[S-GL23] Geologic structure and tectonic history of East Asia and Japanese Islands

[SGL23-01] Age spectra of detrital zircons of Paleogene fore-arc sandstones in Japan and the irreversible major change in mid-Eocene relevant to the initiation of Median Tectonic Line.

*Yukio Isozaki¹, Yukiyasu Tsutsumi², Tomohito Nakano¹ (1.Department of Earth Science and Astronomy, Graduate School of Arts and Sciences, The University of Tokyo, 2.Deprtment of Geology and Paleontology, National Musem of Nature and Science)

Keywords:fore-arc basin, Cretaceous, Plaeogene, Japan, MTL

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.).