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doi:10.2204/iodp.proc.314315316.123.2009 Expedition 315 Site C00011Expedition 315 Scientists2IntroductionIntegrated Ocean Drilling Program (IODP) Site C0001 (proposed Site NT2-03B) targeted the uppermost 1000 meters below seafloor (mbsf) at the seaward slope of the Kumano Basin uplift (outer arc high) where the megasplay fault system branches and may approach the surface (Figs. F1, F2). The upper 1000 m drilled during this expedition provides an opportunity to access the thrust sheets uplifted by several branches of the megasplay fault system, as well as a thin overlying slope basin cover sequence. The nature of the material in these thrust sheets is unknown. The acoustically nonreflective nature of this section suggests that it may be composed of chaotically deformed accretionary wedge sedimentary mélange transported from significantly greater depths. Alternatively, this zone may be composed of highly deformed Kumano forearc basin sediments. It may also be a structurally juxtaposed combination of both. The principal objective during IODP Expedition 315 was to reveal the stratigraphic, physical, structural, and chemical features of materials obtained from the shallow accretionary prism and the overlying slope basin. An integrated interpretation is proposed based on these findings combined with seismic and log data obtained during IODP Expedition 314. Coring at this site also provides critical pilot hole information for future riser drilling. To achieve the ~3500 m total depth objective using riser and weighted drilling mud involves setting multiple casing strings, with the depth of each depending on the least principal stress, the fracture strength of the formation, and the pore fluid pressure gradient. The key part of this casing plan is the “top-hole” portion, where tolerances on mud weight are tight. Planning the casing program, therefore, requires excellent information on physical properties in the uppermost 1000 m. In light of this, our strategy is to utilize riserless coring for this section during a future stage of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE). Furthermore, downhole (in situ) temperature measurements were conducted as deeply as possible, which will enable estimation of deeper temperatures by extrapolation. These measurements will provide critical information for well planning of the 3.5 km riser hole and will aid in the design of sensors and telemetry of the long-term borehole observatory planned for a future NanTroSEIZE stage. |
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