doi:10.2204/iodp.proc.332.104.2011
Site C00021
Expedition 332 Scientists2
Background and objectives
Integrated Ocean Drilling Program (IODP) Site C0002 is slated for deep drilling across the entire plate boundary system, including the megasplay fault at ~5000 meters below seafloor (mbsf) and the décollement at ~7000 mbsf, planned during later stage riser drilling (Tobin and Kinoshita, 2006). The overall scientific objective for the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) observatory is to understand interseismic behavior of the seismogenic zone fault system through in situ monitoring of seismic, geodetic, and hydrological properties.
Site C0002 is located in the southern part of the Kumano Basin (Fig. F1). The Kumano Basin has a generally flat topography at ~2000 m water depth. The sediments in the southern part of the Kumano Basin are tilted northward, truncated by a flat erosional surface, and subsequently cut by normal faults (Park et al., 2002; Moore et al., 2007) (Fig. F2).
This site is situated above the uppermost part of a seismogenic fault of the Tonankai earthquakes, which occurred during an interval 100–150 y from the recorded history of the earthquakes. The seismogenic plate boundary is at ~7000 mbsf, whereas another splay fault propagating with a larger dip angle to the surface is at ~5000 mbsf. Further south (i.e., seaward), the plate boundary is considered aseismic, but the occurrence of very low frequency earthquakes was reported based on observations by a seismic network on land such as Hi-net.
To date, six holes have been drilled at Site C0002, all within 100 m of each other (Fig. F3). Site C0002 was first explored in 2007 during IODP Expedition 314 (Kinoshita et al., 2008), with a full suite of logging while drilling (LWD)/measurement while drilling (MWD) to 1401 mbsf (Hole C0002A; Expedition 314 Scientists, 2009). These measurements revealed the structure, physical properties, and stress state of the Kumano forearc basin, the gas hydrate–bearing zone, and the underlying deformed rocks of the inner accretionary prism (Fig. F4). A prominent bottom-simulating reflector (BSR) interpreted as the base gas hydrate reflection is evident at ~400 mbsf in the log resistivity, gamma ray, P-wave velocity, and density data.
Borehole breakouts indicate northeast–southwest oriented maximum horizontal compression throughout the whole interval, perpendicular to that at IODP Sites C0001 (thrust sheet of megasplay thrust) and C0009 (Kumano forearc basin) (Fig. F4). Fracture and fault orientations also support margin-normal extension of this part of the forearc. This extension is probably driven by uplift of the megasplay/outerarc high causing gravitationally driven extension of the inner wedge (Expedition 314 Scientists, 2009). This contrasts with Site C0001 in the active prism (outer wedge), where convergence-related compression dominates.
During IODP Expedition 315, Holes C0002B, C0002C, and C0002D were drilled. Core samples were taken from 0 to 204 mbsf and 475 to 1057 mbsf (Expedition 315 Scientists, 2009). Based on LWD and core data, the lithology is classified into four units. Unit I (0–135.8 m core depth below seafloor [CSF]) contains sand and silt intercalation and has a fast sedimentation rate. The age of the unit ranges from Quaternary to late Miocene. The forearc basin sequence was divided into two units based on lithofacies; these units corresponded to Units II (135.8–830.4 m CSF) and III (830.4–921.7 m CSF), defined by LWD. Both units are dominated by mud and mudstone. Although core recovery was poor (35% on average for the bottom interval cored with the rotary core barrel [RCB]), the basal unconformity of the Kumano forearc basin was encountered at 922 m CSF, or ~936 m LWD depth below seafloor (LSF), and another 120 m was cored into the accretionary prism. The underlying accretionary prism materials contain more lithified and deformed sediments. Acoustic features in Unit IV (921.7 m CSF and below), characterized by irregular discontinuous horizons, suggest that it may be composed of chaotically deformed accretionary wedge sedimentary mélange transported from significantly greater depths.
Faults and shear zones are clustered around 700, 920–950, and 1000–1050 m CSF. Three deformation phases were recognized by fault analyses. The earliest phase is a thrust fault (and possibly a strike-slip fault) and exhibits northwest–southeast shortening. Two phases of normal faulting occurred subsequent to thrusting. The first is recorded in shear zones and indicates northeast–southwest extension. The second is recorded in normal faults and indicates north–south extension, consistent with the present stress direction acquired from LWD results. A total of 31 whole-round samples were taken for interstitial water analyses. Changes in concentration for most elements seem to be controlled by unit boundaries. A downhole increase of ethane and concomitant decrease of C1/C2 ratios in Unit IV suggest some contribution of thermogenic hydrocarbon gas. Downhole temperature was measured at eight depths to 159.0 m CSF and showed an almost linear downhole increase with a gradient of 0.043°C/m.
IODP Expedition 326 (NanTroSEIZE Stage 3: plate boundary deep riser: top hole engineering) was the first of several project stages that will ultimately drill and core to the boundary between the Philippine Sea and Eurasian plates, the ultimate target of the NanTroSEIZE complex drilling project (Kinoshita et al., 2010). Expedition 332 ran from 19 July to 20 August 2010. Hole C0002F (Fig. F3) was drilled to 868.5 mbsf, and the hole was cased with 20 inch casing. A corrosion cap was set in preparation for future planned drilling.
The primary objective at Site C0002 during Expedition 332 was to install a permanent borehole observatory in the basal forearc basin (Unit III) dominated by condensed mudstone and the upper accretionary prism with interbeds of mudstone, siltstone, and sandstone (Unit IV) (Fig. F5). This long-term borehole monitoring system (LTBMS) will ultimately be connected to a submarine cable network for monitoring earthquakes and tsunamis, the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET).