What does radial anisotropy tell us about the Alpine collision?

The Alpine–Apennine–Dinaric system records the complex interaction between continental collision, subduction, slab rollback, and lithospheric deformation involving the Eurasian and Adriatic plates. While isotropic seismic velocity models have significantly advanced our understanding of Alpine deep structure, they often fail to uniquely resolve deformation styles, slab geometry, and crust–mantle coupling. Here we use seismic radial anisotropy as a tool to investigate how deformation is distributed from the upper crust to the upper mantle across the Alpine collision zone.

Using the AlpRA25 model, a new high-resolution 3-D shear-wave velocity and radial anisotropy model derived from joint inversion of Rayleigh and Love surface waves, we image systematic variations in the radial anisotropy parameter ξ = Vsh²/Vsv² from 5 to 250 km depth. The AlpRA25 model reveals spatially coherent variations in radial anisotropy that correlate with major tectonic features and deformation domains.

In the upper crust, negative radial anisotropy (ξ < 1) spatially correlates with major fold-and-thrust belts, steeply dipping fault systems, and the Eurasian–Adriatic plate interface, indicating the dominance of subvertical fabrics produced by shortening and tectonic stacking. Similar signatures are observed in regions of extended or oceanic crust in the western Mediterranean, consistent with steep faults and dyke intrusions formed during rifting and back-arc extension. In the Northern Apennines, radial anisotropy in the upper crust reflects the overprint by extensional structures in the Tyrrhenian domain (ξ > 1), and the compressive tectonic structures in the Adriatic domain (ξ < 1). 

The lower crust beneath much of the Alps and Northern Apennines is characterized by strong positive radial anisotropy (ξ > 1) likely of Eurasian origin, indicating pervasive subhorizontal fabrics and ductile deformation, consistent with lower-crustal flow and, locally, with crustal thickening related to delamination.

In the upper mantle, pronounced negative radial anisotropy is imaged within the subvertical segments of recent Eurasian and Apenninic slabs, consistent with vertically oriented olivine fabrics produced by slab descent. Surrounding mantle domains including the western Alps, are dominated by positive radial anisotropy.

Overall, radial anisotropy provides independent constraints on deformation geometry, slab dynamics, and crust–mantle coupling in the Alpine region, demonstrating that anisotropy is essential for discriminating between competing geodynamic models of continental collision that cannot be resolved using isotropic velocities alone.