The Japanese Island arc system is flanked to the east by deep trenches, which are subduction boundaries of the Pacific and Philippine Sea plates (Fig. F1). The Nankai Trough is the subducting plate boundary between the Shikoku Basin and the southwest Japan arc (Eurasian plate). The Shikoku Basin is part of the Philippine Sea plate, which is subducting to the northwest under southwest Japan at a rate of 2-4 cm/yr (Karig and Angevine, 1986; Seno, 1977), slightly oblique to the plate margin. Active sediment accretion is presently taking place at the Nankai Trough.
The record of accretion extends landward to Shikoku Island, where older accretionary prism rocks are exposed (Figs. F2, F3). The Cretaceous and Tertiary Shimanto Belt is characterized by imbricated thrust slices of trench turbidites and mélanges composed of ocean-floor basalt, pelagic limestone, radiolarian chert, and hemipelagic shale intermixed with highly sheared scaly shale (Taira et al., 1988). The youngest part of the Shimanto Belt is early Miocene in age. The Shimanto Belt is interpreted as a direct ancient analog of the Nankai accretionary prism (Ohmori et al., 1997).
The subducting oceanic lithosphere of the Shikoku Basin has a history related to the rifting of the proto-Izu-Bonin arc (Taylor, 1992). The rifting started in the Oligocene and culminated in Shikoku Basin seafloor spreading that lasted until ~15 Ma. Three phases of seafloor spreading (Okino et al., 1994) separated the remnant arc, the Kyushu-Palau Ridge, from the active Izu-Bonin arc, creating the Shikoku Basin (Fig. F4).
By 15 Ma, most of the Japan Sea ocean floor had formed (Jolivet et al., 1994; Otofuji, 1996). In southwest Japan, widespread igneous activity took place at 17-12 Ma within the forearc, close to the trench (Kano et al., 1991) (Fig. F2). This episode, including high-Mg andesite emplacement, is generally interpreted as reflecting injection of hot asthenosphere and initial subduction of the young Shikoku Basin seafloor (Takahashi, 1999). From 15 to 10 Ma, the collision of the Izu-Bonin arc against Honshu was apparently not vigorous, but was persistent enough to maintain sediment supply to the collisional trough (Aoike, 1999).
From 8 to 6 Ma, a fold belt developed on the Japan Sea side of southwest Honshu (Ito and Nagasaki, 1997). Fold axis trends are predominantly northeast-southwest, indicating northwest-southeast compression. Widely distributed volcanic activity, starting at ~8 Ma in southern Kyushu and by 6 Ma in southwest Japan, suggests the establishment of a volcanic front and a deeply penetrating subducting slab (Kamata and Kodama, 1994). In the Izu Collision Zone, the accretion of the Tanzawa massif (part of the volcanic front of the Izu-Bonin arc; Fig. F4) seems to have started at ~8 Ma, with the main phase of collision at ~6-5 Ma (Niitsuma, 1989). Subduction of the Shikoku Basin then generated the Nankai Trough, formed a frontal accretionary prism to the southwest Japan forearc (Nankai Trough accretionary prism; Taira et al., 1992; Le Pichon et al., 1987), and shaped forearc basins (Okamura et al., 1987; Sugiyama, 1994).
The Nankai Trough has historically generated earthquakes larger than magnitude (M) 8 at intervals of ~180 yr (Ando, 1975, 1991) (Fig. F5). The most recent one occurred in 1946 and ruptured off shore of Kii Peninsula and Shikoku. Sites 1175 and 1176 are located close to the seaward limit of this rupture zone. Prior to Global Positioning System (GPS) data, Hyndman et al. (1995, 1997) inferred full coupling of the plate boundary based on leveling data. The nationwide permanent GPS network of the Geographical Survey Institute of Japan revealed that the forearc region of Nankai Trough is moving to the west-northwest at 2-5 cm/yr, confirming a full coupling of the plate boundary during the current interseismic period (Le Pichon et al., 1998; Mazzotti et al., 2000). GPS data were also analyzed to show the nature of interplate coseismic slip of the Nankai seismogenic zone (Sagiya and Thatcher, 1999).
Hyndman et al. (1995, 1997) found a correlation between the observed extent of a seismogenic zone and the thermal regime, especially in the Nankai Trough subduction zone. The updip limit of the seismogenic zone was found to coincide generally with the estimated location of the 150°C isotherm, and the downdip limit coincides with the 350°-450°C isotherm. They proposed that the updip limit coincides with the completion of the smectite-illite transition and the downdip limit coincides with the initiation of ductile deformation of quartz or serpentinization of peridotite mantle.
