[S-GL23] 日本列島および東アジアの地質と構造発達史

[SGL23-01] 日本の古第三紀前弧砂岩の砕屑性ジルコン年代組成と始新世中頃の非可逆的変化：MTLの出現時期

*磯崎 行雄¹、堤 之恭²、中野 智仁¹ (1.東京大学大学院総合文化研究科広域科学専攻広域システム科学系、2.国立科学博物館地学研究部)

キーワード：前弧盆地、白亜紀、古第三紀、日本、中央構造線

The unique age spectra of detrital zircons in Paleogene fore-arc sandstones in Japan, and tectonic implications of their irreversible secular change are discussed. The tectono-sedimentary setting of the Palegene fore-arc in Japan can be divied into two distinct phases; i.e. Paleocene to Early Eocene and the Late Eocene to Oligocene. The major change occurred sometime in the mid-Eocene, most probably by the initiation of the Median Tectonic Line (MTL) as a low-angle fault of arc-contraction nature. Recent studies confirmed that the Paleocene sediments unconformably lying on the Ryoke Cretaceous granitoids regionally overthrust the coeval Sanbagawa schists on the south from Kyushu to Kanto (Hasegawa et al., 2020; Isozaki et al, 2020), constraining the tectonic timing to the post-Paleocene (< 56 Ma). U-Pb ages of detrital zircons were measured for Paleocene bench sandstones deposited on the Shimanto accretionary complexes (Domeki Formation) in SW Shikoku. The age spectrum of zircons in the Domeki Fm is almost identical to those of coeval sandstones in the Ryoke belt (Upper Kokawa Fm and Nate Fm of the Izumi Group in Kinki, Kanohara Conglomerate and Yorii Fm in Kanto), suggesting the ubiquitous supply of mootonous terrigenous clastics into the Paleocene fore-arc domain for more than 80 km across the arc from the southern margin of the Ryoke belt (northern margin of the main fore-arc basin) to the central Shimanto belt (bench on slope). This confirms that the stable and continous deposition on the fore-arc crust of SW Japan arc from the Late Cretaceous to Paleocene without any major tectonic disturbance. In contrast, younger Paleogene sandsotnes in Japan show remarkably different age spectra of detrital zircons (Isozaki et al., in prep.).