Geological setting

The Nankai Trough is a convergent plate boundary where the Philippine Sea plate underthrusts the southwestern Japan margin at rates of 4.0–6.0 cm/y along an azimuth of 300°–315°N (Seno et al., 1993; Miyazaki and Heki, 2001; Mazzotti et al., 2000; Henry et al., 2001) down an interface dipping 3°–7° (Kodaira et al., 2000a). The subducting lithosphere of the Shikoku Basin was formed by backarc spreading during a time period of approximately 15–25 Ma (Okino et al., 1994).

The three major seismic stratigraphic sequences identified in the northern Shikoku Basin are the lower and upper Shikoku Basin facies and local spillover of Quaternary trench-wedge turbidites (Fig. F3). The upper Shikoku Basin facies off the Kumano Basin thins toward the north, whereas the lower succession has a more complicated isopach geometry influenced by basement topography (Le Pichon et al., 1987a, 1987b; Mazzotti et al., 2002; Moore, Taira, Klaus, et al., 2001; Moore et al., 2001; Ike et al., 2008b). Throughout the basin, seismic thickness decreases above large basement highs and sand packages of the lower Shikoku Basin pinch out against basement highs. Basement highs are imaged within the subduction zone (Kodaira et al., 2003, Dessa et al., 2004) and influence margin structure (Lallemand et al., 1992) and seismicity (Kodaira et al., 2000b; Park et al., 2004). The mechanical and hydrogeological differences between strata above subducting basement highs and regions with smooth basement topography could be significant for fault zone dynamics and earthquake rupture behavior.

The lower forearc slope of the Nankai Trough consists of a series of thrust faults that have shortened the accreted sedimentary units of the prism (e.g., Moore et al., 2009; Screaton et al., 2009). Swath bathymetry and multichannel seismic (MCS) data show a pronounced and continuous outer arc high extending >120 km along strike, which may be related to slip on the megasplay fault (Moore et al., 2009; Martin et al., 2010). Remotely operated vehicle (ROV) and manned submersible surveys along this feature reveal a very steep slope on both sides of the ridge (Ashi et al., 2002). The other arc high coincides with the updip end of the splaying system of thrust faults that branch from a strong seismic reflector interpreted by Park et al. (2002) as a major OOST. The megasplay fault is hypothesized to represent the mechanical boundary between the inner and outer accretionary wedge and between aseismic and seismogenic fault behavior (Wang and Hu, 2006). At depth, this megasplay produces a high-amplitude reflector (Fig. F2). It branches into a family of thrust splays in the upper few kilometers below the seafloor, including the thrust splay drilled during Stage 1 Expeditions 314, 315, and 316 (Moore et al., 2007, 2009).

The plate boundary, when traced in the downdip direction, eventually ramps down from a sediment/sediment interface to the sediment/basalt or an intrabasalt interface (Figs. F2, F3). This shift in lithologic position of the fault 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 features in basement inherited from backarc spreading in the Shikoku Basin (documented during this expedition) and the changes imparted by increasing pressure-temperature (P-T) conditions and stress changes at depth (documented in the future by deep riser drilling).

Previous drilling achievements

Sites C0011 and C0012

Expedition 322 was designed to document characteristics of incoming sedimentary strata and uppermost igneous basement prior to their arrival at the subduction front (Saito et al., 2009). To accomplish those objectives, coring was conducted at two sites on the subducting Philippine Sea plate. Site C0011 is located on the northwest flank of a prominent bathymetric high (the Kashinosaki Knoll; Ike et al., 2008a), whereas Site C0012 is located near the crest of the knoll (Fig. F2).

The resulting data, which include LWD during Expedition 319, provide a great deal of new information on presubduction equivalents of the seismogenic zone (Figs. F3, F4) (Underwood et al., 2009). Core samples at Site C0011 were obtained by rotary core barrel (RCB) from 340 to 876 m core depth below seafloor (CSF) where the hole was abandoned because of failure of the drill bit. After jetting-in to ~70 meters below seafloor (mbsf), RCB coring at Site C0012 penetrated almost 23 m into igneous basement and recovered the sediment/basalt interface intact at 537.81 m CSF (Fig. F4). This incomplete coring program left major gaps in the stratigraphic coverage, particularly within intervals of the upper Shikoku Basin facies. Nevertheless, the merger of lithofacies and age-depth models shows how correlative units change from an expanded section at Site C0011 to a condensed section at Site C0012. The composite section also captures most of the important ingredients of basin evolution, including a previously unrecognized interval of late Miocene volcaniclastic sandstone designated the middle Shikoku Basin facies. An older (early–middle Miocene) turbidite sandstone/siltstone facies with mixed volcaniclastic-siliciclastic detrital provenance occurs in the lower Shikoku Basin; this unit may be broadly correlative with superficially similar Miocene turbidites on the western side of the basin (Underwood, 2007). The age of basal sediment (reddish-brown pelagic claystone) at Site C0012 is older than 18.9 Ma.

Geochemical analyses of pore fluids on top of the basement high show clear evidence of a seawater-like source (Underwood et al., 2009). The depth of the sulfate reduction zone is also anomalously deep at Site C0012. Chlorinity values increase toward basement because of hydration reactions in the sediment and diffusional exchange with basement fluids. In contrast to Site C0011, where chlorinity decreases with depth, the fluids are largely unchanged by the effects of focused flow and/or in situ dehydration reactions associated with rapid burial beneath the trench wedge and frontal accretionary prism. Thus, Site C0012 finally provides a reliable geochemical reference site, unaffected by subduction processes.