Drilling sites on the incoming plate (proposed Site NT1-07A) (F1B, F2, F3) will capture fundamental geologic properties that are likely to change downdip along the plate boundary (Underwood, 2007). Our goal during Expedition 322 is to document initial conditions at these presubduction reference sites. We acknowledge here that the time allocation for Expedition 322 may be insufficient to complete the entire subduction inputs component of NanTroSEIZE. As implementation of the entire NanTroSEIZE science plan progresses, our knowledge of these initial conditions will expand incrementally and improve the context for interpretation of results from progressively deeper coring depths. Coring will provide samples for critical shore-based studies aimed at evaluating processes hypothesized to govern the transition from stable sliding to seismogenic behavior.
Key scientific questions
How does the physical hydrology of Shikoku Basin respond to variations in primary lithologic architecture and basement structure?
As one moves parallel to strike across the NanTroSEIZE transect area (i.e., away from the northeast flank of the fossil backarc ridge), the bathymetry of Shikoku Basin becomes increasingly complicated because of off-axis volcanic seamounts and remnant fragments of Zenisu Ridge (Le Pichon et al., 1987; Mazzotti et al., 2002). Acoustic thickness generally decreases above larger basement highs (Ike et al., 2008), but the 3-D architecture of the lower Shikoku Basin, its composition, detrital source(s), and directions of gravity flow transport are largely unconstrained by seismic data. Basement relief probably blocked or deflected flow paths during early stages of basin infilling, but the effects remain uncertain in detail. Basement-influenced heterogeneity of the lower Shikoku Basin facies, moreover, carries with it implications for abrupt changes in permeability structure, zonation of fluid pressure, and inconsistent amounts of early diagenesis. Such variations are likely both outboard and inboard of the deformation front (Fig. F7).
To accurately characterize the distribution of porosity and permeability, we need to map the turbidite sand bodies using newly acquired IFREE 3-D seismic data (Park et al., 2008). We also need to characterize their contrasting hydrologic properties directly using cores and logs. The turbidite sand bodies of the Shikoku Basin almost certainly provide high-permeability conduits for fluid flow right up to the time when the pore space is occluded by chemical cement. Updip pinch-outs of sand bodies against basement highs probably create compartments of excess pore pressure even before those strata are buried beneath the trench wedge (Fig. F7). This situation would be expected if overpressures get translated laterally from sediments as they are buried rapidly beneath the trench wedge and/or accretionary prism (e.g., Bredehoeft et al., 1988). To evaluate this possibility quantitatively, our strategy is to measure the differences between hydrologic and geotechnical properties of coeval facies units above basement plain and basement high. The upper boundary of the turbidites is also a likely zone of weakness if fluids migrate out of the turbidite section, are unable to drain vertically through the overlying mudstone aquitard, and create an overpressured horizon at the boundary. As subduction progresses, the presence of sand intervals may simultaneously (1) sustain high pore pressures at the top of the sandy turbidites due to translation of pressure along permeable strata (Bredehoeft et al., 1988; Dugan and Flemings, 2000) and (2) allow improved drainage at their downdip edge, leading to significant changes in effective stress and fault strength in three dimensions (e.g., Saffer and Bekins, 2006).
How do fluids and fluid pressure in the igneous basement affect subduction processes?
Studies of ridge-flank environments show that seawater transport and chemical reactions in upper oceanic basement are complicated (e.g., Wheat et al., 2003). On the other hand, the physical, thermal, chemical characteristics of fluids in the igneous crust of Shikoku Basin remain completely unconstrained by direct sampling. During NanTroSEIZE, we must consider how basement fluids evolve chemically and physically in the downdip direction and determine if or how potentially "exotic" fluids migrate vertically or updip from the basement. Additionally, if heat transfer from the basement is affected by hydrothermal fluid circulation, we need to quantify its effects on temperatures and rock properties both in situ and downdip. Similarly, fluids derived from basement may be focused along fault zones. To adequately characterize the basalt's physical properties and fluid chemistry prior to subduction, reference holes must penetrate significantly into basement. As a longer term NanTroSEIZE goal, we plan to sample basement fluids in sealed boreholes as a component of observatory installations.
How have system-wide patterns of sediment dispersal affected composition within the trench wedge and Shikoku Basin, particularly on the northeast side of the fossil spreading ridge?
