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doi:10.2204/iodp.sp.333.2010

Drilling, coring, and temperature measurements plan

Operations for Expedition 333 are based on the current state of knowledge at the time of writing this Scientific Prospectus. These plans may be modified both before and during the expedition, based on continuing NanTroSEIZE Project Management Team (PMT) discussions. The Expedition 333 operation plan and drilling sequence is shown in Table T1 and Figure F7. The following operations are planned:

1. NanTroSLIDE coring at proposed Site NTS-1A.

2. Formation temperature measurements with advanced piston corer temperature tool (APCT-3), together with hydraulic piston coring system (HPCS), extended shoe coring system (ESCS), and extended punch coring system (EPCS) coring at the two subduction input Sites C0011 and C0012.

3. Basement coring at Site C0012.

Sites C0011 and C0012

Scientific objectives

As mentioned previously, the upper stratigraphic intervals (upper Shikoku Basin facies) were not adequately sampled during Expedition 322. Therefore, one of the priorities for Expedition 333 is to fill in the coring gaps and expand the age-depth models into the Pliocene and Quaternary (Fig. F4). The shallow section is also important for comprehensive profiles of organic and interstitial water geochemistry. Thermal structure, including the effects of fluid circulation in the basement, is another of the critical input variables to document because of its influence on sediment diagenesis and fluid chemistry (Spinelli and Underwood, 2005; Saffer and McKiernan, 2009; Spinelli and Wang, 2008). The age of subducting lithosphere within the Kumano transect area is ~20 Ma (Okino et al., 1994), as verified by coring at Site C0012 (Underwood et al., 2009). The Kashinosaki Knoll lies west of the Zenisu Ridge intraoceanic thrust, which brings backarc basin crust to outcrop at the seafloor (Lallemant et al., 1989; Henry et al., 1997). However, the contribution of active compressive tectonics to the Kashinosaki Knoll morphology is unclear, and the exact timing of volcanic activity responsible for the birth of Kashinosaki Knoll (Ike et al., 2008a) still needs to be established by radiometric dating of the basalt. Dense surface heat flow measurement around the Kashinosaki Knoll indicates significantly higher value than the theoretical value estimated from the age of the Shikoku Basin (Kinoshita et al., 2008a) (Fig. F8). We must study deep thermal structure to document the entire heat flow pattern around the sites with high-quality borehole temperature measurement. As subduction carries Shikoku Basin strata toward and beneath the accretionary prism, we expect fluids and physical properties to change downsection and downdip in response to hydration reactions (e.g., volcanic glass to zeolite + smectite), dehydration reactions (e.g., opal-to-quartz and smectite-to-illite), and crystalline cement precipitation (carbonates, zeolites, and silica). Documenting such changes is an essential ingredient of the NanTroSEIZE science plan. Sharp diagenetic fronts (especially opal-to-quartz) have been linked to anomalous offsets in profiles of porosity, P-wave velocity, and other geotechnical properties (Spinelli et al., 2007). Alteration of dispersed volcanic glass is also potentially important during diagenesis but, as yet, this component of the sediment budget is poorly understood (Scudder et al., 2009). Similarly, hydrous authigenic phases in the basalt (e.g., saponite from ridge-flank hydrothermal alteration) are susceptible to diagenetic reactions at higher temperatures. Updip migration of fluids (including hydrocarbons) toward the Shikoku Basin from landward zones of deeper seated dehydration reactions is a distinct possibility (Saffer et al., 2008), and this idea can be tested through a comprehensive program of geochemical analyses. Interpretation of the geochemistry, however, requires constraints on the in situ temperature.

Characterization of basement composition and structure is a high priority for NanTroSEIZE. Permeability and fluid flow within oceanic basalt are affected by many variables (Fisher, 1998). A long-term goal is to monitor and sample fluids using subseafloor observatories but design of those experiments hinges on coring and logging results. As a prelude, we plan to concentrate first on documenting the basement's structural architecture, hydrologic properties, and early alteration products. Products of early alteration within the uppermost basalt (e.g., saponite and calcite) change the rock's bulk chemistry and physical properties (porosity and permeability). The extent of this alteration is important for constraining the volatile content of subducting crust. In addition, coring at least 200 m into basement and wireline logging during a future expedition will capture heterogeneities in fracture patterns and porosity that might be involved in delamination of the basalt downdip in the seismogenic zone.

The specific set of questions addressed by drilling at input sites are

• Is fluid circulation in basement and permeable sedimentary layers influencing heat flow and diagenesis at Sites C0011 and C0012?

