Shear-Wave Splitting Reveals Layered-Anisotropy Beneath the European Alps in Response to Mediterranean Subduction

Abstract

The European Alps formed at the boundary between the Eurasian plate and Adriatic microplate within a complex system of collision and subduction. However, the large-scale three-dimensional mantle-flow field related to the underlying geodynamic processes has not yet been resolved in detail. In this study, we present the first comprehensive analysis of layered anisotropy for the complete Alpine range from shear-wave splitting measurements at 591 seismic stations of the AlpArray experiment. Our findings suggest a combination of asthenospheric and distinct lithospheric contributions to the splitting observations, which can be seen as a generalization of previously reported models of single-layer anisotropy. The enhanced vertical resolution exposes the impact of successive Mediterranean tectonic episodes, such as the opening of the Provençal-Ligurian and Tyrrhenian Basins alongside the Adriatic slab retreat, as well as the Pannonian Basin opening and the Aegean slab retreat, resulting in deformation of the lithosphere and flow in the asthenospheric mantle. The dominant role of the larger scale Mediterranean subduction systems on mantle dynamics becomes evident. The observations provide supporting evidence that the Eurasian slab has broken off at its boundaries and that the resulting gaps channel flow from the mantle beneath the Eurasian plate to the Adriatic and Aegean subduction systems. The results provide new constraints on geodynamic processes involved in forming the European Alps, as previous tectonic episodes are preserved in the anisotropic fabric of the lithosphere-asthenosphere system. This raises new questions regarding their geochemical and geophysical conditions, and their larger-scale impact on the formation of the Alpine orogeny.