The Apennines orogen parallels Africa-Eurasia convergence, so its origin is not a direct consequence of large-scale plate collision. The orogen has been explained by the progressive eastward retreat of a regional-scale subduction zone trapped between two continents. Along the geologically young Apennines orogen in Italy multiple lines of evidence suggest both a westward subduction of the Adriatic lithosphere and an eastward retreat of the subduction zone via slab rollback. Apennines slab rollback has been characterized as an ongoing process or a stalled process, a distinction relevant to seismic hazard in the region. An alternative scenario removes lithosphere beneath the Apennines through detachment and sinking rather than active subduction.

Slab rollback induces the sub-slab mantle material to flow parallel to the subduction zone and deflect near the slab edge. The deformation of upper mantle rocks will align olivine crystals, and cause seismic wavespeed to have directional dependence (anisotropy). The slab-rollback model can be validated by the detection of strong orogen-parallel fast-polarization of shear-wave splitting on the Adriatic side of Italy, and orogen-normal extensional fabric on the Tyrrhenian side. Alternatively, if a lithospheric fragment detaches and sinks, asthenosphere should flow towards the site of detachment (orogen-normal).

Shear-wave splitting estimates from recordings of portable seismographic stations during the first year of the RETREAT seismic deployment, in combination with broadband data from the Italian national seismic network, seem to exclude a 2-D sub-lithosphere corner flow, associated with the Apennines slab rollback, as the main source of the inferred anisotropy. Surface waves from the great Sumatra-Andaman earthquakes of 2004 and 2005 that cross Italy south of ~44°N display Love-to-Rayleigh scattered waves (quasi-Love phases) diagnostic of sharp lateral gradients in the anisotropic properties of Earth’s upper mantle. Surface waves that traverse Italy further north lack this distinctive phase, documenting a change in the upper mantle fabric that corroborates the shift in the fast polarization of shear wave birefringence. These observations suggest that orogen-parallel asthenospheric extension behind the retreating Apennines slab has limited geographical expression. The anisotropic signal may be generated by a frozen-in fabric of the Adriatic and Tyrrhenian lithosphere domains, or by flow variations induced by episodic and fragmentary slab rollback.