Geotectonic Subdivision of the Japanese Islands Revisited: Categorization and Definition of Elements and Boundaries of Pacific-type (Miyashiro-type) Orogen

Abstract

The geotectonic subdivision and relevant definitions of geotectonic units in the Japanese Islands are revised on the basis of new data, particularly with detrital zircon dating of U-Pb ages and seismic profiling of the deep arc crust across the islands. In addition to the final confirmation of the subhorizontal structures of the Paleozoic to Cenozoic accretionary complexes and their high-P / T metamorphosed equivalents, several new aspects were recognized; i.e., detection of the eastern extension of the collisional suture between the Sino-Korean and Yangtze cratons in the Higo belt with medium-pressure-type metamorphism in SW Japan, and separation of the traditional Sanbagawa belt into two distinct metamorphic belts characterized by mutually different ages of protolith AC-formation and peak metamorphism. The occurrence and consumption of 4 Paleozoic to Mesozoic granite batholiths, as major provenances for the ancient Japanese Islands, are documented by detrital zircon dating of Paleozoic-Mesozoic sandstones. With respect to these new findings, the definitions of unit boundaries were thoroughly revised in terms of chronological spectrum in "ocean plate stratigraphy-metamorphism". The geological significance of 5 major tectonic lines （faults） of the Pacific-type （or Miyashiro-type） orogen in Japan, i.e., the Nagato-Hida marginal TL, Osayama-Omi TL, Ishigaki-Kuga TL, Paleo-Median TL, and Butsuzo TL, is discussed. The current revision of the geotectonic subdivision and definitions of component units and their mutual boundaries leads to the following conclusions, which challenge the conventional understanding of the orogenic history of the Japanese Islands. （1） Proto-Japan in the Early Paleozoic was located closer to the South China （Yangtze） craton rather than the North China （Sino-Korean） craton. （2） Ever since 520 Ma, subduction of past ＊ 東京大学大学院総合文化研究科宇宙地球科学教室 ＊＊ 東京工業大学大学院理工学研究科地球惑星科学教室 ＊＊＊ 成蹊高校 ＊＊＊＊ 富山大学理学部地球科学教室