Mantle deformation beneath the Alps and the physics of the subduction polarity switch: Constraints from thermomechanical modelling, seismic anisotropy and waveform modelling

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The enormous amount of high-quality seismological data that is becoming available from the densely-spaced AlpArray network in combination with new geodynamic modelling capabilities opens up unique opportunities to test current hypotheses about the deep dynamics of continental collision. A major outstanding goal is an understanding of the causes and dynamics responsible for the proposed temporal switch of subduction polarity beneath the Eastern Alps. We pose the hypothesis that in general such a switch is not a coincidental result of long-term plate tectonic history, but the direct result of specific physical conditions at a converging plate boundary. This hypothesis will be tested by studying the physics of such a prescribed or self-consistently evolving polarity switch and by combining thermomechanical modelling with seismological constraints on mantle deformation from observations and modelling of waveform effects due to seismic anisotropy.From a seismological perspective, constraining the current state of mantle deformation beneath the Alps poses a number of difficulties related to the possible influence of the crust on teleseismic waveforms and also due to depth variations of anisotropic structures in the mantle. We propose to tackle these problems by combining analyses of shear-wave splitting from (1) teleseismic XKS phases and (2) converted Psx phases from the Moho and upper-mantle discontinuities to provide a detailed image of mantle and crustal deformation patterns beneath the entire Alpine collision zone. Crustal thicknesses and anisotropies will be determined on the way. Regarding the geodynamical modelling of an orogenic lithosphere-asthenosphere system, the complexity of our time-dependent thermochemical Finite Element and Finite Difference models will be increased stepwise (from 2D to 3D) and include visco-plastic-non-Newtonian rheology, a free surface, erosion and sedimentation, dissipational heating, etc. Consistent tools to determine the evolving lattice-preferred orientations will be applied. From this, different models of seismic anisotropy will be predicted and serve as test models for the comparison with seismological observations and waveform modelling. Additionally, the output of the thermomechanical models will include the evolving topography, uplift-, subsidence and exhumation rates, stresses etc. and will be exchanged with cooperating projects within the SPP MB-4D.