Tectonic framework of continental subduction - testing hypotheses and developing criteria

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The initiation of subduction and the subduction of continental crust are two key processes of global tectonics which are unsufficiently understood. This project will test three alternative hypotheses (intracontinental subduction, continent collision, and ablative subduction), and develop criteria to distinguish between these.In most instances where continental crust has been subducted, this occurred after the subduction of oceanic lithosphere, when a continental margin entered the subduction zone. In some cases it has been proposed that continental subduction started within continental lithosphere, triggered by compressive stresses acting on pre-existing weaknesses or by density heterogeneities. Intracontinental subduction has also been proposed for the subduction of the Eo-alpine (Cretaceous) high-pressure belt of the Eastern Alps, an eclogite-bearing thrust sheet discontinuously cropping out over a distance of ca. 400 km in Austria, Italy, and Slovenia, because of an apparent lack of oceanic crust and the relatively high thermal gradient in the subduction zone. Other authors have interpreted the high-pressure belt as the result of continent collision after subduction of a narrow ocean embayement, or as the result of ablative subduction, that is, down-dragging of upper-plate rocks into the subduction zone. These three models are testable working hypotheses for our project.To test them and to develop criteria to distinguish between them, we will (1) date protoliths of subducted rocks using LA-ICPMS on zircon and characterize them geochemically, in order to find evidence for either a continental or oceanic nature of the subducted crust; (2) determine the P-T-evolution of eclogites using microprobe analysis and thermodynamic modeling; (3) date their prograde metamorphism using Lu/Hf geochronometry in order to increase the accuracy of age information and to find if the terrane was subducted in one piece or in several pieces; and (4) study the kinematics of shear zones delimiting the high-pressure terrane in order to define how it was emplaced after high-P metamorphism. This study builds on our long-standing collaboration with the Cologne isotope geochemistry group and the methods developed in this collaboration, to determine and date the structural and pressure-temperature history of eclogite-facies rocks with the highest precision possible at the present state of the art. We use the Alpine field example because of the unrivaled wealth of structural, petrological, and geochronological data that is already available for this area.