| Co-Investigator(Kenkyū-buntansha) |
YOSHIDA Takeyoshi Tohoku University, Graduate School of Science, Professor, 大学院・理学研究科, 教授 (80004505)
EHIRO Masayuki Tohoku University, Graduate School of Science, Professor, 大学院・理学研究科, 教授 (10124554)
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| Research Abstract |
The Yakuno ophiolite and overlying Maizuru Group constitute an acceretionary prism off the Permian Japan arc. This concept is supported by : pelagic chert resting on the basalt of the Yakuno ophiolite, herripelagic mudstone with silicic tuff, exotic fossiliferous limestones derived from former-accreted seamounts, a coarsening upward sequence ending with terrigenous sandstone and conglomerate, and the mildly deformed latest Permian and Triassic formations of fore-arc basins. The most important aspect of an accretionary prism is however, seaward imbrication and repetition, involving both the Maizuru Group and the Yakuno ophiolite. The D1 deformation structures include an axial-planar foliations (pressure-solution cleavage for the Maizuru Group and metamorphic layering of granulite and amphibolite for the Yakuno Ophiolite),flattering type strain, symmetric pressure shadow and fringe, isoclinal folds with the axial-planar foliations, and a thrust fault at the overturned limb of this fold.
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Exceptional asymmetric structure indicates seaward directed shear near the thrust. The D1 structure of the Maizuru zone is explained by off-scraping, above the basal decollement. The Later Jurassic D2 structure is a kink fold, including a first order asymmetric kink fold with a brittle thrust at its overturned limb. D2 occurs at the time of M2 retrograde metamorphism. M1 prograde metamorphism is the medium-pressure type for the Yakuno Ophiolite that forms metamorphic minerals in layering of granulite and amphibolite ; and also for the Maizuru Group to form illite along pressure solution cleavage of mudstone. The metamorphic grade is controlled by the stratigraphic or ophiolite-stratigraphic position, and such metamorphism is a kind of ocean-floor metamorphism. However, the M1 prograde metamorphism acted only once, progressing contemporaneously with D1 off-scraping. The subduction zone, usually under high P/T regime, was thus, at one time, under abnormal metamorphic conditions. Such a high thermal gradient is likely if hot and young oceanic plate was aproaching and then subducting beneath the Japan arc. The Yakuno Ophiolite was probably formed at the trench-trench-ridge triple junction. Less
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