To reconstruct the tectono-sedimentary history of the CretaceousPaleogene arc-trench system in western Hokkaido, UPb ages were measured of detrital zircons in 11 Cretaceous and Paleogene fore-arc sandstones of the Yezo Group in the Oyubari and Mikasa areas. Age spectra of dated zircons demonstrate that Aptian sandstones of the basal Yezo Group are characterized by the dominant occurrence of Early Cretaceous grains with small amounts of Jurassic, Triassic, Permian, and Precambrian grains (Type 1) . In contrast, those of Cenomanian to Paleocene sand stones of the middle to upper Yezo Group are dominated almost totally by mid-to Late Cretaceous grains (Type 2) . This remarkable change from Type 1 to Type 2 occurred irreversibly during the Albian, indicating that the surface crust in the provenance in western Hokkaido was significantly renewed then. Among nine sandstones of Type 2, a positive correlation exists between zircon peak age and stratigraphic age, suggesting the unidirectional/gradual replacement of exposed magmatic rocks in the provenance, in particular, new volcanics/intrusives (RebunKabato belt to the west) . The pre-Cretaceous zircons in Type 1 sandstones were probably recycled from sand stones in the Jurassic accretionary complexes of the Oshima belt to the west. The coeval and identical turnover in zircon age spectra of the Cretaceous fore-arc sandstones recognized in Southwest Japan, from Kyushu to Kanto district, suggests that the fore-arc basin and its provenance with monotonous arc crustal rocks have ubiquitously developed along the Cretaceous East Asian margin over more than 1,500 km before the Miocene opening of the Japan Sea.

In order to constrain the onset timing of the Median Tectonic Line in SW Japan, detrital zircon geochronology is examined for pre-Miocene undated sandstones at both sides of the subhorizontal MTL in the Mikawa-Ono area, central Japan, where the 3-D configuration of the primary MTL between the Ryoke and Sanbagawa belts is observed. In addition, another Cretaceous sandstone unit (the Idaira Formation) in the Chichibu belt, ca. 10 km to the south, is analyzed for comparison. The following new results were obtained. The undated sandstonedominant unit, previously called the Kawachi Formation or the Nanasato-Isshiki Formation as a whole is, in fact, composed of two distinct units with contrasting characteristics of distribution, rock type, and zircon age spectrum, although both are revealed to be of the Cretaceous. The one to the north of the subhorizontal MTL (in the Ryoke belt) is redefined as the Nanasato-Isshiki Formation sensu stricto, whereas the one to the south (in the Sanbagawa belt) is newly designated as the Rokutazawa Formation. The Lower Cretaceous Idaira Formation forms the third group with a distinct signature in the zircon age spectrum. The Nanasato-Isshiki Formation (together with the associated Atera-Nanataki conglomerate) and the Idaira Formation, overlying the Ryoke granitoids and Chichibu pre-Cretaceous accretionary complex, respectively, form autochthonous units. In contrast, the Rokutazawa Formation alone represents an allochthonous units sitting structurally on the Sanbagawa schists. Age spectra of detrital zircons suggest that both the Nanasato-Isshiki Formation and Atera-Nanataki conglomerate are roughly dated as the Late Cretaceous, and also that they are correlated with the Upper Cretaceous Izumi Group in the Shikoku and Kinki regions of SW Japan. On the other hand, the sandstone of the Rokutazawa

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