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Scientific objectives

Research themes for Expedition 322 include (1) evolution of a deepwater turbidite depositional system and facies architecture within the Shikoku Basin; (2) heat flow, diagenesis, and fluid chemistry of mixed terrigenous and open-ocean sediments; (3) volcanic ash stratigraphy and provenance; (4) physical and hydrogeological properties of hemipelagic and turbidite sediments; and (5) petrology, alteration, and hydrology of Layer 2A of the oceanic crust (basalt). By drilling two sites on the incoming plate (proposed primary Site NT1-07A and proposed contingency Site NT1-01A), we expected to capture most of the fundamental geologic properties that are likely to change downdip along the plate boundary (Underwood, 2007). As the entire NanTroSEIZE science plan moves forward, we will apply our knowledge of the initial presubduction conditions to improve the observational and theoretical context for interpretation of results from forthcoming and progressively deeper coring. A successful coring program during Expedition 322 will segue into a broad range of shore-based studies aimed at evaluating the interwoven factors that collectively lead to transitions from stable sliding to seismogenic behavior (Saito et al., 2009).

Key scientific questions

How does the physical hydrogeology of Shikoku Basin respond to variations in primary lithologic architecture and basement structure?

As one moves across the NanTroSEIZE transect area parallel to strike (i.e., away from the northeast flank of the fossil backarc ridge), the bathymetry of the Shikoku Basin is punctuated by off-axis volcanic seamounts and remnant fragments of a deformed Zenisu Ridge (Le Pichon et al., 1987; Mazzotti et al., 2002). As expected, acoustic thickness of sediments generally decreases above larger basement highs (Ike et al., 2008a), but the 3-D architecture, mineral composition, detrital source(s), and directions of gravity flow transport of the lower Shikoku Basin are largely unconstrained by seismic data (Ike et al., 2008b). Basement relief probably blocked or deflected flow paths during early stages of basin infilling, but the local responses remain uncertain in detail. Basement-influenced heterogeneity of lower Shikoku Basin facies, moreover, carries with it implications for abrupt changes in permeability structure (e.g., because of channels and/or stratigraphic pinch-outs), zonation of fluid pressure, and inconsistent progression of early diagenesis. We expected to uncover evidence of such variations both outboard and inboard of the deformation front as NanTroSEIZE advances through Stages 2 and 3.

To characterize the spatial distribution of porosity and permeability accurately, we still need to map the turbidite sand bodies using recently acquired IFREE 3-D seismic data (Park et al., 2008). More importantly, we also need to characterize hydrologic properties of sandy intervals 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 chemical cement or authigenic clays occluded the pore space. If uncemented sand beds still exist, focused fluid flow could be occurring today. Updip pinch-outs of sand bodies against basement highs also may have created compartments of excess pore pressure; this is particularly likely if overpressures get translated laterally during rapid burial beneath the trench wedge and/or accretionary prism (e.g., Bredehoeft et al., 1988). To evaluate this possibility quantitatively, our strategy is to compare hydrologic and geotechnical properties laterally between 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 zone near the lithologic boundary. As subduction progresses, the presence of sand intervals may simultaneously sustain high pore pressures at the top of the sandy turbidites because of translation of pressure along permeable strata (Bredehoeft et al., 1988; Dugan and Flemings, 2000) and 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 in the igneous basement affect subduction processes?

We know from studies of ridge-flank environments elsewhere that seawater transport and chemical reactions in upper oceanic basement are complicated (e.g., Fisher, 1998; Wheat et al., 2003). On the other hand, the physical, thermal, and chemical characteristics of fluids in the upper igneous crust of the Shikoku Basin remain almost completely unconstrained by direct sampling. The only constraints come from fluid chemistry just above basement (Moore, Taira, Klaus, et al., 2001). During NanTroSEIZE, we must document 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 circulation (Spinelli and Wang, 2008), then we need to quantify those effects on temperature gradients and rock properties at greater depths. Similarly, it is theoretically possible for fluids derived from or modified by basement sources to be focused along fault zones. To adequately characterize the basalt's physical properties and fluid chemistry prior to subduction, reference holes must penetrate deeper into basement (i.e., 100–200 m below the sediment/basalt interface). As a longer term NanTroSEIZE goal, the plans also include sampling basement fluids in sealed boreholes as a component of observatory installations.

How have system-wide patterns of sediment dispersal affected composition within the Shikoku Basin, particularly on the northeast side of the fossil spreading ridge?

