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doi:10.2204/iodp.sp.322.2009

Scientific strategies

Fundamentally, NanTroSEIZE is an ongoing interdisciplinary project that continues to require an unprecedented amount of collaboration and coordination, both within and among all of the individual expeditions. At the same time, scientists on each expedition will be expected to focus on a large number of topical objectives and employ the optimal methods for achieving those objectives. Furthermore, drilling is providing data and samples relevant to several secondary scientific objectives. Some of the ancillary topics include investigation of physical properties and pore water chemistry across a well-defined bottom-simulating reflector, reconstruction of the Kuroshio Current along the Japanese islands, regional tephrachronology, presubduction constraints on the geochemistry of "subduction factories," and sampling the biosphere along a continental margin at subbottom depths down to 1200 mbsf. The following list is just a starting point; we expect members of the invited science party to suggest and pursue additional strategies.

Lithology

Scientific tasks in the field of lithology include integration of seismic stratigraphy with conventional core descriptions, wireline logs, and compositional analyses (grain size, bulk mineralogy, sand petrology, clay mineralogy, and dispersed and layered volcanic ash). This core-log-seismic integration will establish patterns of sediment dispersal, 3-D facies architecture, and their responses to autocyclic and allocyclic forcing. Much of this can be accomplished using standard shipboard protocols, including data from multisensor core loggers. Bulk-powder compositional analysis by X-ray diffraction (XRD) will be essential for core-log-seismic integration. Shore-based work should include detailed XRD analysis of clay minerals, inductively coupled plasma–mass spectrometry bulk chemistry, thin section petrography, environmental scanning electron microscope (SEM)-energy dispersive spectrometry (EDS) of mudstone fabric, chemical analyses for biogenic silica, volcanic glass geochemistry, stable isotope analyses, apatite fission-track geochronology, and radiometric dating of detrital phyllosilicates and K-feldspar. It is particularly important to document the fine details of sediment fabric with high-resolution microscopy and to characterize the "protolith" phyllosilicate composition (e.g., illite crystallinity index, mica polytype, mica b_o value, and timing of crystallization) so we can compare these with incipient metamorphic phases sampled later in the downdip direction within the subduction zone. Such data will help test whether or not mineral changes and/or fabric changes affect the onset of stick-slip behavior during the transformation of mudstone to slate-phyllite.

Physical and chemical hydrology

Aqueous fluids affect virtually all forms of subduction zone behavior (Moore and Vrolijk, 1992; Moore, Taira, Klaus, et al., 2001; Saffer and Bekins, 1998, 2002). Thus, we must consider (1) how spatial distribution of permeability, pore pressure, temperature, and fluid chemistry vary within different sedimentary rocks and igneous basement; (2) how fluid composition and pressure change in response to burial, tectonic consolidation, and diagenesis; (3) how changes in pore pressure affect fault strength; and (d) potential for hydrologic feedbacks to regional strain (e.g., flow of deep-seated fluids along sand layers within the lower Shikoku Basin facies; see Fig. F9). Therefore, horizontal and vertical permeability of the sediment will be measured using a combination of whole-round samples (from both sand-rich and mud-rich units, tested at in situ effective stresses). We will also attempt to measure formation pressure using the sediment temperature-pressure (SET-P) tool (formerly known as Davis-Villinger Temperature-Pressure Probe). Our long-term goal for NanTroSEIZE is to quantify hydrologic properties over a full range of well-characterized lithologies (including basalt), then scale upward from minicore and whole-round specimens to wireline logs and 3-D seismic reflection data. In addition to shipboard interstitial water chemistry, shore-based laboratories will need to concentrate on analyses of trace/minor elements in the pore fluids, together with isotopes (Sr, B, Li, O, H, Cl, and C) that are diagnostic of fluid sources and conditions of low- to medium-temperature fluid-rock reactions. Some of these goals will require installation of long-term borehole observatories, but the design and positioning of such observatories will depend on data from the cores.

Thermal data

Mechanical properties, hydrologic properties, and rock fabrics evolve during compaction and diagenesis. Precise calculations of heat flow and documentation of presubduction diagenetic progress need to be carefully coordinated with parallel efforts in lithology, fluids, and physical properties. Most of the alteration products of early sediment and basalt diagenesis will be documented via shore-based chemical analyses of specific authigenic phases, XRD, transmission electron microscopy, and SEM-EDS. For example, temperature drives the opal-to-quartz reaction, which in turn may dictate where the diagenetic boundary between upper and lower Shikoku Basin facies is located (Spinelli et al., 2007). Thus, a high priority will be to pinpoint thermal structure where the alteration paths begin. These goals can be achieved through relatively high resolution borehole temperature measurements using both the advanced piston coring temperature (APCT-3) and SET-P tools.

Basement coring and logging

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, fluid composition at the sediment/basalt interface, 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 40 m into basement at proposed Site NT1-07A and wireline logging will capture heterogeneities in fracture patterns and porosity that might be involved in delamination of the basalt downdip in the seismogenic zone. Basement penetration at proposed Site NT1-01A is also included in the contingency plan (see "Risks and contingency").

Physical and geotechnical properties

We will document depth-dependent and facies-sensitive variations in bulk density, porosity, void ratio, electrical conductivity, and thermal conductivity using standard shipboard measurements. Wireline logs will measure the physical properties in situ, to be followed by a more time intensive program of shore-based geotechnical and rock mechanical experiments (e.g., one-dimensional consolidation and triaxial, direct shear, ring shear, and flow-through permeability) under a wide range of experimental conditions. Tests on whole-round core samples will define in situ burial conditions, establish whether states of over- or underconsolidation exist on a widespread basis, and pinpoint lithology-, grain size-, and temperature-dependent changes in deformation behaviors, permeability, and moduli (Giambalvo et al., 2000; Burland, 1990). Consolidation tests will also provide lithology-specific solutions to the relation between effective normal stress and void ratio, which will permit estimates of in situ pore pressure and effective stress (Moore and Tobin, 1997; Saffer, 2003; Ge and Screaton, 2005). Shearing experiments on intact and remolded material will document composition- and fabric-dependent variations in frictional strength and velocity dependence (Brown et al., 2003; Kopf and Brown, 2003; Saffer and Marone, 2003).

Downhole logging

Downhole logging is an important method to achieve scientific objectives of the expedition. Resistivity, density, porosity, natural gamma ray, sonic velocity, and borehole image data are critical for core-log-seismic integration in terms of lithology, petrophysics, and sedimentary/basement structures.

Continuous logging data can be used to compensate for uncored intervals or cored intervals with poor recovery. Logging data will provide essential information to identify the critical intervals such as turbidite zones in lower Shikoku Basin and the sediment/basement interface where core quantity and quality are expected to be poor. Borehole images will provide excellent information to characterize possible fluid conduits such as individual sand layers, volcaniclastic successions, and internal structure of igneous rocks.

To fulfill the scientific objectives, natural gamma ray, resistivity, density, porosity, sonic velocity, and resistivity images are essential measurements as highest priority. Photoelectric effect (PEF) and spectroscopy gamma ray logs that provide concentrations of K, Th, and U are useful to identify lithologic determination. Interval velocity data determined by check shot are useful for log-seismic integration.

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