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This section summarizes the science and operations plans for each Stage 1 expedition. The five expeditions that make up Stage 1 will take place on both the SODV and Chikyu (in riserless mode) between September 2007 and April 2008 (Fig. F5). Because both ships are new and these will be among their first expeditions, the exact dates of the individual expeditions are currently tentative and subject to change. In this Stage 1 prospectus, we provide only a brief overview of each expedition. Individual detailed prospectuses for each of the five Stage 1 Expeditions will be published in the coming months (www.iodp.org/expeditions) and each will present the expedition-specific scientific, operational, contingency, and sampling plans.
To maximize the science return, the five Stage 1 expeditions will be implemented as a single science program with samples and data freely shared across all five expeditions (and both drilling platforms). However, each Stage 1 expedition has very different drilling targets and science objectives that must be achieved. The invited co-chief scientists for these Expeditions are members of the NanTroSEIZE PMT to ensure coordination among Stage 1 expeditions toward achieving the overall NanTroSEIZE project science objectives. Furthermore, the PMT has assigned a few scientists ("Specialty Coordinators") to ensure quality and continuity of certain critical data types across the entire project.
On what is slated to be Chikyu's very first scientific ocean drilling expedition, NanTroSEIZE Stage 1 activities will begin with a dedicated drilling and logging program using LWD technology at all six 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 by Ocean Drilling Program (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.
LWD operations consist of continuously drilling one or more holes at each site by drilling down at a controlled rate, with the logging tools incorporated into the bottom-hole assembly a few meters behind the bit. The log 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 downhole and downloaded after the drill string is brought back on board; however, some data will be transmitted to the surface in real time using MWD.
LWD log data will include gamma ray, resistivity, density and porosity, sonic velocity, and 360° borehole resistivity and density imaging. Use of LWD magnetic resonance imaging and "seismic-while-drilling" (a checkshot-like vertical seismic profile collected during drilling with the LWD string) are both currently under discussion and may be incorporated into the logging plan. Additional data to be recorded include quality assessment and drilling parameter logs such as acoustic caliper and annular pressure while drilling (PWD).
The first hole(s) drilled at each site (Fig. F5) in Stage 1 will be the LWD runs, which are intended to extend to the total depth planned for subsequent Stage 1 coring and observatory operations. The LWD operational and science data will be crucial for optimizing the subsequent four Stage 1 expeditions and for planning future stages. LWD plans are as follows:
The shipboard scientific party for this LWD expedition will be focused on scientists with interest and expertise in physical properties, structural analysis, lithostratigraphy, logging interpretation, and log-seismic integration.
Chikyu Expedition 2 targets the uppermost 1000 m at one site (NT2-03) in the outer portion of the Kumano forearc basin. This site provides an opportunity for investigation of (a) the outer forearc basin depositional systems, including possible earthquake-triggered turbidites; convergent margin deformation; likely gas hydrates and a BSR; and (b) physical properties, fluid and gas chemistry, and hydrogeology in the region near the updip edge of the splay fault system.
Stage 1 drilling at this site is the first phase of a two-part strategy: the ultimate objective here is to perform riser drilling to ~3500 msbf during Stage 2, across the megasplay fault at depth, and establish a deep borehole observatory. To achieve the depth objective through riser-based drilling involves setting of multiple casing strings, the depth of each depending 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 requires excellent information on physical properties in the uppermost 1000 mbsf. Our strategy is therefore to drill this section riserless in Stage 1 in this pilot hole, then return for the deeper portion in Stage 2. We plan to core the entire 1000 m drilling range and collect wireline logs if possible to augment the LWD logs with higher quality sonic and resistivity imaging. The second half of this expedition may consist of only engineering and operations to set the riser seafloor structure and uppermost casing in preparation for Stage 2 riser operation.
The exact location of proposed Site NT2-03 as shown in Figure F7 is likely to be modified later in 2006, based on the 3-D survey results, in part to locate it in a region of more gently sloping seafloor. The scientific objectives will remain the same, however. Forearc basin sediments overlie deformed accretionary wedge sedimentary rocks at this site. The forearc basin developed as a response to uplift by the megasplay and hence should preserve a record of that fault system's history. Age-depth relations for the basin sediments will be important. Establishing temperature gradients downhole is also critical. A key scientific question will be to investigate the degree to which forearc basin sediments are incorporated into the developing accretionary wedge and the structural mechanisms by which this happens. The gas hydrate system in this area of apparent weak BSR, adjacent to a strong BSR, will be addressed as well.
The shipboard scientific party for this expedition will be focused on scientists with interest and expertise in physical properties, lithostratigraphy and structure, geochemistry, and age-depth relations (micropaleontology and paleomagnetism).