Diagenesis and porosity reduction in sandstone depend heavily on the initial texture and mineral composition of the sand. The provenance of Miocene turbidites on the northeast side of the Shikoku Basin remains uncertain and must be compared with the basin's southwest side (i.e., offshore Ashizuri and Muroto Peninsulas; Fig. F1). Currently, we know little about clay mineralogy, volcanic ash petrology, or ash alteration on the northeast side of the Shikoku Basin. Preliminary data from Expeditions 315 and 316 (Ashi et al., 2009; Screaton et al., 2009) reveal temporal trends in clay content that are consistent with the Pliocene–Pleistocene sections from offshore of Cape Muroto and Cape Ashizuri (e.g., Underwood and Steurer, 2003; Underwood and Fergusson, 2005), but the older Miocene strata remained largely unsampled prior to Expedition 322. The clay-mineral budget is integral to several important hydration and dehydration reactions (e.g., smectite–illite transition) (Saffer et al., 2008; Saffer and McKiernan, 2009). An abundance of clay-size particles also lowers the coefficient of internal friction regardless of mineral type (e.g., 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). Temporal changes in ocean circulation and climate also need to be evaluated in concert with constraints on the timing of eruptive activity in nearby volcanic arc systems (e.g., Taylor, 1992; Cambray et al., 1995; Kimura et al., 2005). This spatial-temporal view is important because the waxing and waning of eruptive activity may have triggered widespread changes in both sediment composition and sediment supply rates to different parts of the Shikoku Basin at different times.

How do thermal structure and primary sediment/rock composition modulate diagenesis and fluid-rock interactions prior
to subduction?

Thermal structure, including the effects of fluid circulation in the basement, is a critical input variable to document because of its influence on sediment diagenesis and fluid chemistry (Spinelli and Underwood, 2005; Saffer and McKiernan, 2009). The age of subducting lithosphere within the Kumano transect area is ~20 Ma (Okino et al., 1994). Heat flow generally decreases with age and distance from the Kinan Seamounts (Wang et al., 1995; Yamano et al., 2003), but we still need to verify this first-order regional pattern with high-quality borehole temperature measurements. The timing of volcanic activity responsible for the birth of Kashinosaki Knoll (Ike et al., 2008a) also needs to be established by dating the basalt.

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). 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). 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 will be tested through a comprehensive program of geochemical analyses.

Which factor(s) control(s) the décollement's position near the prism toe and at greater depths, together with the fault's mechanical behavior throughout?

The stratigraphy of fault zones can be dictated by a specific stratigraphic interval with low intrinsic strength, perhaps caused 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 are 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 (e.g., Swarbrick and Osborne, 1998). Pinch-outs of highly permeable sand against mudstone aquitards, particularly 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 smectite. If true, those rocks probably occupy the deeper and more landward zones of preferential weakness where the décollement ramps down (Fig. F3). Permeable sand-rich turbidites may also exert a primary control on décollement strength and downstepping via their effect on pore pressure and thus effective stress (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).

Seamount subduction also affects the structure of the frontal accretionary prism (Yamazaki and Okamura, 1989). Evidence from the frontal Kumano transect suggests widespread perturbation of structural architecture due to a subducting seamount just to the southwest of the 3-D seismic coverage (Moore et al., 2009). Among the adjustments to subducting basement highs are the décollement ramping up to the seafloor and oblique thrust development (Screaton et al., 2009). These large-scale tectonic overprints may overwhelm the influences of such inherited compositional factors as cementation or clay mineralogy. Our challenge now is 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).

Drilling plan

To reiterate, the primary plan for Expedition 322 was to sample within the Shikoku Basin and quantify initial conditions in the materials that are tectonically delivered to the subduction system. Geologic materials within the lower half of the Shikoku Basin are what ultimately enter the seismogenic zone and host slip along the deep megasplay and the deep décollement. Preexisting relief on the Shikoku Basin igneous crust obviously affected the spatial distribution of various sediment types and sedimentation rates, so our original plan (Saito et al., 2009) was to drill two sites: a condensed section on a basement high (proposed contingency Site NT1-01 [Hole C0012A]), and a thicker sand-rich section off that high (proposed primary Site NT1-07 [Hole C0011B]). A full program of coring and wireline logging of the sedimentary section was planned for Site C0011, and we benefited greatly from early acquisition of LWD data from Hole C0011A during the final days of Expedition 319. Log-seismic integration after the acquisition of LWD data indicated a depth to basement of ~1050 m SSF. Another change in plan came as a result of premature bit destruction and failure to reach the total depth (TD) target during coring in Hole C0011B. A quick assessment of the time remaining led to our decision to move to the contingency site (Hole C0012A). The coring summary is shown in Table T1.