FOREARC DENUDATION AND TRENCHWARD TRANSPORT OF LARGE CLASTS BY A RIDGE SUBDUCTION: EXAMPLE OF EASTERN ASIAN CONTINENTAL MARGIN DURING THE LATEST CRETACEOUS TO EARLY EOCENE

The Japanese Islands have evolved within a subduction-related orogeny that has existed along the eastern Asian continental margin. This orogeny has continued for 500 Myr and given rise to multiple periods of tectonic events, including igneous activity, high-P/low-T metamorphism, as well as development in the forearc basin and accretionary wedge. The Cretaceous to Paleogene orogeny is one of the largest and most active orogeny in the long history of the Japanese Islands, and formed the Cretaceous to Paleogene Shimanto Accretionary Complex (AC). Although this complex consists mainly of alternating sandstone- and mudstone-dominated sequences with a trench-fill turbiditic origin, the depositional histories of the coarse-grained conglomerates remain poorly understood, despite the fact that they occupy an important structural position along the boundary between the Cretaceous and Paleogene parts of the Shimanto AC. Our new geological, chronological, and Raman spectroscopic analyses for these conglomerates determined the provenance of clasts (felsic igneous rock clasts derived from the Cretaceous igneous belt as well as schist and quartz-rich rock clasts from the Cretaceous high-P/low-T metamorphic belt, that had been situated in the forearc region) as well as the timing of conglomerate deposition (latest Cretaceous to early Eocene), suggesting episodic denudation of the forearc region and transportation of clasts towards the trench during deposition. The latest Cretaceous to early Eocene depositional ages of the conglomerates overlap with the timing of subduction of the Izanagi–Pacific spreading-ridge and associated extensional movements along the major tectonic fault in the forearc region. We propose that regional extension driven by ridge subduction promoted denudation of forearc materials and subsequent transportation and deposition of these conglomerates.