ACCRETIONARY RECORDS OF THE JAPANESE ARCHIPELAGO, NW PACIFIC RIM

Alkaline basalts are thought to be more readily incorporated within accretionary complexes than mid-ocean ridge basalts (MORB) as the former are generally hosted by protruding volcanic edifices within subducting regions of the lithosphere. The Cretaceous tectonic accretion of these seamounts into the Japanese archipelago of the NW Pacific acted as a significant control on the accretionary tectonics of the Pacific Rim. Seamount assemblages consist of alkaline volcanic rocks intercalated with radiolarian and hemipelagic cherts and limestone. The present-day NW Pacific Plate contains a significant number of seamounts that formed around 100 Ma by short-lived hotspot systems. Newly identified group of knolls are also present within the NW Pacific Plate as well, which is petit-spot knolls that formed during the past 10 Ma. The melts from asthenosphere ascended to the surface along brittle fractures within the flexed upper lithosphere. Such magmas have also been recorded along fissures within the Chile and Java trenches. This means that petit-spot volcanic activity is a ubiquitous phenomenon. These young lavas are also likely to be preferentially incorporated into accretionary wedges as they are found in locations stratigraphically above pelagic sediments on the seafloor.

Jurassic to Cretaceous alkaline basalts crop out in the eastern Hokkaido region of the NE Japanese archipelago. Prior to the present-day Pacific Plate, this area recorded the subduction of the Izanagi Plate beneath paleo-Hokkaido, causing the accretion of the Jurassic alkaline basalts into the Tokoro Group and the eruption of fore-arc alkaline basalts onto a forearc sedimentary basin that contains the Late Cretaceous Nemuro Group. This suggests that the subduction of the Izanagi Plate during the Late Cretaceous involved some unusual seamount subduction activity. In addition, radiometric ages for alkaline lavas in the Mineoka ophiolite of the central Japanese archipelago suggest that volcanoes erupted near the paleo-Japan continental arc immediately before the emplacement of this Late Cretaceous to Paleogene ophiolite. The tectonic setting that caused the plate flexure associated with the petit-spot volcanism in this area may have occurred before the emplacement of this ophiolite.