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Recent findings of dismembered ophiolitic bodies in trench landward slopes, especially from the seamounts in the Tonga, Mariana-Yap-Bonin (Ogasawara) and, Guatemala trenches, provide a key to understa...nding the tectonic significance and mechanism of the "ophiolitic melanges" in fold or orogenic belts. Some studies of these "forearc ophiolites" or "island arc ophiolites" are briefly reviewed from the geochemical and tectonic viewpoints, and they are compared with the "Mineoka ophiolite" in the Boso and Miura Peninsulas, which lies in the sandwiched forearc area in central Japan. Three possibilities for the origin of the Mineoka ophiolite are discussed from the field evidence and geochemical and geophysical data: (1) offscraped or underplated ophiolite similar to those in the Shimanto Supergroup; (2) accreted forearc ophiolite from the Izu-Bonin trench landward slope to the Honshu arc side, and (3) obducted oceanic plate from the Pacific side around the triple junction. Geochemical considerations of the abundance of major, minor and rare earth elements suggests that most of the basaltic rocks are of MORB-type with minor hot spot-type alkalic rocks. No distinct island arc-type basalt or boninite was discovered, while calc-alkali and tholeiitic rocks of island arc affinity unconformably covered the ophiolitic rocks The sedimentary rocks between and above the basaltic rocks are of late Eocene age (ca. 40 MaBP), and suggest that a new plate other than the Pacific plate on the north of the North New Guinea plate might have been accreted to the corner of the triple junction after opening of the Shikoku basin from 30 to 17 MaBP. This postulated plate is designated the "Mineoka plate". This may be represented by the ophiolitic rocks emplaced in the area during the early Miocene. The mechanism of emplacement is considered to be obduction and possible delamination by means of simulation which explains gravity anomaly. The right-handed en echelon pattern of fault zones, characterizing the present Mineoka belt, has been developed by the successive shearing under a dextral transpressional stress field.続きを見る
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