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.

Strike-slip motion of a mega-splay fault system in the Nankai oblique subduction zone

“…At the transition zone, the outer ridge is well developed in the trench-parallel direction (Figure 1), where we observe a remarkable topographic lineament with a length of approximately 200 km (Park and Kodaira 2012). A recent drilling campaign of the Integrated Ocean Drilling Program (IODP) used borehole breakouts and core sample observations to show that the stress state (i.e., principal horizontal stress orientation) changes across the outer ridge (e.g., Tobin et al 2009;Lin et al 2010). Because the outer and inner wedges can behave differently during an earthquake cycle (Wang and Hu 2006), the transition zone functions as a stress boundary.…”

“…Borehole breakouts and core sample observations have revealed the change in the stress state (principal horizontal stress orientation) across the WBSF off the Kumano area (e.g., Tobin et al 2009;Lin et al 2010; Figure 4b). The maximum principal horizontal stress is perpendicular to the trench seaward of the WBSF and parallel to it landward of the WBSF, and it changes again further landward to become nearly parallel to plate convergence (red bars in Figure 4b).…”

In situ stress state in the Nankai accretionary wedge estimated from borehole wall failures

“…[7] In this paper, we present results from four boreholes (C0002, C0001, C0004 and C0006, from NW to SE), where the LWD resistivity logs capture images of borehole wall failures (Figure 2). Based on logged physical properties and seismic sections in this site, the boreholes were found to penetrate multiple geologic domains, which are divided by major and minor structural boundaries such as unconformities, boundaries between different thrust sheets, or the megasplay fault [Tobin et al, 2009]. [8] Resistivity imaging logs provide a high-resolution picture of the borehole wall based on resistivity contrasts that allows for the direct observation of borehole breakouts, which look as broad, parallel conductive zones separated by 180°(as shown in the unwrapped images in Figure 2).…”

“…Several vertical boreholes were drilled in the shallow portion of the wedge along a line perpendicular to the plate boundary ( Figure 1) [Tobin et al, 2009]. In these holes, we observed a series of drilling-induced borehole wall failures in logging-while-drilling (LWD) resistivity images of the borehole wall.…”

Stress state estimation by geophysical logs in NanTroSEIZE Expedition 319‐Site C0009, Kumano Basin, southwest Japan

“…Since the borehole deviation of C0009 is kept at less than 0.25 degrees for most of its interval (Figure 2c), the hole deviation effect on the azimuth of breakouts and tensile cracks can be neglected in this analysis. [Tobin et al, 2009;Chang et al, 2010]. Site C0009 drilled in stage 2 of this study is in green diamond.…”