Sediment flow routing during formation of forearc basins: Constraints from integrated analysis of detrital pyroxenes and stratigraphy in the Kumano Basin, Japan

The evolution of sediment flow routing during complete evolution of the Kumano forearc basin is determined through integration of stratigraphic and sediment provenance analyses in the upper Nankai forearc. A new approach uses the compositional variability of detrital clinopyroxenes and orthopyroxenes collected at eight major rivers in Japan and three drill sites in the basin and nearby slope environment, including the first drill cuttings retrieved by the Integrated Ocean Drilling Program (IODP). Joint interpretation of these datasets reveals that the sedimentation history of the basin is characterised by three main phases separated by newly-recognised time-transgressive boundaries. We show that the Kumano Basin initiated as a trench-slope basin in the early Quaternary (∼1.93 Ma) and that it progressively evolved towards an upper slope environment with increased turbidite confinement and influence from climatic forcing. Basin initiation was broadly synchronous with development of the Nankai megasplay fault, suggesting a causal relationship with construction of the Nankai accretionary prism. Unlike preceding studies documenting long-distance longitudinal transport of clastic material along the lower Nankai forearc, only limited longitudinal transport is documented by detrital pyroxenes in the upper forearc. These results suggest that transverse canyons are a major control on the sediment flow routing during maturation of forearc basins and that long-distance longitudinal flows along convergent margins are principally restricted to near-trench environments, even in the presence of large forearc basins.