地学雑誌
Online ISSN : 1884-0884
Print ISSN : 0022-135X
ISSN-L : 0022-135X
論説
関東山地北縁,上部白亜系跡倉層・栃谷層の砕屑性ジルコン年代パタン
―飛騨帯と中央構造線南縁との弧横断方向の関連―
中畑 浩基磯﨑 行雄小坂 和夫坂田 周平平田 岳史
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2015 年 124 巻 4 号 p. 633-656

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 The overall configuration of the Cretaceous subduction-related arc-trench system in Japan is preserved in the current distribution of the relevant orogenic components; i.e., the coeval set of accretionary complexes formed at the trench (Sanbosan and North Shimanto belts) , high-P/T meta-ACs along the Wadati–Benioff zone (Sanbagawa and Shimanto metamorphic belts) , arc batholiths (Ryoke–Sanyo and San-in belts) , and fore-arc basin strata (Ryoseki–Monobegawa and Izumi groups) . To document the sediment distributary pattern within the Cretaceous arc-trench system and the composition of relevant provenance, detrital zircon dating was conducted for the Upper Cretaceous sandstones (Atogura and Tochiya formations) from the northern Kanto Mountains. These strata occur immediately to the south of Median Tectonic Line of SW Japan, and as klippe with unknown origin. The results of U-Pb dating by LA-ICPMS show that three sandstones have common age spectra with four major age groups; i.e., 120–150 Ma (Early Cretaceous) , 170–200 Ma (Jurassic) , 250–300 Ma (Permian) , and 1600–2200 Ma (Paleoproterozoic) , with minor amounts of much older grains up to 2900 Ma (Archean) . These age spectra are unique, when compared to other coeval Cretaceous fore-arc and/or intra-arc sandstones in Japan. The Early Cretaceous grains were obviously derived from a proximal source to the depositional site, probably the Cretaceous volcanic arc of the Ryoke–Sanyo belt in SW Japan. The dominant grains of the Jurassic and Permian ages were probably derived from coeval plutonic belts in the provenance, whereas the Paleoproterozoic grains were probably derived from continental blocks in East Asia with crusts of corresponding ages, such as the North and South China blocks. Except for the Cretaceous arc source, Jurassic and Permian granitoids are extremely rare in major parts of Japan, and the Paleoproterozoic crust is totally absent. The only exception to the occurrence of Jurassic and Permian granitoids exists in the Hida belt of SW Japan. The age spectra of the analyzed Upper Cretaceous sandstones differ remarkably from those of other coeval fore-arc sandstones, whereas they clearly show similarities to Cretaceous sandstones in the Hida belt. This suggests that the analyzed Upper Cretaceous sandstones were primarily deposited in a domain far from the present position but with a close link to the Hida belt, which is located more than 100 km away to the northwest. Tectonic emplacement onto the present position immediately above the Cretaceous high-P/T meta-ACs probably took place during the Paleogene and Neogene, likely in relation to Miocene rifting of the Japan Sea. This study provides the first evidence of the across-arc geotectonic link between back-arc and fore-arc domains of Cretaceous Japan.

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© 2015 公益社団法人 東京地学協会
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