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doi:10.2204/iodp.sp.322.2009

Background

Geologic setting

The Nankai Trough formed by subduction of the Philippine Sea plate to the northwest beneath the Eurasian plate at a rate of ~4 cm/y (Seno et al., 1993). The convergence direction is approximately normal to the trench axis and sediments of the Shikoku Basin and trench-wedge are actively accreting at the deformation front. Great earthquakes during the past 3000 y are well documented in historical and archeological records (e.g., Ando, 1975). The Nankai Trough has been selected as a focus site for studies of seismogenesis by both IODP and the U.S. National Science Foundation (NSF) MARGINS initiative.

The region offshore the Kii Peninsula on Honshu Island was identified through a series of international workshops as the best location for seismogenic zone drilling for several reasons. First, the rupture area of the most recent great earthquake, the 1944 Tonankai M 8.2 event, is well constrained by recent seismic and tsunami waveform inversions (Tanioka and Satake, 2001; Ichinose et al. 2003; Kikuchi et al., 2003). Second, a horizon of significant coseismic slip is reachable by drilling with the Japanese riser drilling vessel Chikyu (Fig. F2). Third, the Kii-Kumano region is typical of the Nankai margin in terms of heat flow and sediment on the incoming plate, whereas the previously drilled area offshore Cape Muroto is anomalous due to stratigraphy associated with basement topography and high heat flow (Moore, Taira, Klaus, et al., 2001). Fourth, data from ocean bottom seismometers and onshore high-resolution geodetic studies indicate significant interseismic strain accumulation (Miyazaki and Heki, 2001; Obana et al., 2001).

Data from existing boreholes (Deep Sea Drilling Project and Ocean Drilling Program [ODP] Legs 31, 87, 131, 190, and 196) show that the décollement in the Nankai Trough propagates through Miocene strata of the lower Shikoku Basin facies near the prism toe (Fig. F2). Seismic reflection data clearly document that the décollement is hosted within this section to at least 25–35 km landward of the trench, where it downsteps to near the sediment/ocean crust interface (Fig. F2) (Park et al., 2002). Thus, this stratigraphic interval, rather than the overlying trench wedge, is the essential one for tracking physical-chemical changes toward seismogenic depths as sediments are exposed to increasing pressure and temperature. Regional-scale analysis of seismic data shows large amounts of complexity and variability in terms of acoustic character and stratigraphic thickness (Ike et al., 2008). Previous drilling also demonstrated that seafloor relief (created during construction of the underlying igneous basement) strongly influenced the basin's early depositional history (Moore, Taira, Klaus, et al., 2001; Ike et al., 2008). For example, the axis of a fossil (middle Miocene) backarc spreading center coincides with a prominent basement high; younger seamounts of the Kinan chain are superimposed on the fabric of the ridge. Evidently, elevation of the seafloor inhibited transport and deposition of sand by gravity flows, so Miocene–Pliocene sediments above the ridge consist almost entirely of hemipelagic mudstone. The seismic reflection response within this facies is nearly transparent. On the flanks of the basement high, coeval strata consist largely of sand-rich turbidites. Semicontinuous high-amplitude reflectors are characteristic of this acoustic facies. The control that basement exerts on stratigraphic architecture provides the strategic backdrop for drilling two reference sites within the NanTroSEIZE transect, at proposed Sites NT1-01A (above a basement high) and NT1-07A (above an adjacent basement plain) (Fig. F3). In the following section, we elaborate on the significance of these drilling targets.

Thematic context for drilling subduction inputs

Frictional sliding

Multiple interrelated controls need to be considered when evaluating downdip changes in frictional properties and their potential for affecting a transition from stable sliding (aseismic) to unstable or conditionally stable fault behavior:

1. A shift to negative frictional velocity dependence (velocity weakening)—potentially caused by alteration of rock composition (e.g., clay-mineral or silica reactions, cementation, or pressure solution), changes in effective normal stress (e.g., Saffer and Marone, 2003), and/or shear localization and fabric development (Marone, 1998), all of which may be driven by incipient metamorphic reactions, cumulative shear strain, cementation, and/or phyllosilicate growth (i.e., phyllitic cleavage);

2. Increasing effective stress that increases the tendency for unstable slip—driven by declining fluid overpressure and coupled (perhaps) with exhaustion of mineral dehydration reactions (Moore and Saffer, 2001); and

3. Elastic stiffness of the fault plane and wall rock—necessary to allow sufficient strain accumulation to generate a recordable earthquake (Moore et al., 2007).
   

Overall, NanTroSEIZE plans to characterize fault rocks and document how the state variables and in situ parameters change as a function of lateral heterogeneity and downdip pressure-temperature (P-T) evolution. In other words, beginning with strata at the seafloor and seaward of the plate boundary, how do the physical (and chemical) properties of wall rocks and shear zones evolve down the "seismic conveyor belt?"

Weak faults

A theoretical case was made decades ago for weak thrusts (Hubbert and Rubey, 1959). Suggested causes include

1. Intrinsic weakness of fault gouge (e.g., high content of clay minerals),

2. Excess pore fluid pressure resulting in low effective stress that may be either localized within the fault (i.e., a weak fault embedded within a "strong" crust) (Rice, 1992) or distributed regionally (i.e., a weak fault within a weak crust) (Davis et al., 1983), and

3. Dynamic weakening during rapid slip events (e.g., Segal and Rice, 2006).
   

In situ measurement of state variables will help quantify the respective contribution of each, but many questions remain unanswered:

• What is the ambient pore pressure in wall rocks and fault zones?

• Are pore pressures localized within faults?

