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Pn data collected within southern Europe and the Mediterranean are used to tomographically image variations in both seismic velocity and seismic anisotropy. Seismic anisotropy is an essential part of the inversion, and without it, several low velocity features within the uppermost mantle could not be properly imaged. The technically active mantle of southern Europe has much lower seismic velocities (7.6–8.1 km/s) than the more stable mantle of the sub‐African plate of the Adriatic sea (8.3 km/s). However, the most dramatic features within Europe's uppermost mantle relate to the Apennine, Dinaride, and Hellenide arcs. These arcs all have extremely low (5%) amounts of arc‐parallel anisotropy. The Tyrrhenian and Aegean back arc regions also show low velocities (7.7–7.9 km/s) but less anisotropy. The same may be true for the Pannonian Basin, but the tomography has poorer resolution there. A model explaining these observations focuses on subducted water metasomatizing the mantle wedge. The addition of water causes melting, creates arc volcanism, lowers the seismic velocity, and enhances the formation of anisotropy due to preferential olivine orientation. For collisional arcs of the northern Mediterranean, arc‐parallel anisotropy has formed in response to compression across them and extension along them. Within back arc regions, water is no longer a major factor. Instead, convection associated with subduction and back arc extension controls the anisotropy.
Thomas M. Hearn (Sat,) studied this question.