Mantle flow below the central and greater Alpine region: insights from SKS anisotropy analysis at AlpArray and permanent stations

Abstract

The Alpine chain in western and central Europe is a complex orogen developed as a result of the African–Adriatic plate convergence towards the European continent and the closure of several Tethys oceanic branches. Seismic tomography studies detected high-wave-speed slabs plunging beneath the orogen to variable depths and a potential change in subduction polarity beneath the Central Alps. Alpine subduction is expected to leave a significant imprint on the surrounding mantle fabrics, although deformation associated with the Hercynian Orogeny, which affected Europe prior to the collision with Adria, may have also been preserved in the European lithosphere. Here we estimate SKS anisotropy beneath the central and greater Alpine region at 113 broadband seismic stations from the AlpArray experiment as well as permanent networks from Italy, Switzerland, Austria, Germany, and France. We compare the new improved dataset with previous studies of anisotropy, mantle tomography, lithospheric thickness, and absolute plate motion, and we carry out Fresnel analysis to place constraints on the depth and origin of anisotropy. Most SKS directions parallel the orogen strike and the orientation of the Alpine slabs, rotating clockwise from west to east along the chain, from −45 to 90∘ over a ∼700 km distance. No significant changes are recorded in Central Alps at the location of the putative switch in subduction polarity, although a change in direction variability suggests simple asthenospheric flow or coupled deformation in the Swiss Central Alps transitions into more complex structures beneath the Eastern Alps. SKS fast axes follow the trend of high seismic anomalies across the Alpine Front, far from the present-day boundary, suggesting slabs act as flow barriers to the ambient mantle surrounding them for hundreds of km. Further north across the foreland, SKS fast axes parallel Hercynian geological structures and are orthogonal to the Rhine Graben and crustal extension. However, large splitting delay times (>1.4 s) are incompatible with a purely lithospheric contribution but rather represent asthenospheric flow not related to past deformational events. West of the Rhine Graben, in northeastern France, anisotropy directions are spatially variable in the proximity of a strong positive seismic anomaly in the upper mantle, perhaps perturbing the flow field guided by the nearby Alpine slabs.