NanTroSEIZE Stage 1: LWD Transect

Abstract

Integrated Ocean Drilling Program (IODP) Expedition 314 is part of the 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, in conjunction with allied laboratory and numerical modeling studies. During Expedition 314, our primary goals are to obtain a comprehensive suite of geophysical logs and other downhole measurements at six sites, using state-of-the-art logging-while-drilling (LWD) technology. All six of these sites are slated for continuous core sampling during subsequent IODP Nan-TroSEIZE expeditions. These sites include sampling of the incoming plate trench sediments, the frontal thrust and toe of the accretionary prism, forearc basin deposits, a major out-of-sequence thrust system (the "megasplay" fault), and highly-deformed rocks of the interior of the accretionary prism. The principal goals of the LWD program are 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.Depth objectives for this expedition range from ~600 to 1400 meters below seafloor. Logging plans are not final, pending negotiation with logging service companies, but we anticipate that the minimum measurements will include 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.In this Scientific Prospectus we present the scientific background and objectives, the drilling and logging operations designed to achieve them, a contingency plan strategy, the currently understood risks and steps taken to mitigate them, and the coordinated Stage 1 plan for sharing samples and data.

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“…Unit I is interpreted to have been deposited from the early Pleistocene to the late Pleistocene as a sediment blanket on the upper slope mainly by hemipelagic settling with minor volcanic ash and sand-silt input. An observed gradual increase in carbonate content upsection is consistent with greater carbonate dissolution at depth and implies that the unit was slowly uplifted to its present water depth of 2632 m. At 78.08 m CSF, an angular unconformity separates Units I and II; this unconformity was predicted by seismic reflection and LWD data (Kinoshita et al, 2008 Structural observation of the upper slope sediments (Unit I) indicates southeastdipping beds and normal faults (Fig. F5).…”

“…The accretionary prism in the frontal thrust region is deformed by thrusting, as visible on the seismic profiles (Moore et al, 2007) and LWD data (Kinoshita et al, 2008). Most of the thrusts inferred from the seismic profile and LWD were confirmed from age reversals, fault zones sampled in cores, and repetition of specific strata, but some additional faults were also defined during Expedition 316 drilling.…”

“…Site C0004 was selected during Expedition 314 (LWD) as an alternate to Site C0003 (proposed Site NT2-01B), where the target could not be reached (Kinoshita et al, 2008). Site C0004 was projected to cross the splay fault at a depth of ~300 mbsf; total depth (TD) was 400 mbsf in order to sample the underthrust material.…”

“…Site C0006 was previously drilled during Expedition 314 (Kinoshita et al, 2008), in which LWD logs were obtained to 885.5 m LWD depth below seafloor (LSF). During Expedition 316, coring was completed to 603 m CSF.…”

“…Subunit IIB (72.06-163.33 m CSF) consists of interbedded fine-grained sand, silty sand, and silty clay in approximately equal abundances. Assessment of this subunit was hampered by poor core recovery, possibly indicating substantial loss of the sand units indicated by LWD logs (Kinoshita et al, 2008). Sand intervals are typically normally graded with indistinct upper boundaries that grade into silty clay.…”