The mid-Cretaceous Oku-Niikappu Dam accretionary complex (ONDC) in the Idonnappu Zone of Hokkaido contains volcanic, plutonic, and ultramafic rocks with island-arc chemical features. The volcanic rocks are overlain by radiolarian chert. This stratigraphic succession represents an ancient island arc, in which activity ceased within a pelagic environment prior to accretion to the continental margin. The ophiolitic rocks of the ONDC, which may have comprised upper mantle, arc crust, and cover sediments, are thus regarded as accreted fragments of an intraoceanic remnant arc. Basaltic to andesitic volcanics, gabbroic to tonalitic plutonics, and serpentinite are constituents common to the ONDC remnant arc and equivalent settings in the modern Philippine Sea. They also have a common cover sedimentary sequence, from volcanic basement through volcaniclastic debrites to pelagic sediments. This sequence may record the history from arc activity through back-arc rifting to intraoceanic isolation. Clasts of serpentinite, plutonic, and metamorphic rocks in the volcaniclastic debrites in the ONDC and those from the Daito Ridge in the Philippine Sea imply extensional exhumation of lower crustal and upper mantle rocks during back-arc rifting.