The geological and geophysical database for the Nankai prism is outstanding. Existing data sets include high-quality industry and academic seismic reflection/refraction data (Aoki et al., 1982; Karig, 1986; Moore et al., 1990, 1991; Stoffa et al., 1992; Park et al., 1999, 2000; Kodaira et al., 2000), complete swath bathymetry, side-scan sonar coverage (Le Pichon et al., 1987; Ashi and Taira, 1992; Taira and Ashi, 1993; Okino and Kato, 1995; Tokuyama et al., 1999), heat flow analyses (Kinoshita and Yamano, 1986; Ashi and Taira, 1993), and three Deep Sea Drilling Project (DSDP)/Ocean Drilling Program (ODP) legs (Legs 31, 87, and 131). Newly acquired 3-D seismic data (Ewing 9907/9908) (Bangs et al., 1999; Moore et al., 1999) were used to locate some of the proposed sites (Fig. F6).
The seismic data provide excellent images of the décollement, proto-thrust zone (PTZ), and various structural domains landward of the frontal thrust that guided our choice of drilling targets, and the well-constrained seismic velocities provide the basis for models of dewatering. The swath bathymetry and side-scan data reveal surficial features that further constrained drill hole locations. One of the prominent topographic features in the vicinity of the Leg 190 sites is an embayment of the trench landward slope. Yamazaki and Okamura (1989) interpreted this embayment as an indentation caused by the collision of seamounts with the prism. Recent seismic reflection work and ocean-bottom seismometer experiments on crustal structure support this interpretation (Park et al., 1999; Kodaira, 2000).
The well-resolved seismic profiles demonstrate several characteristic structural subdivisions across the accretionary prism. Based on 3-D multichannel seismic data obtained by the Ewing 9907/9908 cruise (Fig. F6; also see the "Data Report: Structural Setting of the Leg 190 Muroto Transect" chapter), the accretionary prism along the Muroto Transect can be divided into several tectonic domains from the trench landward (Fig. F7): Nankai Trough axis zone, protothrust zone (PTZ), imbricate thrust zone (ITZ), frontal out-of-sequence thrust (OOST) zone, large thrust-slice (LTS) zone, and landward-dipping reflector (LDR) zone.
Results from Legs 87 (Site 582) and 131 (Site 808) indicate that the stratigraphy of the trench is composed of the following lithologic units in descending order: trench turbidites (Holocene-Pleistocene), turbidite-hemipelagite transition (Pleistocene), hemipelagite with tephra layers (lower Pleistocene-upper Pliocene), massive hemipelagite (mid-Pliocene to middle Miocene), acidic volcaniclastics (15 Ma), and pillow basalts (16 Ma) (Fig. F8). The trench turbidite unit, supplied mostly through an axial transport system from a source region situated in the Izu collision zone mountain ranges, shows a mixture of volcanic, sedimentary, and metamorphic provenances (Taira and Niitsuma, 1986; Underwood et al., 1993). The lower part of the turbidite unit exhibits a finer-grained outer trench turbidite facies in which paleocurrent directions indicate deflected turbidity currents with a transport direction from the outer trench toward the inner trench (Pickering et al., 1993). The basal acidic volcaniclastic deposit was interpreted as recording extraordinarily large felsic igneous activity of the Outer Zone of southwest Japan during the middle Miocene (Taira, Hill, Firth, et al., 1991).
Site 1173 (ENT-01A) is a reference site drilled to basement seaward of the trench axis to provide baseline physical properties and fluid chemistry measurements (Fig. F9). An additional unit is recognized within the surrounding Shikoku Basin sequence that is not present in the local trench stratigraphy near Site 1173. This unit is stratigraphically below the drilled Shikoku Basin hemipelagic units and is characterized by a well-stratified sequence ~0.7 s thick. This section may correlate with the Miocene turbidite unit identified along the Ashizuri Transect and recovered at Site 1177 within the lower Shikoku Basin facies (hereafter called the Pliocene-Miocene Turbidite Unit) (Figs. F8, F10).
This area represents a zone of incipient deformation and initial development of the décollement within the massive hemipelagic unit. Above the décollement, the sediment thickness increases landward, probably because of tectonic deformation with the development of small faults and ductile strain as envisaged by Morgan and Karig (1995a, 1995b).
Site 1174 (ENT-03A), located in the PTZ, sampled a zone of incipient deformation and possible fluid flow (Fig. F9). This site penetrated into the subducting sediment section all the way to basement. A high priority was to sample pore fluids in detail across the protothrust, décollement, and the underthrust sediments.
