MANTLE-LITHOSPHERIC DEFORMATION AND CRUSTAL METAMORPHISM WITH SOME SPECULATIONS ON THE THERMAL AND MECHANICAL SIGNIFICANCE OF THE TAUERN EVENT, EASTERN ALPS

Changes in the thickness of the mantle lithosphere beneath convergent orogens through processes such as the pervasive thickening of the lith osphere and the convective thinning of the thermal boundary layer dict ate the heat flow through the base of the crust and thus may profoundl y influence the thermal evolution of crustal metamorphic terrains. Cal culations based on one-dimensional thermal-energy balances show that w hen mantle lithosphere is substantially thickened and remains intact t hrough the orogenic cycle, then low-temperature facies series, includi ng blueschist and eclogite facies, may be preserved throughout the cru st. In contrast, crust thickened above attenuated mantle lithosphere w ill develop much higher-temperature facies series. Potential-energy ar guments suggest that changes in the thickness in the mantle lithospher e induce changes in the elevation and the potential energy stored with in the lithosphere. Therefore, the response to mantle-lithospheric def ormation observable in crustal metamorphic terrains should not only be recorded in the thermal regime but also in changes in the incremental strain history in as much as it reflects the force balance operating within the orogen. For example, mantle-lithospheric thinning beneath t hickened crust may cause an increase in crustal temperatures synchrono us with the termination of convergent strain, or, for large reductions in mantle-lithospheric thickness, with the onset of extensional defor mation. Such histories are recorded in a number of young metamorphic t errains, for example the Eastern Alps which may provide an important t hermal and isostatic record of the mantle-lithospheric response to con vergent deformation. Here, we show how application of these ideas may lead to new insights into some outstanding problems concerning the the rmal and mechanical evolution of the Eastern Alps during the Tertiary Tauern Metamorphic Event.