A kinematic subduction model for the genesis of back-arc low-K volcanoes at a two-overlapping subduction zone, central Japan: another volcanic front originated from the Philippine Sea plate subduction

Abstract

Systematic geochemical and volumetric variations have been documented across many island arcs. Volcanism in the Shinetsu highland, the northeastern part of central Japan, shows various anomalies in such variations, though located in a subduction zone. In this highland, the distribution density of volcanoes and total volume of erupted materials decrease from the volcanic front toward the back-arc side. However, they increase again in the more distant back-arc area. Lavas from this back-arc area characterized by a low-K nature, are equivalent to those from the trench side of northeastern Japan. This volcanism occurred since 1.7 Ma, taking the place of the usual back-arc-type high-K volcanism. Furthermore, in the fore-arc region of this highland, the volcanic front had retreated about 20 km toward the back-arc side, during 4 – 3 Ma. Presently, the depth to the Wadati-Benioff zone beneath the volcanic front is 20–30 km deeper than that beneath the main part of northeastern Japan. To explain these features, a kinematic subduction model is proposed here which assumes the northward subduction of the Philippine Sea (PHS) plate over the previously subducting Pacific (PAC) plate. In this model, the retreat of the Pliocene volcanic front in the fore-arc region is considered to have resulted from the migration of a high-temperature region of the wedge mantle (> 1400 °C) toward the back-arc side by the sweeping effect of the subducting PHS plate at about 4-3 Ma. Generation of the low-K volcanism in the back-arc side of this highland is interpreted to have been caused by the increase of the supply rate of H 2O related to the dehydration of amphibole and chlorite dragged by subduction of the PHS plate which occurred slightly before 1.7 Ma.