Opening of the Kuril Basin deduced from the magmatic history of Central Hokkaido, North Japan

Abstract

The Kuril Basin is a fan-shaped back-arc basin in the northwestern Pacific rim. The basin is separated by the Kuril arc from the Pacific Ocean to the southeast, and is bounded by the subsided Okhotomorsk Block to the north and by the Late Eocene to Early Miocene Hidaka magmatic arc to the west. The clastic sediments constituting the Late Cretaceous to Early Eocene Proto-Kuril arc, which underlies the modern Kuril arc, were transported from the Okhotomorsk Block; between them the Kuril Basin now intervenes. This suggests that the Kuril Basin had not opened and that the Proto-Kuril arc had been located in the southern margin of the Okhotomorsk Block at least until Early Eocene time. It also indicates that the Kuril Basin was not entrapped by the Proto- or modern Kuril arc. The Tertiary Hidaka magmatic arc in Central Hokkaido was formed along a N-S trending subduction zone where the Pacific plate was subducting beneath the Eurasian plate. Therefore, the Kuril Basin was not yet present on the east of Central Hokkaido during the period when the Hidaka magmatism was taking place. Based on the fact that the Hidaka arc magmatism ceased at 17-16 Ma and coevally along its N-S trending extension, it is suggested that the Kuril Basin opened just after 16 Ma by the southward retreat of the Proto-Kuril arc-trench system. In Central Hokkaido, an extensional stress regime during the Middle Miocene is inferred from the occurrence of A-type magmatism and graben-like sedimentary basins. This stress field can be attributed to subsidence of the Pacific slab covered by the retreated Proto-Kuril arc during the opening of the Kuril Basin. The occurrence of an undeformed and horizontal sedimentary cover on the rugged basement of the Kuril Basin is consistent with rather rapid opening at 16-15 Ma. During the Middle Miocene, the Kuril Basin and Japan Basin were formed by a similar process i.e., rotation and oceanward retreat of an arc-trench system. Opening of the two back-arc basins along different plate boundaries i.e., the Pacific-North American boundary for the Kuril Basin and the Pacific-Eurasian boundary for the Japan Basin, may have been caused by the migration of a hot region across the overriding plate boundary.