can have an important influence on shear zone behaviour at depth. The resulting complexities need to be accounted for in models of crustal strength-depth relationships and shear zone rheologies.