Abstract
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.
Subject
Paleontology,Stratigraphy,Earth-Surface Processes,Geochemistry and Petrology,Geology,Geophysics,Soil Science
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