Oblique Plate-Slab Subduction Causing Various Anomalous Distributions of Volcanoes and Geothermal Systems in Japan: Simplified Modeling and Simulation

Abstract

Various anomalous phenomena are observed at the parts of island arcs and their junctions where oblique plat-slab subduction occurs. In Japan consisting of five island arcs, where the old thick Pacific plate and the young thin Philippine Sea plate have been subducting, various anomalous distributions of volcanoes and geothermal systems are observed at the parts of the oblique subduction as follows: (1) The Kyushu region where the distributions of volcanoes and related geothermal systems have migrated fairly systematically and concentrated in late Neogene to Quaternary time. (2) The Kinki region where high-temperature hot springs distribute on a line in a non-volcanic environment. (3) The Kanto region where a large coastal plain of very low geothermal gradient distributes adjacent to active volcanic belts.
A simple geometry model and simulation method for calculating the slab movement underground have been developed, and applied to the above three regions for better understating the origins of the anomalous phenomena using various kinds of open electronic Earth-sciences information. The corresponding mechanisms have been estimated to be (1) segmentation and steepening of the Philippine Sea plate-slab beneath the north part of the Ryukyu arc, (2) mantle upwelling caused by the deep subduction of the Pacific plate-slab, and segmentation and steepening of the shallow Philippine Sea plate-slab affected by the mantle upwelling under the central part of the Southwest Japan arc, and (3) covering of the segmented Philippine Sea plate-slab from the cold fore-arc region of the Izu-Ogasawara arc over the Pacific plate-slab, respectively.