A detailed study of seismic wave propagation in the Fiji-Tonga region shows that there exists in the upper mantle an anomalous zone whose thickness is about 100 km and whose upper surface is approximately defined by the zone of seismic foci extending to depths of about 700 km. Attenuation of seismic waves within the zone is anomalously low and velocities are high. Other island arcs appear to be associated with similar zones.The anomalous zone in Tonga can be interpreted as the westernmost portion of a block of lithosphere that has been thrust, or dragged, or has settled beneath the island arc. Such mobility of the lithosphere suggests a key role in geotectonics for this layer of strength and raises a number of possible new solutions to long standing problems. For example, assuming that deep earthquake zones throughout the world are a measure of the amount of underthrusting during the last 107 years, an average rate of spreading over the entire worldwide rift system can be obtained as a check, but not a proof, of the hypothesis. The half-velocity obtained is 1.3 cm/year and is reasonable in light of current knowledge.