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 Seismo genic Zone Experiment, recovered samples of mud and mudstone from the Kumano forearc basin, the inner and outer accretionary prisms, the over lying slope apron, and the Shikoku Basin (subduction inputs). This unprece dented suite of cores and cuttings captures an unusually complicated history of subductionzone tectonics, erosion and dispersal of suspended sediment from multiple sources, and sedimentation in diverse environments. Our Xray diffraction analyses of 1567 samples show that claymineral assemblages shifted gradually throughout the subduction system, from a smectiterich as semblage during the Miocene to a more illite and chloriterich assemblage during the Pliocene and Quaternary. Miocene muds in the Shikoku Basin (Sites C0011 and C0012) originated primarily from weathering of anomalous, neartrench felsicvolcanic 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 accre tionary prism (Sites C0001 and C0002) and coeval mudstones from the Shi koku Basin. We suggest that frontal accretion during the earlylate Miocene was a product of Pacific plate subduction rather than subduction of the Phil ippine 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 arctrench 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 vol canoes 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 IzuHonshu collision zone, supplied suspended sediment through a combination of transverse and trenchparallel (axial) rout ing. At the same time, the northeastdirected Kuroshio Current intensified at ca. 3.5 Ma. That regionalscale oceanographic transition probably resulted in more illitic clays moving from offshore Taiwan through the Okinawa Trough, although its compositional signal is masked by simultaneous enrichment of illite from the Outer Zone sources. Accreted trenchwedge deposits in the frontal prism (Sites C0006 and C0007) originated mostly from the IzuHonshu collision zone, and most were transported down the axis of the trench by sedi ment gravity flow. Hemipelagic deposits in the Kumano Basin (Site C0002) were homogenized from a combination of transverse gravity flows, north eastdirected surface current, and thermohaline bottom currents. Slopeapron and slopebasin deposits (including masstransport deposits) likewise show uniform claymineral assemblages indicative of northeastdirected transport by the Kuroshio Current, transverse resedimentation, and bottomwater cir culation. Collectively, these differences in sediment composition in both time and space set the NankaiShikoku depositional system apart from other sub duction zones, and they are important to consider when assessing the mar gin’s hydrogeology and frictional and/or geotechnical properties.