Looking for layered anisotropic structures in the mantle beneath the northern Apennines

- Layered seismic anisotropy in the northern Apennines is found only at a few stations.
- Shallow-layer anisotropy consistent with plate drag in Po-Plain area.
- Shallow-layer anisotropy related to a remnant lower-crust deformation in Tuscany.
- Deeper anisotropy in both areas consistent with the slab-retreat mantle flow.

Competing geodynamic scenarios proposed for northern Apennines (Italy) make very different predictions for the orientation of strain in the upper mantle. Constraints on the pattern are offered by observations of seismic anisotropy. Previous study of the anisotropy beneath the northern Apennines used birefringence of core-refracted shear waves (SKS phases), and demonstrated the presence of two domains: Tuscan and Adria. In the transition between the two domains, across the Apennines orogen, anisotropy measurements reflect a complex deep structure. To define better the upper-mantle structure beneath this area we analyze seismological data recorded by a set of seismic stations that operated for 3 years, between 2003 and 2006, located in the outer part of the Apennines belt, in the Adria terrane, collected by the RETREAT Project. Directionally distributed sets of SKS records were inverted for layered anisotropic structures with a well-tested method, adding new results to previous hypotheses for this area. New data analysis argues for two-layer anisotropy for sites located on the Apennines wedge and also one site in the Tuscan terrane. Beneath the wedge an upper layer with nearly north-south fast polarization pervades the lithospheric mantle, while at depth a nearly NW-SE Apennines-parallel direction is present in the lower layer. Beneath Tuscany a shallower NW-SE direction and a deeper E-W one suggest the deeper strain from active slab retreat, with a mantle-wedge circulation (i.e. an east-west corner flow), overlain by an Apennines-parallel fast polarization that could be a remnant of lower-crust deformation.