Evolution of quartz microstructures and textures during polyphase deformation within the Tauern Window (Eastern Alps)

The interior of the Tauern Window exposes underplated Penninic continental lithosphere and the overlying obducted Penninic oceanic crust within a larg e antiformal dome in the internal zone of the Eastern Alps. These units hav e been affected by a polyphase deformation history. Generally, three deform ation events are distinguished. D-1 is related to underplating of, and top- to-the-N nappe stacking within, the Penninic continental units of the Tauer n Window. Deformation stage D-2 is interpreted to reflect the subsequent co ntinent collision between tile Penninic continental units and the European foreland, D, is related to the formation of the dome structure within the T auern Window. During thickening of continental lithosphere and nappe stacki ng (D-1), and sub sequent intracontinental shortening (D-2), these tectonic units have been ductilely deformed close to a plane strain geometry. Condi tions for the plastic deformation of the main rock-forming mineral phases ( quartz, feldspar, dolomite, calcite) have prevailed during all three phases of crustal deformation. Generally, two types of quartz microstructures tha t are related to D-1 are distinguished within the Tauern Window: (a) Equili brated and annealed fabrics without crystallographic preferred orientations (CPO) have only been observed in the central part of the southeastern Taue rn Window, corresponding with amphibolite-grade metamorphic conditions. (b) In the northeastern and central part of the Tauern Window microstructures are characterized by quartz grains that show equilibrated shape fabrics, bu t well preserved CPO with type-I cross girdle distributions, indicating a d eformation geometry close to plane strain. During D-2, two types of quartz microstructures are distinguished, too: (a) Quartz grains that show equilib rated shape fabrics, but well-preserved CPO. The c-axes distributions gener ally are characterized by type-I cross girdles, locally by type-II cross gi rdles, and in places, oblique single girdle distributions. (b) A second typ e of quartz microstructure is characterized by highly elongated grains and fabrics typical for dislocation creep and grain-boundary migration, and str ong CPO. This type is restricted to the southern sections of the western an d eastern Tauern Window. The c-axis distributions show type-I cross girdles in the western part of the Tauern Window and single girdles in the southea stern part. In the western part of the Tauern Window,, a continuous transit ion from type (b) microstructures in the south to type (a) microstructures in the north is documented. The microstructural evolution also documents th at the dome formation in the southeastern and western Tauern Window has alr eady started during D-2 and has continued subsequent to the equilibration d uring amphibolite to greenschist facies metamorphism. D-3 is restricted to distinct zones of localized deformation. D-3-related quartz fabrics are cha racterized by the formation of ribbon grains; the c-axes show small-circle distributions around the Z-axis of the finite-strain ellipsoid. During exhu mation and doming (D3) deformation occurred under continuously decreasing t emperatures.