Accretion of a large number of seamounts or fragments of huge oceanic plateaus is recorded in the geology of Japan. Major accretion occurred twice; a late Early Carboniferous seamount chain accreted in latest Permian to Middle Jurassic time, and a Late Jurassic oceanic plateau accreted in the Early Cretaceous. The Late Jurassic plateau now in northern Japan and Sakhalin is here named the ''Sorachi Plateau'' and a Tithonian age for it is well documented on the basis of microfossils from pelagic cherts and limestones. Petrochemical characteristics of basalts suggest two kinds of sources; depleted mantle under oceanic lithosphere and enriched mantle related to plume upwelling. All the accreted fragments in Japan can be traced back to the central part of the Panthalassa by using paleomagnetic data and plate ''trajectory analysis.'' The timing of generation of the accreted seamount chain and the plateau is consistent with a time of global change. Their place of origin, in the mid-Panthalassa, retains a positive residual geoid anomaly and slow Vp in the lower mantle, which suggest a past upwelling of hot material from the core-mantle boundary. ¿ American Geophysical Union 1994