Clay-mineral assemblages across the Nankai-Shikoku subduction system, offshore Japan; a synthesis of results from the NanTroSEIZE project

The Integrated Ocean Drilling Program, as part of the Nankai Trough Seismogenic Zone Experiment, recovered samples of mud and mudstone from the Kumano forearc basin, the inner and outer accretionary prisms, the overlying slope apron, and the Shikoku Basin (subduction inputs). This unprecedented suite of cores and cuttings captures an unusually complicated history of subduction-zone tectonics, erosion and dispersal of suspended sediment from multiple sources, and sedimentation in diverse environments. Our X-ray diffraction analyses of 1567 samples show that clay-mineral assemblages shifted gradually throughout the subduction system, from a smectite-rich assemblage during the Miocene to a more illite- and chlorite-rich assemblage during the Pliocene and Quaternary. Miocene muds in the Shikoku Basin (Sites C0011 and C0012) originated primarily from weathering of anomalous, near-trench felsic-volcanic rocks along a broad swath of the Outer Zone of Japan. The middle Miocene, however, was also a time of sediment transport into the Shikoku Basin by turbidity currents emanating from the East China Sea (Kyushu Fan). Interfingering of clays from those two sources resulted in considerable compositional scatter. Our results also reveal large discrepancies in contents of smectite between Miocene mudstones from the inner accretionary prism (Sites C0001 and C0002) and coeval mudstones from the Shikoku Basin. We suggest that frontal accretion during the early-late Miocene was a product of Pacific plate subduction rather than subduction of the Philippine Sea plate. Routing of sand through the East China Sea was effectively cut off by ca. 7 Ma due to rifting of the Okinawa Trough and the buildup of topography along the Ryukyu arc-trench system. Subduction of Shikoku Basin restarted at ca. 6 Ma, and denudation of the Outer Zone continued through the Pliocene and Quaternary. Those adjustments in weathering, from volcanoes to exposures of plutons and metasedimentary rocks, gradually increased the concentrations of illite and chlorite. By the late Pliocene, multiple sources, including the rapidly uplifted Izu-Honshu collision zone, supplied suspended sediment through a combination of transverse and trench-parallel (axial) routing. At the same time, the northeast-directed Kuroshio Current intensified at ca. 3.5 Ma. That regional-scale oceanographic transition probably resulted in more illitic clays moving from offshore Taiwan through the Okinawa Trough, although its compositiona