Conglomerates in the Shimanto Accretionary Complex, SW Japan: Origin, Deposition, and the Influence of Subduction of the Izanagi-Pacific Spreading-Ridge

The Shimanto Accretionary Complex in SW Japan consists mainly of alternating sandstone- and mudstone-dominated sequences with a trench-fill turbiditic origin. However, depositional histories of the coarse-grained Ohyamamisaki (Shikoku) and Nyunokawa conglomerates (Kii Peninsula) remain poorly understood, despite the fact that they occupy an important structural position along the boundary between the Cretaceous and Paleogene to Neogene parts of the Shimanto Accretionary Complex. Our new geological, chronological, and Raman spectroscopic analyses indicate that the Ohyamamisaki conglomerate contains clasts of the Cretaceous rhyolitic lapilli tuff, together with the Cretaceous psammitic schist that experienced peak metamorphic temperatures of 316-333 degrees C. The Nyunokawa conglomerate contains clasts of the Cretaceous rhyolitic lapilli tuff and granitic rocks. The lithological characteristics, ages, and metamorphic temperatures of clasts indicate that they were derived from volcanic and plutonic rocks in the Ryoke and San-yo belts or (in the Ohyamamisaki conglomerate) from the high-P/low-T Sanbagawa Metamorphic Complex. This suggests episodic denudation of the forearc region and transportation of clasts toward 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 Median Tectonic Line, 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 the Ohyamamisaki and Nyunokawa conglomerates.