NATURE OF PERMIAN OPS MÉLANGE IN AKIYOSHI BELT, SOUTHWEST JAPAN

The accretionary complexes of Japan are divided into Carboniferous-Permian, Jurassic-Early Cretaceous, Late Cretaceous-Paleogene, and Neogene complexes. Among them the Permian mélanges of the Akiyoshi belt is characterized by large bodies of reef limestone ranging in age from Early Carboniferous to Middle Permian, and is one of the Permian accretionary complexes formed along the eastern subduction margin of the Asian continent. It consists mainly of basalt, limestone, chert, rhyolitic tuff, mudstone and sandstone in ascending order. The composition and stratigraphic order show the typical “ocean plate stratigraphy” (Wakita, 2014 IGR accepted). The Akiyoshi belt locally includes mélanges in some places, e.g. the Tsunemori Formation. These mélanges are very different from the mélanges of Jurassic mélanges of the Mino belt and from the Late Cretaceous mélanges of the Shimanto belt, Southwest Japan, which are highly sheared during subduction tectonics in relatively deeper levels. On the other hand, the mélange matrices of the Permian accretionary complex of the Akiyoshi belt are less sheared during accretion, probably due to in relatively shallower levels. Similar nature of mélanges is recognized in the other Permian mélanges the Kurosegawa belt and in the Early Cretaceous mélanges of the Sanbozan belt. The reason why the Permian and Early Cretaceous mélanges are different from other ages is because they may indicate the end of accretionary prism development as products of last subduction stage before the tectonic setting changes, when large bodies of basalt and limestones are stuck in relatively shallower levels to make the subduction zone jump further outboard. The nature and cause of the Permian mélange of the Akiyoshi belt is key to understand the ancient tectonic history and plate tectonic setting.