• How does contrasting pore fluid chemistry at Sites C0011 and C0012 relate with in situ diagenesis and fluid flow?

• Can a change of physical properties between 200 and 250 mbsf at Site C0011 (Fig. F9) be related to lithologic variation or diagenesis? Does the same transition occur at Site C0012?

• Is magmatic activity heterogeneous in composition and age on a backarc basin basement high?

• Is alteration of the upper oceanic basement heterogeneous and how does it influence geochemical and fluid budgets?

Understanding relationships between the physical and chemical evolution of basement and sediment is a key objective of Expedition 333, relevant both to seismogenic zone and subduction factory studies.

Drilling strategy

At Sites C0011 and C0012, HPCS cores with APCT-3 measurements will be collected from the seafloor to refusal, and ESCS/EPCS coring will continue as time permits to the target depths of 350 mbsf at proposed Hole C0011C and 150 mbsf at proposed Hole C0012C or to the refusal depths (Figs. F5, F6, F10, F11). RCB coring will collect basement cores at proposed Hole C0012D from 520 to 740 mbsf after washing down to 520 mbsf.

Time was insufficient to deploy HPCS coring during Expedition 322, so temperature measurements were not made at Sites C0011 and C0012. The sediment temperature-pressure tool was successfully tested in the drill string but was not deployed in the formation. Expedition 333 plans to make formation temperature measurements using the APCT-3 during HPCS coring operations at a target spacing of every third core.

NanTroSLIDE Ancillary Project Letter

Scientific objectives

Expedition 333 will drill and sample the slope basin seaward of the megasplay that is characterized in 3-D seismic data by stacked mass transport deposits (MTDs) (Strasser et al., 2009) (Figs. F12, F13, F14). This coring has the aim of establishing the submarine landslide history and reconstructing transport dynamics. Core from proposed Site NTS-1A will be integrated with 3-D seismic interpretation and data from nearby NanTroSEIZE sites to determine the relation of submarine landslides to the tectonic evolution. By establishing a better physical understanding of tectonic processes and slope failures, we will also gain a general understanding of failure-related sedimentation patterns and the significance of episodic mass transport events. Ultimately, this could help us assess the tsunamigenic potential of tectonic landslides. The primary goals of drilling the proposed site (NTS-1A) are

1. To establish a well-dated Quaternary mass-movement event stratigraphy and

2. To sample the distal part of an exceptionally thick MTD for analyzing its rheological behavior to constrain sliding dynamics and tsunamigenic potential.

These aim at providing answers to the following questions:

1. What is the frequency of submarine landslides?

2. What is the source materials of the MTDs?

3. What is the importance of accretionary wedge remobilization versus surficial processes?

4. What controls type, size, and magnitude of turbitides and MTDs and how do they change through time?

5. How do large MTDs relate to the timing of splay fault activity as inferred from NanTroSEIZE Stage 1 drilling (Strasser et al., 2009)?

6. What are the dynamics of large submarine landslides and can we infer their tsunamigenic potential?

By addressing these questions, we aim to isolate tectonic processes influencing magnitude and occurrence of submarine landslides along active subduction zone margins and to understand their potential for triggering catastrophic events in terms of both hazards (tsunamigenic landslides) and sediment mass transfer within the context of margin evolution.

Drilling strategy

Proposed Site NTS-1A (water depth = 3100 m) is located on a margin-perpendicular transect 4.5 km southwest of the NanTroSEIZE Stage 1 drilling transect (Fig. F12). It is located 5 km south-southwest of Site C0008, which was drilled into a small slope basin seaward of the megasplay fault (Kimura et al., 2008). Site C0008 results show the utility of using the ages of MTDs to reconstruct slope failure activity related to megasplay fault movements (Strasser et al., 2009). Apart from the deepest section, Site C0008 lacks clear evidence for MTDs, potentially due to a significant hiatus in its upper part, suggesting erosion or nondeposition likely related to a prominent slope collapse structure seaward of the megasplay fault (Kimura et al., 2008). On the basis of new 3-D seismic data interpretation, the Ancillary Project Letter proponents have identified a lower slope basin that (1) better represents the depocenter for downslope mass transport, (2) is clearly characterized by stacked MTDs as seismically imaged by acoustically transparent to chaotic bodies with ponded geometries (Fig. F12), and (3) includes a large, as thick as 150 m, MTD. Expedition 333 will drill at a location where the MTD bodies wedge out and where basal erosion is minimal. Continuous coring with HPCS and ESCS/EPCS to ~350 mbsf will allow for sampling the MTDs across the most complete and longest stratigraphic succession.

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