Integrated Ocean Drilling Program (IODP) Expedition 314 was a milestone, both as the inaugural scientific drilling mission of the new vessel, Chikyu, and as the first step in the multistage Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), an ambitious, coordinated, multiplatform, and multiexpedition drilling project designed to investigate fault mechanics and seismogenesis along subduction megathrusts through direct sampling, in situ measurements, and long-term monitoring.

        During Expedition 314, our primary goals were to obtain a comprehensive suite of geophysical logs and other downhole measurements at sites along a transect from the incoming plate to the Kumano forearc basin using state-of-the-art logging-while-drilling (LWD) technology. Drilling and logging was successfully completed at four sites, ranging in depth below the seafloor from 400 to 1400 m, with partial success at a fifth site. These sites included the frontal thrust and toe region of the outer accretionary prism near the trench, the fault zone and associated thrust sheet of a major out-of-sequence thrust system (the "megasplay" fault), 1 km thick forearc basin deposits, and highly deformed rocks of the interior of the accretionary prism. The principal goals of the LWD program were to document in situ physical properties; stratigraphic and structural features; sonic to seismic scale velocity data for core-log-seismic integration; and stress, pore pressure, and hydrological parameters through both scalar and imaging log measurements. All four sites are slated for continuous core sampling during subsequent IODP NanTroSEIZE expeditions.

        Depth objectives for this expedition ranged from ~400 to 1400 meters below seafloor. Logging included the measurement of natural gamma radiation, azimuthal gamma ray density, neutron porosity, full waveform sonic velocity, azimuthal resistivity imaging, zero-offset vertical seismic profile, ultrasonic caliper, and annular fluid pressure, though not all logs in this suite were collected at all sites.

        Principal results of Expedition 314 include the following:

1.           The megasplay thrust sheet is composed of highly deformed and fractured rocks that are anomalously well indurated relative to their present depth.
2.           Present-day stress varies markedly along the NanTroSEIZE transect, and stresses in the upper 1.4 km are strongly compressional in the outer, active accretionary prism but extensional in the forearc basin.
3.           The occurrence of gas hydrate as a cement preferentially located in sandy portions of turbidite beds above a bottom-simulating reflector was quantified.
4.           Drilling at Sites C0001 and C0002 also provides important pilot hole information that will help prepare for the planned deep-riser sites for later stages of NanTroSEIZE drilling.

        For more information: http://www.jamstec.go.jp/chikyu/nantroseize/e/expedition_314.html