The Miocene Salzachtal-Ennstal-Mariazell-Puchberg (SEMP) strike-slip fault in Austria allows study of the internal structure of a fault zone from the near surface to ∼30 km depth. As it enters the Tauern Window along the Rinderkarsee shear zone, the SEMP fault passes from a dominantly brittle to a dominantly ductile structure. The shear zone consists of three 1- to 100-m-wide zones of brittle-ductile and ductile deformation separated by 500-m-wide zones of less deformed rocks. The southern shear zone is mylonitic, with ductile amphibole and plagioclase; weak crystal preferred orientations imply that the main deformation mechanism was dislocation-accommodated grain boundary sliding. The northern and central shear zones are characterized by discrete millimeter-wide shear zones with ductile quartz, muscovite, and biotite and brittle feldspar. Shear zone nucleation at the grain scale involved dislocation creep and the transformation of plagioclase to muscovite; strain then localized in muscovite-rich grain boundary shear zones that linked to form throughgoing shear zones.