Seismic evidence for a mantle plume oceanwards of the Kamchatka-Aleutian trench junction

A non-linear iterative P-wave traveltime tomography has revealed a mantle p lume originating at a depth of nearly 1000 km, rising across the 600 km dis continuity, and deflecting subhorizontally in the uppermost mantle presumab ly by shear flow due to the overlying moving plate. Data from the Geophysic al Survey of Russia (1955-1997) were inverted jointly with the catalogues o f International Seismological Centre and USGS National Earthquake Informati on Centre (1964-1998). The result shows a 300-500 km-wide cylindrical low-v elocity anomaly (similar to -2 per cent) that extends from a depth of great er than 900 km to shallower than 200 km. The anomaly is almost vertical at depths up to similar to 400 km and rises obliquely to the north up to simil ar to 200 km under the ocean floor near the northern end of Emperor seamoun ts. Above similar to 300 km depth a subsidiary anomaly extends subhorizonta lly to the NW in fair agreement with the direction of movement of the Pacif ic Plate. The overlying seafloor is characterized by anomalously high heat flow, which may be attributed to the thermal effect of the mantle plume.