Secular Changes in Fore-arc Sandstones and Their Provenance in Cretaceous Southwest Japan: U-Pb Dating of Detrital Zircons

To reconstruct the tectono-sedimentary history of the Cretaceous fore-arc in Japan, U-Pb dating is conducted of detrital zircons in Upper Jurassic and Cretaceous shallow-marine sand-stones from the Chichibu belt in Kyushu, Shikoku, and Kii Peninsula, Southwest Japan. The U-Pb ages of detrital zircon in 18 analyzed sandstone samples revealed the following new facts. The predominance of Jurassic grains, with a minor amount of Paleozoic and Precambrian grains, in the Upper Jurassic and pre-Hauterivian Lower Cretaceous sandstones indicates that the fore-arc has been fed with abundant terrigenous clastics from the provenance composed of the Jurassic arc granitoids/accretionary complexes. More Permian grains were supplied probably from the older Permian accretionary complex in a limited way during the Hauterivian, nonetheless, Jurassic grains again dominated in the Aptian sandstones. In contrast, Albian and Upper Cretaceous sandstones are characterized by the remarkable predominance of Cretaceous zircons with almost a complete absence in pre-Cretaceous zircons. This suggests a major reorganization of the arc provenance in the mid-Cretaceous, probably during the Albian; i.e., from the previous regime with dominant Jurassic orogenic elements to a new one solely with Cretaceous arc granitoids. Coeval sandstones in Kanto share the same age spectra and secular changes, indicating not only the monotonous nature of the provenance and fore-arc basin for nearly 1000 km along arc-trench system, but also the appearance of regional tectonism in mid-Cretaceous SW Japan. In particular, the total shut-down of pre-Cretaceous grains into the fore-arc domain probably reflects the appearance of a significant/effective topographic barrier in the arc crust with respect to mid-Cretaceous extensive arc magmatism along the active margin in East Asia, prior to the opening of the Japan Sea in the Paleogene-Miocene.