P and S wave finite-frequency tomography reveals the impact of slab interference on mantle flow beneath the greater Alpine region

The Cenozoic evolution of the tortuous Adria-Europe plate boundary zone suggests potential interference between opposite-dipping slabs in the upper mantle, with potential perturbation of the surrounding mantle flow. However, compelling seismic evidence of slab interference beneath the greater Alpine region is still lacking. Here we use P- and S-wave velocity models based on the Finite-Frequency Tomography method to reveal the interactions between slabs imaged beneath the greater Alpine region and their relationships with prominent low-velocity anomalies in the upper mantle. We document high-velocity anomalies beneath the Alps, the Apennines and the northern Dinarides. A velocity gap is imaged between the Alpine and Apenninic slabs. Low-velocity anomalies are found to the west of the Alps, beneath the Po Plain, and on either side of the central-southern Apennines. Because of slab interference, low-velocity anomalies of different origins, either due to asthenospheric upwelling or slab fluids, coexist in nearby regions of the upper mantle. The lack of major gaps between the Western Alpine slab and the retreating Apennine slab makes a potential toroidal flow around the northern tip of the Apennine slab during slab rollback difficult, with consequent activation of an asthenospheric counterflow and associated mantle upwelling to the west of the Alps. Beneath the Eastern Alps, the Alpine slab is torn and overturned by the NE-dipping Dinaric slab. The asthenospheric mantle beneath the Po Plain is confined by slabs and shows a prominent low-velocity anomaly likely due to melting triggered by carbon-rich supercritical fluids generated above the Alpine slab. Toroidal flow may occur farther south around the Apenninic slab, in a region where the mantle flow is not affected by interaction between slabs. Similar complexities may characterize other plate-boundary zones where the interplay between opposite-dipping slabs is suggested by geophysical and geological data, such as the Taiwan orogeny or the Hindu Kush - Pamir - Tianshan tectonic knot in Central Asia.