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doi:10.2204/iodp.proc.314315316.133.2009 Expedition 316 Site C00041Expedition 316 Scientists2Background and objectivesIntegrated Ocean Drilling Program (IODP) Site C0004 (proposed Site NT2-01I; Kimura et al., 2007) targeted the uppermost 400 meters below seafloor (mbsf) at the seaward edge of the Kumano Basin uplift (outerarc high) where the megasplay fault system branches and approaches the surface (Figs. F1, F2) (Moore et al., 2007). The scientific objectives at Site C0004 are to clarify the character and behavior of the shallow portion of the megasplay, characterize its slip and deformation mechanisms and the evolution of the region updip of the (inferred) unstable seismogenic fault, and investigate the relationship between fluid behavior, slip, and deformation along and adjacent to the megasplay fault. As described in the IODP Expedition 316 Scientific Prospectus (Kimura et al., 2007), proposed Site NT2-01B was intended to penetrate the splay fault at ~600 mbsf and had a planned total depth (TD) of 1000 mbsf. Because of drilling difficulties at this site during IODP Expedition 314, Site NT2-01I was selected as an alternate site where the splay fault could be drilled at a shallower depth (~300 mbsf). TD was 398 mbsf, allowing examination of the slope sediments, prism, splay fault, and underthrust sediments. During Expedition 314, Hole C0004B was drilled using logging while drilling (LWD) to obtain downhole geophysical information and images (see Fig. F1 in the “Expedition 314 Site C0004” chapter). Interpretation of gamma ray, resistivity, sonic velocity, and caliper log responses during Expedition 314 suggested three logging units corresponding to the slope sediment, accretionary prism, and underthrust sediments. Resistivity-at-the-bit (RAB) images were used during Expedition 314 to define three structural domains that differed in depth ranges from the inferred logging units. The upper domain was characterized by a lack of fractures and weak breakouts, the middle domain was characterized by a series of zones with conductive fractures and intensive development of borehole breakouts, and the deepest domain has gentle dips on bedding planes, less common fractures, and narrower breakouts. Core samples collected during Expedition 316 provide a vital addition to the LWD data. The recovered cores allow description of lithology and structure, provide age control through paleontological and paleomagnetic analyses, and provide samples for interstitial water, microbiology, and shipboard and physical property and geotechnical studies. In addition to core samples, in situ temperature measurements provide an important control for thermal models of the subduction zone. The results, when integrated with data from other Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) expeditions, will characterize physical properties, strength, composition, and structure of the slope sediments, hanging wall, and footwall of the splay fault. Development of the splay fault at this site will also be compared to later drilling of the splay fault at greater depths at Sites C0001 and C0002. |
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