Some remarks on deformation and P–T conditions of the Cretaceous regional metamorphic belts in southwest Japan: reply to comment by M. Brown on ‘Thermal effects of ridge subduction and its implications for the origin of granitic batholith and paired metam

Deformation and P–T conditions of the Cretaceous Ryoke–Sambagawa metamorphic belts have been analyzed based on recent numerical modeling of plastic flow at convergent plate boundaries. The analyses suggest that a three-dimensional corner flow in the forearc wedge associated with subduction with moderate obliquity (more than 45° for the angle between the subduction direction and the trench) can explain the observed prolate strain: a large strain (more than 10), with the angle of 10–30° between the elongation axis and the strike of the regional metamorphic belts, is attributed to a combination of (1) stretching in the subduction direction above the subducting plate, and (2) stretching and rotation towards the along-trench direction at the wedge corner. Therefore, the observed large strain is not a consequence of a significant lateral displacement of the two plates. Based on the analyses, the contemporaneous Ryoke (low-P/T)–Sambagawa (high-P/T) metamorphic belts are suggested to have been formed within a single trench–forearc system by a series of events, not by accidental amalgamation of two distinct terranes. The contrasting low- and high-P/T metamorphic conditions are demonstrated to be simultaneously achieved in the forearc region during the dynamic flow associated with ridge subduction, where a large temperature gradient is produced.