Crustal evolution of the southwestern Kuril Arc, Hokkaido Japan, deduced from seismic velocity and geochemical structure

The southwestern Kuril Arc, which is not affected by a series of accretion and collision processes, is one of the most important experimental sites among the world arc–trench systems for investigating the evolution of arc crust. Published petrological and seismic studies have shown that, in south-central Hokkaido, the upper part of the Kuril Arc crust is thrusting upward toward the west while the lower part is descending downward in the same direction. Among several hypotheses for formation of continental crust, we discuss the role of delamination of the island arc lower crust based on geochemical and seismic velocity structure models. Seismic velocity structure is obtained from an onshore–offshore seismic survey across the southwestern Kuril Arc–Trench system. The structure in the back-arc area shows a thick accretionary complex formed by Paleogene accretion and collision. Based on published seismic results, this accretionary complex is considered to continue on land to the Cretaceous accretionary complex observed in the Hidaka Belt. The structure of the Kuril Arc appears characteristic of upper crustal andesitic rocks (6.0–6.2 km/s) and thick middle to lower crust (6.5–7.3 km/s). Based on published geochemical study, we consider a geochemical model of simple crustal evolution explaining the mechanism of generation of andesitic rocks on the Kuril Arc. Each volume of andesitic, basaltic, and mafic layers estimated from the geochemical model is consistent with those obtained from the seismic velocity model. Crustal composition estimated from the geochemical model and major-element chemistry of the volcanic rocks in the Kuril Arc indicate that mechanical delamination is insufficient to cause the oceanic island arc crust to evolve into continental crust.