Permo-Triassic lithospheric extension in the Alpine realm

Research project P 14525 Permo-Triassic lithospheric extension in the Alpine realm Peter FAUPL 26.6.2000 During Permo-Triassic time (290 - 205 million years ago) the Austrialpine unit which forms the major part of the Eastern Alps, and the former neighbouring units were affected by extensional regime due to the clockwise rotation of Africa in the south with respect to Europe in the north. The extensional event is recorded in the Permo-Triassic sediments as well as in the metamorphic basement. Sedimentary piles up to 4 km in thickness show, that the landscape was formed by grabens, alluvial plains and a hypersalinar marine sea during the arid Permian period (290 - 250 Ma). In Triassic time a tropical sea and subsequent 240 Ma the Meliata Ocean opened in the south of the Austroalpine realm. Periods of tectonic activity which caused the formation of basins and reef-plateaus alternated with calm periods of undisturbed cyclic sediments. On the other hand extension caused magmatic underplating and intense volcanic activity. The basement was affected by a high temperature / low pressure metamorphic event with peak conditions at about 260 Ma and a long term cooling history until 200 Ma. The aim of the proposed project is to learn more about the evolution of the lithosphere in an extensional environment, the acting geodynamic processes and the plate tectonic framework. We want to study the interaction between the lithospheric mantle, the continental crust and the contemporaneous sediments during this process. The project includes a study of the best preserved Permo-Teriassic crustal succession in the Alps. The Strieden- Goldeck-Drauzug section (Carinthia, Austria) exhibits a more or less continuous profile from the Permo-Triassic sediments to the metamorphic basements. The facies and subsidence evolutions of the sediments shall be compared with thermal and subsidence history of the underlying continental crust by means of sedimentological, geochronological and geothermobarometrical P-T data. Studies on the metamorphic history of the Hungarian basement (Trans-danubian Central Range unit and Tiszia Superunit) and on metamorphic pebbles from the Cretaceceous Gosau Group will yield further information about the extend and the Permo-Triassic metamorphic event. More precise data on the timing of the metamorphic peak shall be derived by Sm-Nd age data on garnets from metapelitic and pegmatitic rocks. All these data will be integrated in a refined model on the Permo-Triassic geodynamic evolution of the Alpine realm.

In Permo-Triassic times, at about 300 to 200 Million years (Ma), movements of the African Plate with respect to the European Plate caused extension in the lithosphere (continental crust and lithospheric mantle) in between. Beside others the Austroalpine unit, which built up large parts of the Eastern Alps was located within this area. In the frame of the project it was tried to learn more about the evolution of the Lithosphere in an extensional regime. Especially the interaction of the lithospheric mantle, the crust and the contemporaneous sedimentary successions were of interest. The investigations comprise one of the best preserved Permo-Triassic crustal sections in the Alps, located between the Gail and upper Möll valley in Carinthia. Further additional data about the extension, the timing and the pressure and temperature conditions of the Permo-Triassic event were collected in the Eastern Alps, in Hungary and in Croatia. Based on the available data the Permo-Triassic evolution can be summarized as follows: In the early Permian an extensional environment caused the formation of basaltic melts in the lithospheric mantle. These about 1000 C hot melts migrated upwards and underplated the continental crust. Due to the input of thermal energy, acidic melts were generated within the crust and can be found as granites, pegmatites and volcanic rocks today. The latter extruded at about 280 Ma and can be found between the western part of the Southern Alps of Italy to the basement of the Great Hungarian Plain. Contemporaneously high temperatures within the crust caused the formation of new mineral assemblages (metamorphism). After that slow cooling of the lithosphere caused a rise in density and subsidence of the surface. At about 245 Ma large areas where at sea level and a shallow marine basement developed. The opening of the Meliata-Hallstatt ocean to the south of the Austroalpine unit occurred at about 235 Ma. Continous subsidence of the Austroalpine crust caused the sedimentation of more than 3000 m of shallow marine reef and lagoonal sediments on the shelf areas. At about 200 Ma the continental crust was cooled down to a relaxed geothermal gradient of c. 25 C/km depth.