Global Mapping of the Uppermantle by Surface Wave Tomography

Surface wave tomography compliments detailed body wave studies by providing a global framework for the lateral variability of the uppermantle. In particular the method allows one to map the mantle beneath the lithosphere and to discuss the fate of overridden oceanic plates. Midocean ridges appear to extend to at least 400 km. By contrast, the very high velocities associated with shields are primarily much shallower. The Red Sea-Afar region is a pronounced and deep low-velocity anomaly. A significant uppermantle anomaly has been found in the central Pacific. This "Polynesian Anomaly" is surrounded by hotspots; Hawaii, Tahiti, Samoa and the Caroline Islands. This may be the site of the extensive Cretaceous volanism which generated the plateaus and seamounts in the western Pacific. Anisotropy indicates deep upwellings, >300 km depth, under midocean ridges, the Afar and the Polynesian Anomaly and downwelling under the western Pacific and the northeastern Indian Ocean. The large fast anomaly under the south Atlantic may represent overridden Pacific plate