Eastern Alpine tectono-metamorphic evolution: Constraints from two-dimensional P-T-t modeling

We use two-dimensional (2-D) P-T-t modeling to constrain the thermal and rheological aspects of different scenarios for the late Mesozoic and Cenozoic tectonic evolution of the Eastern Alps, inferred from excellent data sets from the Tauern Window (TW). Models invoking subduction of the South Penninic (SP) oceanic lithosphere during thrusting and subsequent erosion of the Austro-Alpine (AA) upper plate nappe stack are inconsistent with the observed thermal evolution within the AA and Penninic units. In these models, predictions for the AA peak thermal conditions are lower than observed. After exhumation and cooling to midcrustal levels and subduction of the continental Middle Penninic (MP) block, the AA undergoes a phase of renewed heating to almost the previous peak temperatures. Simultaneously, the Penninic units experience a phase of heating upon subduction, followed by cooling after onset of subduction of the North Penninic (NP) basin. The model predictions are inconsistent with the observed nearly isothermal uplift path of the SP after subduction and cannot explain observed inverted metamorphic peak conditions in the deeper AA (amphibolite facies) down to the higher Penninic unit (greenschist facies). A model with the beginning of subduction of the SP occurring after crustal thickening of the AA and subsequent return to normal crustal thicknesses is compatible with the P-T-t data. In this model, peak temperature conditions are higher in the AA, followed by a phase of strong cooling in the AA upper plate with the onset of underthrusting. This model also explains successfully nearly isothermal exhumation of the MP and inverted metamorphic peak conditions in the deeper AA. Material accreted to the hanging wall from the oceanic crust (SP) experiences a phase of cooling during ongoing subduction of oceanic lithosphere and begins to heal up to its thermal climax after the subduction of trailing continental lithosphere. The subsequent PT path of the Penninic units strongly depends on the timing and rates of underthrusting by the foreland. Observed PT paths in the MP within the TW require continuous subduction of the NP and the trailing European foreland under the exhuming MP block. Documented rapid cooling in the final uplift phase of the Penninic units in the TW requires exhumation rates up to approximately 4 mm/yr. Predictions of slightly elevated pre sent-day geothermal gradients in the TW area are consistent with available heat flow data. As a result of Mesozoic rifting followed by late Mesozoic crustal thickening of the AA, paleorheological reconstructions are characterized by a contrast between relatively strong oceanic lithosphere and adjacent weak continental lithosphere. Predicted decoupling of weak continental and oceanic lithosphere during subsequent subduction of the Penninic units can explain observed Late Cretaceous crustal extension in the AA units in terms of gravitational spreading. Ongoing subduction leads to an overall strength increase due to underthrusting of cool oceanic lithosphere, whereas subduction of continental lithosphere causes a strength decrease in the upper levels of the lithosphere. Continuous crustal thickening and relaxation of the depressed isotherms reduce the strength of the lower lithospheric, mantle beneath the central orogen, further enhanced by rapid late-stage uplift. Predictions for the present-day rheological structure of the Eastern Alps support the existence of a strong upper crustal layer, two wedge-shaped strong upper mantle layers to the north and the south of the orogen, and a weak upper mantle underlying the central orogen.