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doi:10.2204/iodp.pr.322.2009

Abstract

Integrated Ocean Drilling Program (IODP) Expedition 322 is part of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) and was designed to document characteristics of incoming sedimentary strata and igneous basement prior to their arrival at the subduction front. To accomplish these objectives, coring was conducted at two sites in the Shikoku Basin on the subducting Philippine Sea plate. Site C0011 is located on the northwest flank of a prominent bathymetric high (the Kashinosaki Knoll), whereas Site C0012 is located near the crest of the knoll. The resulting data, which include logging while drilling during IODP Expedition 319, provide a wealth of new information on presubduction equivalents of the seismogenic zone. Unfortunately, coring at Site C0011 began at 340 m core depth below seafloor (CSF) and failed to reach the total depth target because of premature destruction of the drill bit at 876 m CSF. Coring at Site C0012, however, penetrated almost 23 m into igneous basement and recovered the sediment/basalt interface intact at 537.81 m CSF. The age of basal sediment (reddish-brown pelagic claystone) is >18.9 Ma. This recovery of basement was a major achievement, as was the comprehensive integration of core-log-seismic data at Site C0011. The merger of lithofacies and age-depth models from the two sites spans across the Shikoku Basin from an expanded section (Site C0011) to a condensed section (Site C0012) and captures all of the important ingredients of basin evolution, including a heretofore unrecognized interval of late Miocene tuffaceous and volcaniclastic sandstone designated the middle Shikoku Basin facies. An older (early–middle Miocene) turbidite sandstone/siltstone facies with mixed detrital provenance occurs in the lower Shikoku Basin; this unit may be broadly correlative with superficially similar Miocene turbidites on the western side of the basin. When viewed together, the two sites around the Kashinosaki Knoll not only demonstrate how basement relief influenced rates of hemipelagic and turbidite sedimentation in the Shikoku Basin, they also build the complete lithostratigraphic template on which all of the postexpedition laboratory results can be placed. Those forthcoming details will include mineral and volcanic ash composition, geotechnical properties, frictional properties, and hydrological properties. Another triumph came from geochemical analyses of pore water and hydrocarbons at Site C0012. Unlike other so-called reference sites in the Nankai Trough, pore fluids on top of the basement high show clear evidence of a seawater-like source, with chlorinity values increasing toward basement because of hydration reactions and diffusion; the fluids are largely unchanged by the effects of focused flow and/or in situ dehydration reactions associated with rapid burial beneath the trench wedge and frontal accretionary prism. Thus, Site C0012 finally provides a reliable geochemical reference site for the subduction zone.

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