The Cenozoic basaltic rocks in Sikhote-Alin and Sakhalin exhibit a change over 55 m.y. from subduction-zone-related to continental-rift-related volcanism as the Japan Sea opened between eastern Sikhote-Alin and the Japan arc. The temporal geochemical trends in volcanic rocks erupted before and during the opening of the Japan Sea, suggesting that there was a change in magma source from the lithosphere to the depleted asthenosphere as the Japan Sea opening progressed. Two geochemical groups can be identified within the basalts that postdate the Japan Sea opening: tholeiitic basalts and alkalic basalts. As the tholeiitic basalts have EMI-type signatures, we propose that the tholeiites had a significant contribution from an EMI-type Precambrian subcontinental lithospheric mantle. The alkalic basalts, yielding geochemical characteristics similar to enriched oceanic island basalts, may have been derived from the subcontinental asthenospheric mantle. The opening of the Japan Sea was triggered by lateral migration of asthenospheric mantle from beneath northeast China toward the Japan arc.