Geotectonic subdivision and areal extent of the Sangun belt, Inner Zone of Southwest Japan

Abstract

The Sangun belt has long been considered to be a major coherent glaucophanitic terrane of Permian to Triassic age, and to be paired with the low‐P/T Hida belt to the north. However, recent progress in geochronology, metamorphic geology, and tectonics has revealed that the belt is in fact comprised of two geologic units of different ages and with contrasting conditions of formation. The older unit is named the Renge belt and the younger the Suo belt. The Renge belt is the oldest of the high‐P/T metamorphic belts in the Japanese Islands and extends from northern Kyushu, through the San‐in coastal regions, to the Hida marginal belt. It is characterized by 330–280 Ma ages and the association of glaucophane–schist to epidote–amphibolite facies schists. The Renge belt is also typically associated with meta–ophiolite sequences (470–340 Ma) including serpentinite. The Suo belt is characterized by 230–160 Ma high‐P/T schists closely related to weakly metamorphosed Permian accretionary rocks of the Akiyoshi belt. Metamorphic facies series is from the prehnite–pumpellyite facies through the pumpellyite–actinolite and glaucophane–schist facies to the epidote–amphibolite facies. The belt is widespread in west Kinki to north and central Kyushu via Chugoku, but also stretches further to the southwest and is present in the Ishigaki‐Iriomote Islands of the southern Ryukyu Arc. Throughout this belt, there are scattered small blocks or lenses of meta–ophiolite, whose K–Ar ages of relict hornblendes are 590 to 220 Ma. Bounded by low‐angle faults and thrusts, both belts define subhorizontal nappes dipping gently north. The geotectonic framework in the Inner Zone of Southwest Japan is made up of, from north to south, the Hida‐Oki, Renge, Akiyoshi, Suo, Maizuru plus ultra‐Tamba, Mino‐Tamba, and Ryoke belts, with a tectonically downward‐younging polarity. This has resulted from stepwise accretions during Palaeozoic to Mesozoic time.