岩石の弾性波速度に基づく島弧地殻深部及び最上部マントルの構成岩石の推定:伊豆弧と東北本州弧

摘要

Recent geophysical investigations provide the high-resolution seismic velocity structure of the crust and uppermost mantle of island arcs. This paper summarizes lithological investigations of the Northeast Honshu Arc and the Izu Arc by comparing the seismic velocity structures with high-pressure and high-temperature laboratory ultrasonic measurements of elastic wave velocity for rocks (e.g. Ichino-megata xenoliths) that are derived from the deep crust and upper mantle. The velocity structure of the lower crust of the Northeast Honshu Arc is well correlated with the tectonic history of the arc. The lower crust of the Northeast Honshu Arc is inferred to be composed mainly of pyroxene hornblende gabbro (Miocene rift basin), hornblende gabbro (Ou Backbone Range,Kitakami Lowland), and quartz-bearing rock (Kitakami Belt). Anomalously low-Vp/Vs uppermost mantle in the Kitakami Belt is inferred to be composed of orthopyroxenite, which can form by a reaction between slab-derived silicic melts and mantle peridotite. The results indicate that the lower crust and the uppermost mantle of the Izu Arc and the Northeast Honshu Arc are more heterogeneous than previously thought.