Volcano-tectonic interactions during rapid plate-boundary evolution in the Kyushu region, SW Japan

摘要

Evolution of the local plate tectonic and volcanic system relationship at Kyushu Island is defined by major changes in tectonics and volcanic style at ca. f5, 10, 6, and 2 Ma. Plate reconstructions presented here suggest that prior to 15 Ma, the Pacific plate subduction dominated Kyushu tectonics. From 15 to 6 Ma, the evolving relative plate motions shifted the triple junction between the Pacific plate, Philippine Sea plate, and southwest Japan northwards, so that the Philippine Sea plate was subducted beneath Kyushu. We suggest that a lack of subduction-related volcanism from 10 to 6 Ma is due to shallow subduction of the young Shikoku Basin lithosphere. By 6-5 Ma, changes in the Philippine Sea plate motion led to more rapid, nearly trench-normal, subduction of the Eocene west Philippine Basin crust beneath Kyushu. This model is supported by an increase in arclike geochemistry of lavas since ca. 6.5 Ma. Subduction of fluid-rich features such as the Kyushu-Palau ridge introduced large volumes of fluids into the Kyushu arc system, leading to voluminous volcanism across Kyushu, focused particularly in areas where the ridge subduction occurs in tandem with local extensional tectonics. Key issues, such as the timing of Izu arc collision with central Japan and the history of motion of the Philippine Sea plate, are reassessed here, resulting in a model that favors Izu arc-central Japan collision at ca. 8-6 Ma, rather than the more widely accepted date of ca. 15 Ma.