Mantle Wedge Anisotropy and Slab Subduction in the Southern Apennines

Teleseismic Receiver Functions (RFs), from a permanent broadband station in the southern Apennines (Italy) reveal and constrain the presence of anisotropy in the mantle wedge and supra-slab layers. We analysed the back-azimuth (baz) variation of the Ps converted phases using RFs computed from 337 teleseismic events recorded at MedNET station CUC. Harmonic angular analysis of the RFs dataset shows the presence of a strong first harmonic component, which is characteristic of dipping layer and/or anisotropic layer with a dipping symmetry axis. Three strong and broad phases between 4.5 and 10.5 s dominate the radial RFs data-set. For each on these phases, a polarity reversal occurs on the transverse RFs at about N45°E (the trench-normal direction). The back-azimuth dependence of the timing and amplitude of these pulses rules out a simple isotropic slab model, suggesting instead combined effects of dipping interfaces and anisotropy. The first phase can be clearly seen, on radial RFs, from all baz direction, but its polarity flips at about N120°E and N330°E. Transverse energy associated to this phase reaches its maximum from NE sector, where a clear polarity reversal is present. The second broader phase is almost always negative, on the radial, but can be clearly seen only from NE sector, where it shows a strong back-azimuth dependence of its arrival time. The third phase is always positive between N0°E and N120°E on radial RF data-set, and can also be followed in the same baz interval on transverse RFs. It disappears for events from other baz directions. We modelled the Ps converted phases using a stack of anisotropic layers with dipping symmetry axes bounded by dipping discontinuities. Stable features of our models include: (1) a thin isotropic mantle lid under the crust; (2) an anisotropic low-velocity zone (LVZ) in the mantle wedge, with plunging fast symmetry axis aligned with the trench-normal direction; (3) two anisotropic layers with lower velocity than the above mantle wedge; and (4) an isotropic mantle lid of the subducting slab.