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Tobin and Kinoshita (2006a, 2006b) provide the overall goals and plans of the NanTroSEIZE project as a whole, and the reader is referred to those publications for details. Here we briefly summarize the overall goals.
IODP will attempt to drill into, sample, and place instruments into the seismogenic portion of a plate boundary fault, or megathrust, within the Nankai Trough subduction zone in the offshore Kumano-nada region, near the Kii Peninsula of Honshu Island (Fig. F1). The most ambitious objective is to access, sample, and place instruments into the Nankai plate interface within the seismogenic zone using riser drilling at proposed Sites NT2-03 and NT3-01 at depths of ~3500 and ~6000 m, respectively (Fig. F2). The science plan entails sampling and long-term instrumentation of the following:
Conditions for stable versus unstable sliding, which define seismic versus aseismic behavior, have long been the subject of research and debate, as has the frictional strength of likely fault zone material. Fault zone composition, consolidation state, normal stress magnitude, pore-fluid pressure, and strain rate may affect the transition from aseismic to seismic slip (e.g., Saffer and Marone, 2003). At NanTroSEIZE, we will sample the following:
We will also install a series of borehole observatories to provide in situ monitoring of these critical parameters (seismicity, strain, tilt, pressure, and temperature) over time and test whether interseismic variations or detectable precursory phenomena exist prior to great subduction earthquakes. The overarching hypotheses to be addressed are as follows (refer to the Complex Drilling Program [CDP] proposal document at www.iodp.org/NanTroSEIZE for more details):
2. Subduction zone megathrusts are weak faults.
3. Within the seismogenic zone, relative plate motion is primarily accommodated by coseismic frictional slip in a concentrated zone.
4. Physical properties, chemistry, and state of the fault zone change systematically with time throughout the earthquake cycle.
5. The megasplay (OOST) thrust fault system slips in discrete events, which may include tsunamigenic slip during great earthquakes.
Sediment-dominated subduction zones such as the East Aleutian, Cascadia, and Nankai margins are characterized by repeated occurrences of great earthquakes of ~Mw 8.0 (Ruff and Kanamori, 1983). Although the causative mechanisms are not well understood (e.g., Byrne et al., 1988; Moore and Saffer, 2001; Saffer and Marone, 2003), the updip limit of the seismogenic zones at these margins is thought to correlate with a topographic break along the outer high (e.g., Byrne et al., 1988; Wang and Hu, 2006). At Nankai, high-resolution images of the outer high from seismic reflection profiles clearly document a major OOST fault system that branches from the plate boundary décollement within the coseismic rupture zone of the 1944 Tonankai M 8.2 earthquake and splits into several subsplays near the seafloor (Park et al., 2002) (Fig. F2). As stated above in the fifth hypothesis, one of the first-order goals in characterizing the seismogenic zone along the Nankai Trough, and which bears on understanding subduction zone megathrust behavior globally, is therefore to document the role of the megasplay fault in accommodating plate motion and characterize its mechanical and hydrologic behavior. The NanTroSEIZE strategy is to drill into the basal décollement fault at two locations and the megasplay fault system at three locations to comprehensively study the updip transition from aseismogenic to seismogenic fault activity.
During the development of the NanTroSEIZE drilling proposals, it was recognized from the outset that achieving these very ambitious goals would require a carefully planned and managed effort over multiple years and a number of individual drilling expeditions. The IODP SAS accordingly developed a new designation, the CDP, to recognize the need to organize multiple expeditions for a unified scientific purpose. The three Chikyu expeditions now planned for 2007 to early 2008 and the deferred operations described above comprise Stage 1 of the NanTroSEIZE CDP. Expedition 314 is the first of these three and is dedicated to acquiring high-quality downhole logging information from the six drill sites planned for Stage 1 as a whole. The Stage 1 overarching prospectus (Tobin and Kinoshita, 2006b) describes the overall NanTroSEIZE objectives and unified Stage 1 plan in greater detail, and readers should familiarize themselves with this plan.
