Microstructural and textural development of calcite marbles during polyphase deformation of Penninic units within the Tauern Window (Eastern Alps)

The evolution of calcite microstructures and crystallographic preferred orientations (CPOs) is well understood due to well constrained experimental studies. However, the interpretation of naturally deformed calcite marbles is more difficult because of less constrained strain paths, a multiphase deformation history, and variable P–T conditions. The Penninic units within the Tauern Window (Eastern Alps) have been affected by several deformation events and metamorphic overprint. Generally, three major deformational events can be distinguished. D1 is related to underthrusting of Penninic units beneath the Austroalpine nappe complex, and top-to-the-N nappe stacking within the Penninic continental units. Deformation stage D2 is interpreted as reflecting the subsequent continent collision between the Penninic continental units and the European foreland. D3 is related to the formation of the dome structure of the Tauern Window. This polyphase deformation history can be partly reconstructed by the evolution of calcite microfabrics and CPOs.

Three types of calcite-fabrics are distinguished within the Penninic units of the Tauern Window and the Lower Austroalpine unit. D1-fabrics are characterized by equilibrated microstructures and LT-CPOs. The CPOs are generally strong and symmetric, with one well developed cluster near the Z-axis of the finite strain ellipsoid. These fabrics have locally been overprinted by subsequent amphibolite to greenschist facies metamorphism. Generally, the occurrence of LT-fabrics coincides with the occurrence of amphibolite facies metamorphic mineral assemblages in the central part of the Tauern Window, while HT-fabrics have been observed outside this area. Fabrics from the central part of the Tauern Window have likely been strengthened during subsequent thermal equilibration, while the fabrics that have been observed at the peripheral parts have been less affected by subsequent metamorphic overprint. Therefore, D1-fabrics do not reflect D1 conditions, but subsequent thermal equilibration. Similar observations have been made for the evolution of D2-fabrics. LT-fabrics dominate inside the amphibolite facies isograde, HT-fabrics occur outside (greenschist facies metamorphic conditions). The fabrics are characterized by high finite strains near the margins of the Tauern Window, the intensity of which decreases towards the central parts, where microstructures are characterized by recrystallization and thermal equilibration due to amphibolite facies metamorphic overprint. The strong CPOs document this influence. The HT-fabrics and microstructures within peripheral areas indicate that they have less been affected by this thermal event, and, therefore, are more indicative for the deformational conditions during D2. D3 is restricted to distinct shear zones along the tectonic boundaries of the Tauern Window. From the central parts to the shear zone boundaries, a clear evolution of microfabrics can be observed. In internal parts, microstructures are characteristic for intracrystalline plasticity with a dominating activity of r−-glide. Twinning was less important during the final phases of deformation. On approaching the shear zone boundaries, the grain size decreases due to dynamic recrystallization and secondary grain size reduction until ultramylonites are formed. Within these domains grain boundary sliding seems to have been dominant.

In conclusion, calcite CPOs from polyphase areas do not only include information on the deformation conditions, but also bear information about the thermal overprint subsequent to the main deformational event.