Abstract During the TRANSALP project, deep seismic surveys and accompanying geophysical and geological projects were carried out to better understand the deep structure of the Eastern Alps south of Munich. The TRANSALP field trip series roughly follows the route in three parts: the Tauern Window, the Northern Calcareous Alps and its foreland, and the Southern Alps including the Dolomites. In this Tauern Window field trip, we will visit most of the geologically important sites along the middle part of the traverse or in its vicinity. The main topics covered in the Tauern Window will be the early Alpine paleogeographic situation on an extending European continental margin, Alpine nappe stacking and ductile rock deformation, metamorphism, uplift modes and exhumation, lateral escape, and the Brenner normal fault.

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 réduction in plagioclase-rich horizons locally produced rocks with an average grain size of <30 μ m and 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.