日本におけるプレート造山論の歴史と日本列島の新しい地体構造区分

抄録

The Japanese Islands consist fundamentally of late Paleozoic to Cenozoic accretionary complexes that formed in situ in a subduction zone along the East Asian continental margin, i.e. 2.0 Ga Yangtze craton (South China) and 450 Ma fore-arc ophiolite. Recent research utilizing microfossil and radiometric dating has distinguished several major accretionary complexes, including high-P/T metamorphosed parts, and subordinate ophiolites. In particular, recognition of oceanic plate stratigraphy and age of subduction-related metamorphism for individual accretionary complex allows the geotectonic subdivision of the Japanese Islands be emended with a new definition of geotectonic units and their mutual boundaries. Removing the effect of arcrelated magmatism and secondary tectonic modification by microplate activities such as backarc basin opening, fore-arc sliver movement, and arc-collision, a remarkable oceanward younging polarity is recognized among the accretionary complexes. This polarity in growth is well observed in Southwest Japan where seven distinct units occur, i.e. from the Japan Sea side to the Pacific side: 400-300 Ma high-P/T schists, Permian (250 Ma) accretionary complex, 230-180 Ma high-P/T schists, Jurassic (180-140 Ma) accretionary complex, 100 Ma high-P/T schists, Late Cretaceous (80 Ma) accretionary complex, and Tertiary (50-20 Ma) accretionary complex. The sinuous surface trajectories of these geotectonic boundaries and occurrence of several tectonic outliers and windows indicate that all these complexes, including high-P/T schists, occur as subhorizontal (or gently northward dipping) thin tectonic unit, i. e. nappe. Thus the Japanese Islands form a huge pile of nappes that become younger structurally downward to the modern Nankai accretionary complex. What is remarkable in this subhorizontal orogen is that high-P/T units are tectonically intercalated between low-P units, e. g. the thin nappe of 100 Ma Sanbagawa blueschists between Jurassic and Late Cretaceous accretionary complexes of the prehnite-pumpellyite facies. Uplift of the Sanbagawa high-P/T unit appears to correlate with the arrival of the Kula/Pacific spreading ridge at the trench, suggesting that this high-P/T accretionary complex may have been extruded and uplifted into low-P domain in fore-arc by buoyant subduction of the spreading ridge at the trench. Evidence of ridge subduction at that time is supported by reconstructed paleoplate motion and the coeval climax of arc-related Ry-oke magmatism associated with low-P/T regional metamorphism. Formation of older high-P/T blueschist nappes sandwiched between low-P units can be explained likewise. Subduction of major spreading ridges seems most critical for the episodic oceanward development not only of subhorizontal high-P/T nappes but also of continent side granitic belts.