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Post-Palaeogene volcanism in Japan is closely related to the evolution of the Japanese island-arc system that was initiated with the Japan Sea opening (JSO) in Early–Middle Miocene times, and is reflected in the distribution of volcanoes and geochemical variations of volcanic rocks presently observed. In the NE Japan Arc at the beginning of the JSO, the volcanic field migrated towards the oceanic side and the isotopic composition of magma temporally changed in the back-arc side of the system. This was likely caused by injection of hot asthenosphere into the mantle wedge and damming of the subducting slab. During the JSO, rift volcanism in the back-arc region was characterized by numerous grabens that filled with voluminous volcanic rocks derived from bimodal low-K tholeiitic basalt and silicic magmas. In the SW Japan Arc, a period of Middle Miocene forearc volcanism at 147–12 Ma was fed by high-magnesium andesite and caldera-forming silicic magma, and is interpreted as a result of subduction initiation associated with the oceanwards migration of the SW Japan Arc and under-thrusting of the hot Shikoku Basin.

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