Mantle-lithospheric deformation and crustal metamorphism with some speculations on the thermal and mechanical significance of the Tauern Event, Eastern Alps

Abstract

Changes in the thickness of the mantle lithosphere beneath convergent orogens through processes such as the pervasive thickening of the lithosphere and the convective thinning of the thermal boundary layer dictate the heat flow through the base of the crust and thus may profoundly influence the thermal evolution of crustal metamorphic terrains. Calculations based on one-dimensional thermal-energy balances show that when mantle lithosphere is substantially thickened and remains intact through the orogenic cycle, then low-temperature facies series, including blueschist and eclogite facies, may be preserved throughout the crust. In contrast, crust thickened above attenuated mantle lithosphere will develop much higher-temperature facies series. Potential-energy arguments suggest that changes in the thickness in the mantle lithosphere induce changes in the elevation and the potential energy stored within the lithosphere. Therefore, the response to mantle-lithospheric deformation 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 thickened crust may cause an increase in crustal temperatures synchronous with the termination of convergent strain, or, for large reductions in mantle-lithospheric thickness, with the onset of extensional deformation. Such histories are recorded in a number of young metamorphic terrains, for example the Eastern Alps which may provide an important thermal and isostatic record of the mantle-lithospheric response to convergent deformation. Here, we show how application of these ideas may lead to new insights into some outstanding problems concerning the thermal and mechanical evolution of the Eastern Alps during the Tertiary Tauern Metamorphic Event.