Subdivision of the hida metamorphic complex, central Japan, and its bearing on the geology of the Far East in pre-Sea of Japan time

Abstract

Petrological and structural characteristics of the regional metamorphic rocks in south-west Japan and in the Korean Peninsula make it possible to speculate on the geological correlation between Japan and the Asian continent prior to the opening of the Sea of Japan, a typical marginal sea. The Hida metamorphic complex, situated on the Sea of Japan side of southwest Japan, is subdivided into two distinct geological units, the Hida gneisses and the Unazuki schists. The Hida gneisses are polymetamorphosed Precambrian rocks, while the Unazuki schists occurring on its eastern and southern sides are low- to medium-grade metamorphic rocks originating from Upper Paleozoic deposits. Sedimentary facies and other geological features suggest that the Hida gneisses were the basemen of the Unazuki schists. Consequently, the geotectonic framework of southwest Japan is revised from north to south as follows: Hida gneiss region (Precambrian massif) Unazuki zone (late Permian intermed. P T metamorphic belt) Circum-Hida tectonic zone (Mid- to Late Paleozoic melange zone) Mino-Tanba terrain (Late Paleozoic to Mesozoic geosynclinal region) and so on. The Unazuki zone is similar to the Okcheon zone in the Korean Peninsula in respect of age, lithology and biofacies of sedimentary rocks as well as the age and type of regional metamorphism. Furthermore, the Hida gneisses and the Gyeonggi polymetamorphosed Precambrian gneisses in the Korean Peninsula are similar in the apparent baric type of metamorphism, radiometric ages and the relationship with the overlying metamorphosed formations. The similarity of the geotectonic frameworks of southwest Japan and the Korean Peninsula suggests that the Unazuki zone and the Okcheon zone once formed a continuous geotectonic unit. Thus we have a new geological coordinate in reconstructing the paleogeography prior to the opening of the Sea of Japan.