Active and fossil mantle flows in the western Alpine region unravelled by seismic anisotropy analysis and high-resolution P wave tomography

The anisotropy of seismic velocities in the mantle, when integrated with high-resolution tomographic models
and geologic information, can be used to detect active mantle flows in complex plate boundary areas, providing
new insights on the impact of mantle processes on the topography of mountain belts. Here we use a densely
spaced array of temporary broadband seismic stations to analyze the seismic anisotropy pattern of the western
Alpine region, at the boundary between the Alpine and Apenninic slabs. Our results are supportive of a poly-
phase development of anisotropic mantle fabrics, possibly starting from the Jurassic to present. Geophysical data
presented in this work, and geologic evidence taken from the literature, indicate that: (i) fossil fabrics formed
during Tethyan rifting may be still preserved within the Alpine and Apenninic slabs; (ii) mantle deformation
during Apenninic slab rollback is not compensated by a complete toroidal flow around the northern tip of the
retreating slab; (iii) the previously observed continuous trend of anisotropy fast axes near-parallel to the western
Alpine arc is confirmed. We observe that this arc-parallel trend of fast axes is located in correspondence to a low
velocity anomaly in the European upper mantle, beneath regions of the Western and Ligurian Alps showing the
highest uplift rates. We propose that the progressive rollback of the Apenninic slab, in the absence of a coun-
terclockwise toroidal flow at its northern tip, induced a suction effect at the scale of the supraslab mantle. The
resulting mantle flow pattern was characterized by an asthenospheric counterflow at the rear of the unbroken
Western Alps slab and around its southern tip, and by an asthenospheric upwelling, mirrored by low P wave
velocities, that would have favored the topographic uplift of the Alpine belt from the Mont Blanc to the
Mediterranean sea.