New Constraints on the Distributary Pattern of Clastics in Fore-arc and Tectonics in Paleogene SW Japan: U–Pb Ages of Detrital Zircons of the Domeki Formation in the Shimanto Belt, Western Shikoku

― 707 ― Abstract U ­ Pb ages of detrital zircons are measured for two sandstones of the Domeki Formation （ Fm ） in the Shimanto belt, western Kochi, which represent bench deposits on the Paleogene fore-arc slope in SW Japan. The results show that both samples are replete with Paleocene and Late Cretaceous zircons, and they have extremely small quantities of pre-Cretaceous grains. This indicates that the provenance of the Domeki Fm was occupied mainly by felsic igneous rocks of the San-in belt in SW Japan, i.e., coeval volcanic arc. The age spectrum of zircons in the Domeki Fm is almost identical to those of coeval sandstones deposited in the main fore-arc basin （ uppermost Izumi Group in the Kii peninsula, and Kanohara Conglomerate/Yorii Fm in Kanto ） . This suggests the ubiquitous supply of terrigenous clastics of the monotonous composition into the Paleocene fore-arc domain, including both the main fore-arc basin at the continent side and minor bench basins at the trench side, for more than 80 km across the arc from the southern margin of the Ryoke belt to the central Shimanto belt. Also confirmed is the stable and continuous deposi-tion of voluminous arc-derived clastics on the fore-arc of SW Japan from the Late Cretaceous to Paleocene for more than 1000 km along the arc. The present data constrain the timings of two large-scale tectonic episodes in Paleogene SW Japan, i.e., the initiation of the Median Tectonic Line between the Ryoke and Sanbagawa belts and the first surface exposure of high-P/T Sanbagawa schists, to have been no earlier than Paleocene/Eocene boundary （ ca . 56