Abstract
Integrated Ocean Drilling Program (IODP) Expedition 333 returned to two sites drilled during IODP Expedition 322 on the ocean side of the Nankai Trough to pursue the characterization of the inputs to the Nankai subduction and seismogenic zone, as part of the Nankai Trough Seismogenic Experiment (NanTroSEIZE) multi-expedition project. Site C0011 is located at the seaward edge of the trench and Site C0012 on a basement high, Kashinozaki Knoll (Fig. 1). The main objectives of drilling again at these sites were to fill coring gaps in the upper part (<350 m) of the sedimentary sequence, to measure heat flow, and to core the oceanic basement to a greater depth on the Knoll. New results include the observation of a diagenetic boundary within the Shikoku Basin sediments that may be compared to one documented further west by ODP Legs 131, 190 and 196 but occurs here at a lower temperature. Borehole heat flow measurements confirm spatial variations in the Shikoku Basin that were indicated by short probe surveys. Heat flow variations between topographic highs and lows may be related to fluid convection within the basement. This expedition also included the objectives of the Nankai Trough Submarine LandSLIDE history (NanTroSLIDE) Ancillary Project Letter (APL) and cored at Site C0018 a pile of mass transport deposits on the footwall of the megasplay fault, a major out of sequence thrust that presumably slips coseismically during large subduction earthquakes. This brought new insight on the timing of these mass wasting events and on the deformation within the sliding slope sediments. <br><br> doi:<a href="http://dx.doi.org/10.2204/iodp.sd.14.01.2012" target="_blank">10.2204/iodp.sd.14.01.2012</a>
Highlights
Introduction and GoalsSite C0018Site C0011 Site C0012Philippine Sea Plate / Japan ≈ 50 mm y-1 Eurasian Plate Pacific Plate Depth (A) Site C0009Site C0002 Site C0001 Kumano BasinSites C0003-C0005, C0008, and C0010
The estimated heat flow value at this site is 141 mW m-2, amounting to ~50% higher than the 89.5 mW m-2 determined for the adjacent Hole C0011C that was drilled during this expedition
The only two samples that contained both methane and ethane were found at depths of ~501 mbsf and ~520 mbsf in Sediments cored at Site C0018 (Fig. 6) are subdivided into two lithologic subunits (Expedition 333 Scientists, 2012c), which correlate with seismic Unit 1a and 1b of Kimura et al (2011) and Strasser et al (2011)
Summary
Oceanic basement km Figure 1. [A] Location map of Kumano, Muroto, and Ashizuri borehole transects across the Nankai Trough. Interpretation of 3D reflection seismic data and drilling results document a complex temporal and spatial evolution of the shallow mega-splay fault zone (MSFZ) and anticline structures in the underlying accretionary prism This is characterized by a general decrease of thrusting motion with time since 2 Ma and alternating periods of high and low structural activity on individual structural segments (Strasser et al, 2009; Kimura et al, 2011). The chlorinity decrease, which continues to the deeper Methane was the only hydrocarbon gas detected in holes interstitial waters of Hole C0011B taken during Exp. 322 C0011C and C0011D It occurs only in low concentrations (Underwood et al, 2010), may reflect the updip migration of (2 ppmv) in the uppermost 260 m of the cored sequence and rises to reach the highest. Habutaki ash ash Pink sulfate-methane transition zone occurs deeper, in the 250–300 m depth range
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