Structure of the upper mantle under the western Mediterranean basin

The structural interpretation of phase velocity data for the western Mediterranean basin reveals a low shear velocity channel with velocity 4.10±.05 km/sec below the central part of the basin; along the margins of the basin the channel shear velocity is 4.43±.10 km/sec. The roof of the central low-velocity channel is at 50±6 km and drops to 100±20 km around the margins. The extent of the central low-velocity channel is not determined, but its low shear velocity correlates well with the channel velocity under the Alps. Data from refraction surveys indicate that material with a P-wave velocity of 7.70 km/sec lies at a depth of about 11 km under the central basin, and surface wave data show it to lie at 31±4 km under the margins. The surface wave data also show that higher-velocity material does lie between the 7.7-km/sec layer and the low-velocity channel. This material, with a shear velocity of 4.80±.15 km/sec, occurs at sufficient depth to indicate that the crust of the central part of the basin is transitional rather than oceanic in nature.