Accretion complexes and related forearc basins in the Cretaceous of central Japan, with special reference to the continuity of accretion tectonism and associated metamorphism

Abstract

The Sambagawa, Chichibu and Shimanto belts in the Kanto Mountains of central Japan have been shown to be formed by accretion and the rocks of these belts have undergone various amounts of deformation and metamorphism. The three belts were successively accreted from north to south during the Jurassic to Neogene and the ages of deformation and metamorphism young towards the south. The different grades of deformation and metamorphism depend mainly on the depth of accretion of each belt. Two thin Cretaceous sedimentary units, the Atokura Formation and the Sanchu Cretaceous System are found on or near the boundary of the belts in the Kanto Mountains. These Cretaceous sediments were deposited in the forearc basins developed along the boundary of the belts and were deformed (or partly metamorphosed) by the differential movement of each belt. The deposition, deformation and metamorphism of the geological units of the Kanto Mountains in central Japan occurred serially in an accretionary regime and can be explained by accretionary tectonics. The tectonic events took place almost continually from the Jurassic to the Palaeogene.