In brief, Stage 1 of the NanTroSEIZE program includes three coordinated riserless drilling expeditions to drill at several sites across the continental slope and rise offshore the Kii Peninsula, within the inferred coseismic slip region of the 1944 Tonankai M 8.2 earthquake (Figs. F1, F2) (Tobin and Kinoshita, 2006a, 2006b). This prospectus is concerned with the first of these, an all-LWD expedition to all of the Stage 1 drilling sites to define physical properties, lithostratigraphy, and structural information in advance of coring operations. Detailed plans are described in the following sections. This will be followed by a coring expedition (Expedition 315, Megasplay Riser) aimed at sampling the materials and characterizing in situ conditions within the accretionary wedge to a depth of ~1000 mbsf at proposed Site NT2-03. This site will also serve as a pilot hole for later Stage 2 riser drilling targeting the megasplay fault at ~3000–3500 mbsf. A third riser Chikyu expedition (Expedition 316, Shallow Megasplay and Frontal Thrusts) targets two shallow fault zones: (1) the frontal thrust near the trench (proposed Site NT1-03) and (2) the older accretionary prism and megasplay fault at ~1000 mbsf (proposed Site NT2-01). Two additional NanTroSEIZE riserless expeditions, originally scheduled for the JOIDES Resolution during FY2007 but now removed from the schedule, sought to characterize the sedimentology, physical properties, hydrogeology, and in situ conditions of the incoming sediment and ocean crust at proposed Sites NT1-01 and NT1-07 and will document the long-term slip history of the megasplay fault at proposed Site NT3-01 based on stratigraphic relationships in the Kumano forearc basin by sampling ~1000 m of basin sediments and as much as 300 m of the underlying accretionary wedge. See www.iodp.org/expeditions for updated information on the dates of these expeditions.
Expedition 314, slated to be Chikyu's first IODP expedition, will kick off NanTroSEIZE Stage 1 activities with a dedicated drilling and logging program, using LWD technology at all six Stage 1 sites. The overall science plan calls for continuous coring, downhole measurements, and geophysical logging at all Stage 1 sites. In the typically unstable formations associated with riserless drilling in the accretionary prism environment, LWD is the only option to obtain high-quality geophysical logs, as demonstrated at other convergent plate margins during ODP Legs 156 and 171A (North Barbados Ridge), 170 and 205 (Costa Rica), and 196 (Nankai Muroto transect). Note that the full set of LWD tools cannot be run simultaneously with coring, so no coring is planned for this expedition. For operational and budgetary efficiency, the LWD logging portion of the science plan for all six proposed sites has been grouped together to form this stand-alone expedition, whereas coring and other activities will take place on the two subsequent Stage 1 expeditions and one expedition in the future.
LWD technology permits logging in these challenging environments because holes are drilled rapidly and continuously, without slow coring operations, and because the logging data are recorded just behind the bit when the hole has been freshly cut and conditions are as close to in situ as possible. The planned LWD operation for all sites during Expedition 314 consists of drilling one or more holes at each site by continuously washing down at a controlled rate with the logging tools incorporated into the bottom-hole assembly from 1 to ~40 m above the drill bit. The logging data are therefore acquired very soon after the hole is cut, providing the best possible data quality. The majority of the data are recorded in memory mode downhole and are downloaded after the drill string is brought back on board; however, some limited-resolution MWD data will be transmitted to the surface in real time. LWD logging measurements now planned for Expedition 314 will include spectral natural gamma radiation, resistivity, gamma density and neutron porosity, 360° borehole resistivity and density imaging, sonic velocity, interval seismic velocity, and APWD. Additional data to be recorded include logs for quality assessment and environmental correction (mud temperature, mud resistivity, etc.) as well as drilling parameter logs such as weight-on-bit and torque. More detailed information about the logging instruments to be deployed is given in "Measurement and logging-while-drilling tool capabilities."
