Uppermost mantle velocities and anisotropy beneath Europe

Pn data collected within southern Europe and the Mediterranean are used to tomographically image variations in both seismic velocity and seismic aniso tropy. Seismic anisotropy is an essential part of the inversion, and withou t it, several low velocity features within the uppermost mantle could not b e properly imaged. The tectonically active mantle of southern Europe has mu ch lower seismic velocities (7.6-8.1 km/s) than the more stable mantle of t he sub-African plate of the Adriatic sea (8.3 km/s). However, the most dram atic features within Europe's uppermost mantle relate to the Apennine, Dina ride, and Hellenide arcs. These arcs all have extremely low (<7.8 km/s) man tle velocities beneath them and considerable (>5%) amounts of are-parallel anisotropy. The Tyrrhenian and Aegean back are regions also show low veloci ties (7.7-7.9 km/s) but less anisotropy. The same may be true for the Panno nian Basin, but the tomography has poorer resolution there. A model explain ing these observations focuses on subducted water metasomatizing the mantle wedge. The addition of water causes melting, creates are volcanism, lowers the seismic velocity, and enhances the formation of anisotropy due to pref erential olivine orientation. For collisional arcs of the northern Mediterr anean, are-parallel anisotropy has formed in response to compression across them and extension along them Within back are regions, water is no longer a major factor. Instead, convection associated with subduction and back are extension controls the anisotropy.