The third expedition on Chikyu will sample two major thrust fault systems at relatively shallow depths where they are accessible to riserless drilling. The first is the main frontal thrust at the seaward edge of the entire accretionary wedge (Figs. F2, F6). Based on seismic data and submersible diving studies, this thrust is thought to have placed moderately consolidated clastic rocks over the weak and unlithified late Quaternary trench section clastic sediments (Ashi et al., 2002). Propagation of an underlying décollement zone into the trench section is not clearly imaged, raising the alternative hypothesis that this frontal thrust fault is the main detachment, which propagates straight to the seafloor horizon. However, 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. Reflection amplitude of the fault plane is variable near this planned site, but generally it is a negative polarity reflector (Fig. F6). The objectives of drilling at proposed Site NT1-03 are comprehensive characterization of the deformation, evaluation of the inferred depth of detachment and structural partitioning at the décollement, physical properties of the fault zone and surrounding wallrocks, diagenesis, and chemical and other evidence for fluid flow in the frontal thrust fault. This will be accomplished by complete coring and a suite of downhole measurements (temperature, pressure, and packer tests) of the 600 m of planned drilling, with the fault zone targeted at 350400 mbsf, along with the LWD logs from Chikyu Expedition 1. Later stages of drilling may target deeper intervals at this site, depending on what is discovered in Stage 1.
Operations at proposed Site NT2-01 will target the shallow portion of the megasplay fault system. On this expedition, we will conduct a coring and downhole measurement program, similar to that described above, to obtain samples of the fault rock and wallrock and characterize it as completely as possible in conjunction with the previously acquired logging data. We anticipate crossing the main fault at ~800 mbsf, in addition to possibly numerous subfaults and a generally deformed zone. Work on this expedition will lead to subsequent installation of a pore pressure and strain monitoring system in Stage 2.
The shipboard scientific party for this expedition will be focused on scientists with interest and expertise in lithostratigraphy and structure, physical properties and hydrogeology, geochemistry, and age-depth relations (micropaleontology and paleomagnetism).
The purpose of this expedition is to sample at two reference sites within Shikoku Basin: NT1-01 and NT1-07 (Figs. F2, F6). The program of coring and downhole measurements 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 hosts fault slip. There is significant preexisting relief on the Shikoku Basin igneous crust that has affected the distribution of sediments, and therefore the lithostratigraphy and fluid content of the sediment column vary spatially. In particular, seismic reflection analysis shows that 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. 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.
Our plan is to drill proposed Site NT1-01 on a basement high and 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. Key scientific themes will include deepwater turbidite depositional system and facies architecture, heat flow, diagenesis and fluid chemistry of mixed terrigenous and oceanic sediments, volcanic ash stratigraphy, physical properties of hemipelagic and turbidite sediments, and igneous petrology, alteration, and hydrology of the uppermost oceanic crust. On this expedition, complete coring and wireline logging of the sedimentary section is planned at both sites, plus 100 m penetration into the igneous basement at Site NT1-01 only. Casing of Site NT1-07 is planned, with packer testing for hydrologic properties of the lower Shikoku Basin turbidites.
The shipboard scientific party for this expedition will be focused on scientists with interest and expertise in lithostratigraphy, physical properties and hydrogeology, geochemistry, and age-depth relations (micropaleontology and paleomagnetism).
The plan for this expedition is to core, perform downhole measurements, and install a long-term monitoring observatory at proposed Site NT3-01 to a depth of up to 1340 mbsf, for both scientific and engineering reasons (Figs. F2, F5, F7). This is the site slated for deep drilling across the entire plate boundary system to 5500+ mbsf during Stage 3 riser drilling. Scientifically, the Stage 1 drilling at this site will (a) document the depositional and uplift history of outer Kumano Basin sediments which will shed light on the long-term slip history of the megasplay fault system and deformation in the accretionary wedge; (b) sample the interior of the accretionary wedge in the mid-slope region; (c) establish a thermal structure at the position of the updip limit of co-seismic slip; and (d) monitor strain, tilt, seismicity, and pore fluid pressure in this key position above the aseismicseismic transition zone, where VLF earthquakes have recently been observed (Obara and Ito, 2005; Ito and Obara, 2006).
Plans include continuous coring of the entire sedimentary section through the Kumano Basin and as much of the underlying older accretionary wedge rocks as possible to 1340 mbsf. Downhole temperature and pressure measurements will be performed, and a VSP will be acquired, possibly as an offset VSP with a second ship shooting to the borehole seismometers. Installation of casing and a CORK-style system will follow; current plans for the borehole observatory include monitoring systems for pore fluid pressure at several depths, tensor volumetric strain, borehole tilt, and temperature. All components of the long-term monitoring package are subject to availability of funds and equipment at the time of this writing.
As with Site NT2-03, this site will serve as a primary hole for subsequent Stage 2 riser drilling to much greater depth. Complete characterization of seismic velocity, density, porosity, thermal conductivity, rock strength parameters, and pore pressure gradient are high priority, and the data will be merged with LWD logs from Chikyu Expedition 1. Additionally, the observatory systems to be installed here will serve in part as a prototype and testbed for strategies that might be applied to conduct multiple measurements efficiently in a single cased borehole.
The shipboard scientific party for this expedition will be focused on scientists with interest and expertise in the long-term observatory science, downhole measurements, physical properties, lithostratigraphy, geochemistry, and age-depth relations (micropaleontology and paleomagnetism).