Proposed Site NT1-01 targets the Shikoku Basin sedimentary section in a location where it has been deposited on a prominent basement high (Fig. F7). Characterization of the sediments, fluids, and basement composition at this site is part of a two-part strategy to document the material inputs to the seismogenic zone. Our plan is to drill proposed Site NT1-01 on a basement high and proposed Site NT1-07 in a thicker section off that high in order to show how basement relief influences the presubduction geometry of sedimentary facies, temperature, permeability, sediment and basement alteration, and fluid flow. Significant preexisting relief on the Shikoku Basin igneous crust has affected the distribution of sediments; therefore, the lithostratigraphy and fluid content of the sediment column vary spatially. In particular, most of the basin area includes a large proportion of turbidites in the deeper part of the stratigraphic column, but basement highs lack much or all of this deep turbidite section, based on seismic data analysis. Presence or absence of these facies and attendant fluid and smectite clay content may strongly affect downdip physical properties and initial conditions as sediments enter the seismogenic zone. Basement highs have been suggested to act as asperities in earthquake slip.
The program of LWD at these two sites (along with subsequent coring and observatory installation in the future) will quantify initial conditions in the material that is tectonically delivered to the subduction system; this material ultimately is what enters the seismogenic zone and initially hosts fault slip. Key scientific themes for proposed Site NT1-01 LWD will include deepwater turbidite depositional system and facies architecture through integration of logs with 3-D seismic data and physical properties (especially porosity and density to quantify mass flux) of the anticipated hemipelagic and turbidite sediments.
LWD of the entire sediment section to just above the top of basement is planned, to an estimated depth of ~600 mbsf (IODP Environment Protection and Safety Panel [EPSP] approved maximum penetration at this site is 800 mbsf). Because we wish to preserve basement hydrologic conditions in anticipation of observatory installation during a subsequent stage, no basement penetration is planned for this site with LWD.
Paired with proposed Site NT1-01, proposed Site NT1-07 targets the incoming section, as described above. At this location off the basement high, the deep portion of the Shikoku Basin section includes reflectors interpreted as regionally extensive deepwater turbidites (Fig. F8). LWD drilling of the entire sediment section to the top of basement is planned, at an estimated total depth of ~1200 mbsf (EPSP approved maximum penetration at this site is 1200 mbsf). As with proposed Site NT1-01, no basement penetration is planned for LWD operations during this stage.
Proposed Site NT1-03 targets the main frontal thrust at the toe of the entire accretionary wedge (Fig. F9). Based on seismic data and submersible dive studies (Ashi et al., 2002), this thrust is thought to have placed moderately consolidated clastic rocks over the weak and unlithified late Quaternary trench section clastic sediments. Propagation of an underlying décollement zone into the trench section is not clearly imaged, raising the hypothesis that this frontal fault is the main detachment, which, in this case has propagated all the way to the seafloor. On the other hand, detailed analysis of the seismic data suggests that substantial footwall deformation exists in the first few hundred meters below the fault at this site location, implying that strain decoupling is not total across this fault. Reflection amplitude of the fault plane is variable near this site, but generally it is a negative polarity reflector.
The overall objectives of drilling at proposed Site NT1-03 are comprehensive characterization of the lithology, deformation, and physical properties of the wall rocks and the fault zone, as well as documentation of the fluid chemistry. LWD drilling will contribute in situ physical properties and borehole imagery for this characterization. Drilling is planned to ~600–900 mbsf in a faulted turbidite and hemipelagic sediment setting, with the fault zone targeted at 350–400 mbsf (EPSP approved maximum penetration at this site is 1800 mbsf). A substantial porosity and velocity inversion is anticipated beneath the frontal thrust fault reflector. Later stages of drilling may target deeper intervals beyond 600 mbsf at this site, depending on what is discovered during Stage 1.
Goals at proposed Site NT2-01 entail LWD drilling of ~1000 m (EPSP approved maximum penetration at this site is 1200 mbsf) of the midslope region, across at least one major strand of the megasplay fault system (Fig. F10). This site will begin the downdip transect of the megasplay fault system by sampling a relatively shallow, presumably aseismogenic point on the fault zone at ~800 mbsf. The anticipated lithology is deformed terrigenous sediment, faults, and possible gas hydrate, though there is no clear bottom-simulating reflector (BSR) at this site. Beneath the near-surface slope deposits, the acoustically transparent zone above the reflective thrust fault may be composed of highly deformed and faulted accretionary mélange and/or disrupted stratigraphy of slope deposits. This material may have been transported a substantial distance up the splay thrust, in which case it would be likely to be anomalously well consolidated for its depth, with attendant low porosity and high density and seismic velocity. Beneath the fault reflector, 3-D seismic data suggest we will penetrate deformed but stratigraphically intact slope sediments that have been overridden by the splay thrust fault.
