The crystalline basement of the Adria microplate in the eastern Alps: a review of the palaeostructural evolution from the Neoproterozoic to the Cenozoic

The frontal northern part of the continental Adriatic microplate is formed by a stack of Austroalpine nappes and by the Southalpine basement and cover units. Both basement portions are now side by side, separated by the Periadriatic (Insubric) fault system, but they became juxtaposed only during the Alpine orogeny. Permian HT-LP metamorphism is now recognised also within the Austroalpine basement of the eastern Alps, whilst it was known since long in the Southalpine basement. High thermal heat flow responsible for HT-LP metamorphism is related to the extensional tectonics and lithospheric thinning during the Permian and by concurrent magmatic underplating. During the Variscan orogeny, eclogite facies metamorphism is documented within the A-tztal and the Silvretta basements (350 Ma), as well as within the Nonsberg-Ulten unit (336 Ma). The HP events in the A-tztal and the Silvretta basements correlate temporarily with the closure of the Rheic Ocean between the Hanseatic and the Galatian terranes. Docking of the Galatian-Hanseatic composite terrane with Laurussia resulted in large-scale strike-slip tectonics and potentially in the formation of mega-folds (Schlingen) within the Austroalpine basement during the Visean. Palaeo-Tethys ridge subduction may be responsible for fast decompression and isobaric heating documented within the A-tztal basement. The Palaeozoic sedimentary sequences of the Austroalpine and the Southalpine phyllitic complexes testify that from Silurian to Devonian, both basement portions were in a passive margin position. During the Cambrian-Ordovician, the Austroalpine basement occupied a position along the leading edge of Gondwana facing the Proto-Tethys Ocean. The compositional variation of the intruded granitic suites confirm an evolution from a magmatic arc towards a collisional-type geodynamic setting related to the closure of the previously opened back-arc basins.