• Are coseismic slip and fluid expulsion coupled?

• Are bound fluids in shear zones chemically distinct from those in adjacent wall rocks?

• Do such fluids provide chemical proxies to unveil their sources?

• Does resolved shear stress change gradually or abruptly downdip?

One of the key elements of this test will be to determine which stratigraphic intervals, if any, serve as conduits for focused fluid flow or seals for overpressured compartments. Fundamentally, we cannot determine how or why fault gouge, wall rock, or fluids change downdip without first completing the "baseline" characterization of the starting materials at the presubduction reference sites. Similarly, we cannot resolve whether elevated pore pressures (if present at all) are localized within faults or regionally pervasive without a series of downhole measurements distributed throughout the plate boundary system, including the Kumano Basin and uppermost accretionary wedge.

Basement structure

Subduction zones respond differently to basement highs (e.g., seamounts as potential asperities) and smooth basement plains (Cloos, 1992; Cloos and Shreve, 1996), but several questions need to be answered within this context:

• Are the overlying sedimentary rocks fundamentally different because of autocyclic adjustments of sedimentary systems to basement/seafloor topography (i.e., in terms of texture, mineralogy, grain fabric, frictional properties, permeability, cohesion, etc.)?

• Do hydrologic properties within the upper igneous crust change from basement highs to plains, thereby influencing patterns of fluid flow, temperature, localization of excess pore pressure, strength variations on the décollement, and possibly earthquake behavior downdip?

• Does basement topography control patterns of heat and/or fluid flow?

• Do the presubduction histories of basalt alteration and/or diagenesis of the lowermost sedimentary strata make any difference?

  • If so, will superimposed products of sediment diagenesis, slaty fabric, and rock deformation change the fault strength?

  • If so, can those changes at depth be linked to inherited differences in texture, composition, or earlier mineral dehydration?

This expedition will attempt to address these questions by drilling through the basal sediments and into the upper basalt.

Position of décollement

Seismic reflection profiles indicate that the Nankai plate boundary fault ramps downsection near the updip limit of the seismogenic zone (Fig. F2). Shifting a fault's position in the downdip direction from a sediment/sediment interface to the sediment/basalt or intrabasalt interface must be influenced by changes in the rock's mechanical properties or pore pressure, but the precise causative mechanisms remain unclear:

• Are some of the governing properties inherited from the primary depositional environments, or are they all imparted on the rock within a critical P-T or stress window regardless of primary composition?

• Is there any hydrologic control?

• Are evolutions of secondary porosity, fragmentation, cementation, and/or hydrothermal alteration key processes within the upper 100–200 m of basalt?

Our goal is to begin this assessment during Expedition 322.

Seismic studies/site survey data

Site survey data have been collected in the drilling area over many years, including multiple generations of two-dimensional seismic reflection (e.g., Park et al., 2002), wide-angle refraction (Nakanishi et al., 2002), passive seismicity (e.g., Obana et al., 2004; Obara and Ito, 2005; Ito and Obara, 2006), heat flow (Yamano et al., 2003), side-scan sonar, swath bathymetry, and visual observations from submersible and remotely operated vehicle (ROV) dives (Ashi et al., 2002). In 2006, Japan and the United States conducted a joint three-dimensional (3-D) seismic reflection survey over a ~11 km x 55 km area, acquired by PGS Geophysical, an industry service company (Moore et al., 2007). This 3-D data volume was used to refine selection of drill sites and targets in the complicated megasplay fault region, define the regional structure and seismic stratigraphy, analyze physical properties of the subsurface through seismic attribute studies in order to extend information away from boreholes, and assess drilling safety. A smaller 3-D survey was conducted over proposed Sites NT1-01A and NT1-07A in 2006 by the Japan Agency for Marine-Earth Science and Technology–Institute for Frontier Research on Earth Evolution (JAMSTEC-IFREE) (Park et al., 2008). Prestack depth migration of those data led to refined velocity models and revised estimates of sediment thickness and total drilling depths.

Proposed Site NT1-07A

Shikoku Basin, overlying flanks oceanic basement high

Water depth is ~4060 m at this site, which is located at the intersection between IFREE 3-D Line 95 (Fig. F4) and CDEX Line ODKM03-101 (Shotpoint 2520) (Fig. F5). Compared to the overlying sand-rich trench-wedge facies, strata in the Shikoku Basin are dominated by hemipelagic mud and mudstone. At ODP Sites 808 and 1173, the boundary between upper Shikoku Basin facies and lower Shikoku Basin facies was defined by a combination of lithology and diagenesis, with abundant layers of volcanic ash above the boundary and an abrupt reduction in porosity below the boundary because of opal cement dissolution (Spinelli et al., 2007). At proposed Site NT1-07A, we see prominent reflectors at ~270 and 400 mbsf, which may coincide with a comparable lithologic boundary. Interpretations of acoustic stratigraphy are not straightforward, however, and another interval of interest is related to potential cementation of mudstone by diagenetic opal. Based on the modeled thermal history of the sediment, opal cementation of the section is expected from ~270 to 515 mbsf (Fig. F6). Prominent reflectors at the top and bottom of the interval of inferred cementation indicate sharp contrasts in physical properties. Deeper in the section, the lower turbidite facies thickens and displays outstanding acoustic continuity. The interval of inferred turbidites begins at ~750 mbsf and clearly laps onto the basement high toward the southeast, thereby offering an optimal target for overpressured sands near the updip pinch-outs. Recent analysis of 3-D seismic data indicates that the depth to basement at proposed Site NT1-07A is ~1200 mbsf.

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