Stratigraphic review of Eocene to Oligocene successions along the eastern Japan Sea: Implication for early opening of the Japan Sea

Stratigraphic review of the on-land successions exposed on the back-arc side of Japan and Sakhalin, combined with supplemental radiometric dating, reveals the existence of Late Eocene or Oligocene marine sediments along the eastern coast of the Japan Sea. This finding implies that the origin of the Japan Sea dates back to the Oligocene or earlier time, much older than ever supposed. The host Eocene to Oligocene subaerial to shallow-water successions distribute immediate west of the volcanic front of that time and contain volcanic rocks, the chemistry of which is characteristic of continental rift or volcanic arc. They are associated with parallel dike swarms or normal faults, and could be infillings of an incipient back-arc rift. Collective evidence suggests that the rift was rapidly subsided and spread during the following Early to Middle Miocene time. However, a back-arc-wide unconformity was produced during a short period from the latest Oligocene to the earliest Miocene and was succeeded by extensive bimodal volcanism with rapid migration of the volcanic front to the fore-arc side. These short-term events could have been induced by upwelling or inflow of hot asthenosphere beneath the arc-rift system, followed by rapid subsidence and spreading of the Japan Sea.