Expedition 314 summary

Abstract

Integrated Ocean Drilling Program (IODP) Expedition 314 was a milestone, both as the inaugural scientific drilling mission of the new vessel, the D/V 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 total depth below seafloor from 400 to 1400 m, with partial success at a fifth site. These sites included (1) the frontal thrust and toe region of the outer accretionary prism near the trench; (2) the fault zone, associated thrust sheet, and foot wall of a major out-of-sequence thrust system (the “megasplay” fault); and (3) 1 km thick forearc basin deposits and highly deformed rocks of the interior of the older accretionary prism. The principal objectives 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 image logging measurements. All four sites have since undergone core sampling during subsequent IODP NanTroSEIZE Expeditions 315 and 316. Depth objectives for this expedition ranged from ~400 to 1400 meters below seafloor. Logging included measurement of gamma ray values, azimuthal gamma ray density, neutron porosity, full waveform sonic velocity, azimuthal resistivity imaging, zero-offset vertical seismic profile, ultrasonic caliper, and annular fluid pressure and temperature, although not all logs in this suite were collected at all sites. Principal results of Expedition 314 include the following: (1) 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. (2) The megasplay thrust sheet is composed of highly deformed and fractured rocks that are anomalously well indurated relative to their present depth. (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 provided important pilot hole information that will help prepare for deep-riser sites planned for later stages of NanTroSEIZE drilling. (5) The main frontal thrust and main splay fault zone, imaged at Sites C0004 and C0006, respectively, are intervals of strong fracturing and brecciation several to ten meters in thickness. At these two sites, there are differing contrasts in physical properties within and below the fault zones.