Abstract
The primary goals during Expedition 322 of the Integrated Ocean Drilling Program were to sample and log the incoming sedimentary strata and uppermost igneous basement of the Shikoku Basin, seaward of the Nankai Trough (southwestern Japan). Characterization of these subduction inputs is one piece of the overall science plan for the Nankai Trough Seismogenic Zone Experiment. Before we can assess how various material properties evolve down the dip of the plate interface, and potentially change the fault’s behavior from stable sliding to seismogenic slip, we must determine the initial pre-subduction conditions. Two sites were drilled seaward of the trench to demonstrate how facies character and sedimentation rates responded to bathymetric architecture. Site C0011 is located on the northwest flank of a prominent basement high (Kashinosaki Knoll), and Site C0012 is located near the crest of the seamount. Even though significant gaps remain in the coring record, and attempts to recover wireline logs at Site C0012 failed, correlations can be made between stratigraphic units at the two sites. Sedimentation rates slowed down throughout the condensed section above the basement high, but the seafloor relief was never high enough during the basin’s evolution to prevent the accumulation of sandy turbidites near the crest of the seamount. We discovered a new stratigraphic unit, the middle Shikoku Basin facies, which is typified by late Miocene volcaniclastic turbidites. The sediment-basalt contact was recovered intact at Site C0012, giving a minimum basement age of 18.9 Ma. Samples of interstitial water show a familiar freshening trend with depth at Site C0011, but chlorinity values at Site C0012 increase above the values for seawater toward the basement contact. The geochemical trends at Site C0012 are probably a response to hydration reactions in the volcaniclastic sediment and diffusional exchange with seawater-like fluid in the upper igneous basement. These data are important because they finally establish an authentic geochemical reference site for Nankai Trough, unaffected by dehydration reactions, and they provide evidence for active fluid flow within the upper igneous crust. Having two sets of geochemical profiles also shows a lack of hydrogeological connectivity between the flank and the crest of the Kashinosaki Knoll. <br><br> doi:<a href="http://dx.doi.org/10.2204/iodp.sd.10.02.2010" target="_blank">10.2204/iodp.sd.10.02.2010</a>
Highlights
Introduction and GoalsSubduction megathrusts are responsible for some of the world’s deadliest earthquakes and tsunamis
To improve our understanding of these hazards, an ambitious project known as the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) was initiated along the subduction boundary of southwestern Japan, with the overarching goal of creating a distributed observatory spanning the up-dip limit of seismogenic and tsunamigenic behavior (Tobin and Kinoshita, 2006a, 2006b)
We were able to make confident correlations between the logs and subsequent data from rotary core barrel (RCB) cores, with a vertical offset of ~4 m between the coring hole and the logging hole
Summary
Introduction and GoalsSubduction megathrusts are responsible for some of the world’s deadliest earthquakes and tsunamis. Lithologic Unit I was not cored, so its character is inferred from LWD data (Underwood et al, 2009) and by analogy with the upper part of the Shikoku Basin at ODP Sites 808, 1173, 1174, and 1177 (Taira et al, 1992; Moore et al, 2001b). The unit extends from the seafloor to ~151 mbsf, below which we recovered the first volcaniclastic sandstone of middle Shikoku Basin facies (Fig. 6).
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