Mantle deformation during opening of the Japan Sea back-arc: Insights from peridotite xenoliths, Kawashimo, southwest Japan

Abstract

For a better understanding of plate dynamics such as the opening of Japan Sea back-arc basins, it is critical to know why and how back-arc spreading/rifting occurs adjacent to convergent plate boundaries. Two models have been proposed for the formation of Japan Sea back-arc basins: the “slab rollback model” and the “plume model”. We report here on 12 spinel peridotite xenoliths in the alkali basalt of the Cenozoic Kawashimo volcano in southwest Japan. These xenoliths are dominantly harzburgite with some lherzolite. The harzburgites and lherzolites show coarse-grained or porphyroclastic textures that record variable degrees of deformation. Olivine and spinel compositions indicate the samples are residual mantle peridotites formed under various degrees of partial melting. Olivine crystallographic preferred orientations in the xenoliths have orthorhombic patterns characterized by a strong [010] maximum, and this orthorhombic pattern implies that the olivine deformed by dislocation creep on the (010)[100] slip system. Using a sub-grain size piezometer, the maximum differential stress varies from 3 to 13 MPa in the samples. We also found that the peridotite xenoliths deformed under relatively high strain-rates, such as 10−13 to 10−10 s−1, using olivine flow laws and the obtained stress and grain size. The range of high strain-rates is comparable to that predicted using a thermomechanical model of back-arc spreading/rifting. Therefore, it is probable that the peridotite xenoliths preserve textures that formed during the mantle deformation that accompanied the Japan Sea back-arc spreading/rifting. We estimated an equilibrium temperature of 1238 ± 20 °C using the two-pyroxene geothermometer, and this temperature is higher than any temperature reported previously for peridotite xenoliths in southwest Japan. The obtained equilibrium P–T condition for the xenoliths implies that the hot mantle plume hypothesis best explains the mechanism of formation of Japan Sea back-arc basins.