Sandstone diagenesis and porosity reduction depend heavily on the initial texture and mineral composition of the sand. Currently, we do not know whether the anticipated Miocene turbidites on the northeast side of Shikoku Basin shared a common provenance with coeval sand bodies on the southwest side (i.e., offshore Ashizuri and Muroto Peninsulas; Fig. F1A). If armed with core samples, pore water, and thermal data from the reference sites, scientists will be able to forecast the onset of cement precipitation (e.g., quartz, calcite, and zeolite), framework grain dissolution, and formation of pseudomatrix by compaction and/or tectonic deformation of ductile rock fragments and phyllosilicates. Currently, we know little about clay mineralogy, volcanic ash stratigraphy, or ash alteration on the northeast side of Shikoku Basin. Data from Expeditions 315 and 316 reveal temporal trends in clay content that are consistent with the Pliocene–Pleistocene sections of Muroto and Ashizuri (e.g., Underwood and Steurer, 2003; Underwood and Fergusson, 2005), but the older Miocene strata remain largely unsampled. The clay-mineral budget is integral to several important hydration and dehydration reactions (e.g., smectite to illite transition) (Saffer et al., 2008), and an abundance of clay-size particles lowers the coefficient of internal friction regardless of mineral type (Brown et al., 2003). One prediction to test is the enrichment of both detrital and authigenic smectite (an unusually weak expandable clay) in response to larger amounts of volcanogenic input from the Izu-Bonin arc and a weaker northeast-directed proto-Kuroshio Current during the Miocene (Underwood and Steurer, 2003; Underwood and Fergusson, 2005).
How do thermal structure and primary sediment/rock composition modulate diagenesis and fluid-rock interactions?
Thermal structure is a critical input variable to document because of its effect on sediment diagenesis and fluid chemistry. Subducting lithosphere within the Kii transect is ~20 Ma (Okino et al., 1994). Heat flow generally decreases with age and distance from the Kinan Seamounts (Fig. F8) (Wang et al., 1995; Yamano et al., 2003), but we must verify this first-order regional pattern with high-quality borehole temperature measurements. We expect fluids and physical properties to change downsection and downdip in response to both hydration reactions (e.g., volcanic glass to zeolite + smectite) and dehydration reactions (e.g., opal-to-quartz and smectite-to-illite), together with precipitation of crystalline cements (carbonates, zeolites, silica). Sharp diagenetic fronts (especially opal-to-quartz) may be responsible for anomalous offsets in profiles of porosity and other geotechnical properties (Spinelli et al., 2007). The contribution of dispersed volcanic glass is potentially important during diagenesis but, as yet, is poorly understood. Similarly, hydrous authigenic phases in the basalt (e.g., saponite from ridge-flank hydrothermal alteration) are susceptible to diagenetic reactions at higher temperatures. Migration of fluids from zones of deeper seated dehydration reactions is a distinct possibility (Fig. F9), and this can be tested through a comprehensive program of geochemical analyses.
Which factor(s) control(s) the décollement's position near the prism toe, as well as the location of ramps and flats and mechanical behavior throughout?
One generic possibility to consider is a reduction in shear strength along a specific stratigraphic interval with low intrinsic strength, caused perhaps by unusually high contents of clay-size particles and/or smectite-rich clay (e.g., Vrolijk, 1990; Deng and Underwood, 2001; Kopf and Brown, 2003). Another generic possibility is a reduction of effective stress because of excess pore pressure. Causes of excess pore pressure could be as diverse as rapid updip migration of pore fluids from deep-seated sources, in situ mineral dehydration within poorly drained mudstone, or compaction disequilibrium caused by rapid loading of an impermeable mudstone beneath the landward-thickening trench wedge. Pinch-outs of highly permeable sand against mudstone aquitards, if combined with compaction disequilibrium and pressure-driven fluid flow, could lead to a complicated 3-D geometry of stratigraphically controlled compartments of excess pore pressure. Strata near the basalt/sediment interface, moreover, may contain abundant volcaniclastic debris and smectite. If true, those rocks may form the deeper and more landward zones of preferential weakness where the décollement ramps down. The permeable sandy turbidites may also exert a primary control on décollement strength and downstepping via their effect on pore pressure and thus effective stress (Moore and Byrne, 1987; Saffer and Bekins, 2006). Other possibilities to consider include changes in the hydrologic properties of basal sedimentary rock and/or the upper igneous crust (i.e., heterogeneities in permeability might localize overpressures) and changes in rock fabric as a function of protolith (i.e., the uneven development of slaty or phyllitic fabric may respond to inherited variations in sediment texture, composition, and earlier diagenetic-metamorphic history).
Does the plate boundary fault, near its updip limit of seismicity, shift its position from a sediment/sediment interface (stable sliding) to the sediment/basalt interface (stick-slip)?
The plate boundary when traced in the downdip direction eventually ramps down from a sediment/sediment interface to the sediment/basalt or intrabasalt interface (Fig. F2). This shift in lithologic position must coincide with fundamental changes in the rock's mechanical and/or hydrologic properties, but how so? Shore-based studies indicate that systematic fragmentation of upper basement and incorporation of basalt slabs into shear-zone mélanges could be controlled by primary layering of the igneous rock (Kimura and Ludden, 1995). Our challenge will be to discriminate between the presubduction factors inherited from Shikoku Basin (documented during this expedition) and the changes imparted by increasing P-T conditions and stress changes at depth (documented in the future by deep riser drilling).