Opening of the Sea of Japan back-arc basin by asthenospheric injection

Abstract

The position of a volcanic front on an arc is governed by dehydration reactions of amphibole + chlorite in down-dragged hydrated peridotite at the base of the mantle wedge. As these reactions are essentially pressure-dependent (ca. 3.5 GPa), the paleo-position of a volcanic front offers an estimate for the depth of the subducted oceanic lithosphere beneath it. This paper evaluates volcanism in the Northeast Japan arc since the Oligocene and discusses the mass transportation in the mantle wedge during the episode of back-arc extension in the Sea of Japan with reference to the migration of the volcanic front. The volcanic front during the Oligocene (ca. 30 Ma) was located along the western coast of the present Northeast Japan are and migrated about 200 km trenchwards at 23 Ma before the opening of the Sea of Japan back-arc basin. It follows that the angle of subduction became steeper during the period of 30-23 Ma, triggered by the injection of the asthenosphere into the mantle wedge. The back-arc extension of the Sea of Japan between 21 and 14 Ma is a manifestation of the dynamic process of the lithosphere due to the asthenospheric injection. The temperature of the injecting asthenospheric mantle was higher than that of the surrounding upper mantle or lithosphere. This caused the characteristic volcanism at 23-22 Ma: (1) High-magnesian andesite volcanism took place in the near-trench region; (2) across-arc compositional variation was not observed; (3) the volume of volcanic materials increases towards the back-arc side of volcanic arc. The volcanic front has migrated over a short distance inwards (less than 100 km) since 15 Ma, possible as a result of cooling in the mantle wedge, because both amphibole and chlorite break down at slightly higher pressures at lower temperatures. The upper mantle materials which injected into the mantle wedge and caused back-arc spreading are identical to those of the mid-oceanic ridge basalt source in Sr-Nd isotopic compositions, because volcanic rocks in the back-arc region of the Northeast Japan arc show secular variation in isotopic compositions from enriched to depleted isotopic characteristics during the last 30 Ma.