As with proposed Site NT1-03, the logging data will be used to characterize the material properties, deformational features, and conditions in the fault zones, wall rocks, and sediments. The logging data will guide coring operations during the subsequent Expedition 315, as well as subsequent installation of a pore pressure and strain monitoring system during Stage 2.
LWD drilling at proposed Site NT2-03B targets the uppermost 1000 mbsf (EPSP approved maximum penetration at this site is 1250 mbsf) at the seaward edge of the Kumano Basin uplift (outerarc high) where the megasplay fault system branches and approaches the surface (Fig. F10). Stage 1 drilling at this site is the first phase of a two-part strategy. The ultimate objective is to perform riser drilling to ~3500 mbsf during Stage 2, across the megasplay fault at depth, and establish a long-term deep borehole observatory. The upper 1000 m to be drilled during this expedition provides an opportunity to access the thrust sheets uplifted by several branches of the megasplay fault, as well as a thin overlying slope basin cover sequence. The nature of the material in these thrust sheets is unknown. As with proposed Site NT2-01, the acoustically nonreflective nature of this section suggests that it may be composed of chaotically deformed accretionary wedge sedimentary mélange transported from significantly greater depth. Alternatively, this zone may be composed of highly deformed Kumano forearc basin sediments. Possibly, it is a structurally juxtaposed combination of both. Together with later core samples, logs from this zone will discriminate among these possibilities and provide data on physical properties, strength, composition, and structure of the hanging wall of the main megasplay branch. It is likely that the upper 1000 m of drilling at this site will also penetrate one or more subsidiary splay branches near the updip end of the splay system, affording an opportunity to compare fault development with proposed Site NT2-01.
Stage 1 LWD will also provide critical "pilot hole" information for later riser-based drilling. To achieve the ~3500 m total depth objective using the riser and weighted drilling mud involves setting multiple casing strings, the depth of each of which depends on the least principal stress, fracture strength of the formation, and pore fluid pressure gradient. The key part of this casing plan is the "top-hole" portion, where tolerances on mud weight are tight. Planning the casing program, therefore, requires excellent information on physical properties in the uppermost 1000 mbsf. In light of this, our strategy is to utilize riserless drilling for this section in Stage 1 in this pilot hole and then return for the deeper portion in Stage 2.
At proposed Site NT3-01, we plan LWD drilling of as deep as 1400 mbsf (EPSP approved maximum penetration at this site) of the Kumano forearc basin section and the underlying formations, interpreted as older rocks of the accretionary prism and/or early slope basin sediments deposited prior to the development of the megasplay fault and the Kumano Basin (Figs. F11, F12). Scientific objectives include investigation of the outer forearc basin depositional systems, including possible earthquake-triggered turbidites, convergent margin deformation, likely gas hydrate, and a BSR. This is the site slated for deep drilling across the entire plate boundary system to >5500 mbsf during Stage 3 riser drilling. Expedition 314, along with later coring in the future, will accomplish the following:
Plans include continuous logging of the entire sedimentary section through the Kumano Basin and as much of the underlying older accretionary wedge rocks as possible to 1400 mbsf. Depth to the base of the Kumano Basin section is estimated to be ~900 mbsf, based on preliminary 3-D prestack depth migration velocities. The objective is to penetrate to this unconformity and 100–300 m below it to sample the underlying formation. Characterization of seismic velocity, density, porosity, stress, rock strength parameters, and pore pressure are all high priorities.
The overarching objective of the LWD program is to provide borehole data that will be used in conjunction with cores to document the geology, physical properties, mechanical state, fluid content, and stress conditions at the drilling site locations. Specifically we want to document the following: