Alpine metamorphism of the Eastern Alps

The metamorphic overprint of the Eastern Alps during Alpine times shows sig nificant differences between the two main tectonic units, the Penninic and the Austroalpine, in terms of pressure, temperature, time and tectonic evol ution. The Penninic units are characterized by mainly oceanic rocks and fra gments of continental polymetamorphic basement and metasedimentary cover wh ich underwent high-pressure metamorphism of low geothermal gradients betwee n c. 7-13 degrees C/km typical for a subduction zone. This metamorphism cau sed eclogite formation at maximum crustal depths of about 70 to 85 km (18 t o 25 kbar at 600 degrees C) which is restricted to the Tauern Window and su bsequently the formation of blueschists at 10 to 15 kbar and 400 to 450 deg rees C in the Tauern Window and 6 to 8 kbar at similar to 350 degrees C in the Engadine and Rechnitz Windows. The dominant rock-forming mineral assemb lages however are due to a subsequent greenschist to amphibolite facies ove rprint. In the Engadine and Rechnitz Window only lower greenschist facies c onditions of similar to 350-450 degrees C at similar to 2-4, kbar are recor ded whereas in the Tauern Window a concentric metamorphic zonation reaching a maximum of 7 kbar at similar to 550-600 degrees C in the centre is obser ved. For the Tauern Window the geothermobarometric results indicate a disco ntinuous two-stage PT-loop. The age of this metamorphic evolution of the Pe nninic realm is well known for the late greenschist to amphibolite facies e vent which took place shortly after 30 Ma and is recorded by cooling ages a nd fission track studies. The exact age of the high-pressure event is still unknown. The oldest calculated age for the onset of Alpine metamorphism in the Tauern Window however is 62 Ma (CHRISTENSEN et al., 1994) which is old er than the proposed age of eclogite formation in the Central Alps reported by GEBAUER ct al. (1992) but coincides with Eocene Ar/Ar-ages reported by ZIMMERMANN et al. (1994) for the Tauern Window. The Lower Austroalpine unit, the paleogeographic link between the Penninic units and the Austroalpine continental plate shows a metamorphic evolution similiar to that of the Penninic realm. In the Lower Austroalpine units adj acent to the western Tauern Window a high-pressure event in the range of si milar to 10 kbar and similar to 350 degrees C was overprint ed by greenschi st facies conditions of similar to 4 kbar and 400 degrees C during Eocene t imes, whereas east of the Tauern Window thorough PT-estimates are still mis sing. In the Lower Austroalpine units at the eastern end of the Alps a diff erent metamorphic evolution is recorded with greenschist- to amphibolite fa cies conditions of 8-9 kbar and 500-550 degrees C, reflecting higher geothe rmal gradients than in the Penninic high-pressure event, and characterized by Cretaceous mineral ages of less than or equal to 80 Ma. Cretaceous ages have also been recorded for the Middle Austroalpine units t hat represent the polymetamorphic basement and sedimentary cover of the con tinental plate below which the Penninic units were subducted. The Alpine me tamorphic overprint increases from north towards the south from lower green schist to amphibolite facies. Locally preserved relic eclogite facies condi tions of 12 kbar and 550 degrees C in the west (Otztal Alps) and 18-20 kbar and 700 degrees C in the east (Koralpe) represent geothermal gradients of around 20 degrees C/km. The subsequent amphibolite facies overprint is inte rpreted as due to uplift and exhumation after a collisional episode in a su bduction regime. The reason for this could be a continent-continent collisi on at the western end of the Tethys in the late Cretaceous due to the closu re of the Hallstatt-Meliatta ocean basin about 100-90 Ma in age, predating the sedimentation of the late Cretaceous Gosau group sediments. The Upper A ustroalpine and Southalpine sediments of the Alpine sedimentation cycle sta rting in the late Carboniferous/Permian however are slightly effected by ve ry low- to low-grade metamorphism with decreasing intensity from north to s outh, due to internal thrusting in the course of a collisional tectonic env ironment at the western end of the Tethys during Surassic/Cretaceous times.