Landward of the PTZ, a zone of well-developed seaward-vergent imbricate thrusts can be recognized. The thrusts are sigmoidal in cross section with a mean dip of ~30° and a typical thrust spacing of 0.5 km. The frontal thrust forms the seaward edge of the ITZ. At Sites 583 and 808 we cored the frontal part of the imbricate thrust zone.
Site 583 is situated on the hanging wall of the frontal thrust. Although drilling failed to penetrate the décollement zone, good quality physical properties measurements were obtained from all of the holes, providing evidence that sediments dewater under tectonic stresses as they are accreted (Bray and Karig, 1988). The pore-water concentration depth profiles from these sites are far from being detailed enough to provide insight into the nature of fluid flow at this segment of the Nankai Trough (Kastner et al., 1993). The significant geochemical findings were that organic-fueled diagenesis is intense and that methane concentrations and the C1/C2 ratios abruptly decrease at ~600 mbsf. Interestingly, similar abrupt decreases were observed at the décollement zone at Site 808. Fluid flow from a deep-seated source could explain these observations.
Site 808 (Figs. F8, F9), which penetrated the prism and reached oceanic basement at 1290 mbsf, was particularly successful in terms of physical properties and structural geology measurements because of relatively high core recovery and also because the sediments yielded consistently high-quality paleomagnetic data (Taira et al., 1991, 1992). These data allowed individual core sections and, in some cases, individual structural samples to be oriented relative to the present geographic coordinates. Physical properties generally varied smoothly downhole, except for sharp discontinuities across the frontal thrust and décollement zones. Discrete structures were concentrated in the vicinity of the fault zones as well as in several horizons above the décollement zone.
Pore waters were recovered throughout the section at Site 808, including the frontal thrust, décollement zone, and underthrust package. Depth profiles for chemical concentrations and isotopic ratios (particularly D, O, Sr, and He) do not indicate active fluid flow along the décollement or along the frontal thrust. They may, however, indicate a component of lateral fluid flow (1) below the décollement at the approximate depth of the minimum in Cl concentration (~1100 mbsf) and (2) above the décollement along a horizon marking the lithologic boundary between the volcanic-rich and -poor members of the Shikoku Basin sediments (~820 mbsf). Pore-water freshening may also result from in situ diagenetic reactions, such as thermally driven smectite dehydration. Detailed two dimensional numerical models of fluid flow and solute transport, which incorporate fluid sources from compaction and smectite dehydration, show that measured smectite abundances at Site 808 are insufficient to account for observed freshening, and episodic, focused fluid flow is necessary to match both measured fluid expulsion rates at the seafloor and observed downhole chlorinity values at Site 808 (Saffer and Bekins, 1998). Cores recovered from Site 808 also revealed that fractures within the décollement zone have not been mineralized. This contrasts with observations at Barbados and Peru where the major tectonic structures have been mineralized, implying continuous or episodic fluid flow.
About 20 km landward from the deformation front, the imbricate thrust packages are overthrust by a younger generation fault system. Because this fault system cuts the preexisting sequence of imbricate thrusts, it is called an OOST zone. Important and significant deformation also appears within the underthrust Shikoku Basin hemipelagite. The hemipelagic unit seems to be tectonically thickened, probably as a result of duplexing.
Landward of the frontal OOST is a zone characterized by the development of at least four distinctive out-of-sequence thrusts that separate tectonic slices of either previously imbricated packages or relatively coherent sedimentary sequences. The coherent slices are composed of ~0.7-s-thick (maximum) stratified layers that closely resemble the Pliocene-Miocene Turbidite Unit recognized in depressions in the Shikoku Basin. Slope sediment in this zone shows landward tilting suggesting recent active uplift. Bottom-simulating reflectors (BSRs) are weakly and patchily developed in this zone.
Site 1175 (ENT-07A) penetrated the slope sediments that cover the LTSZ. Investigation of the age and lithologic characteristics provided information on the history of accretion and deformation of the prism (Fig. F11).
Site 1176 (ENT-06A) was designed to penetrate an OOST, to sample and investigate the lithology, deformation, physical properties, and potential fluid from the seismogenic portion of the accretionary complex (Fig. F11). Difficult drilling conditions, however, led to the site being abandoned above the OOST.
Landward-dipping, semicontinuous strong reflectors characterize this zone. Relatively coherent slope sediments cover the sediments exhibiting landward-dipping reflectors (LDRs). This zone appears to be divided into several discrete packages by thrust faults. A BSR is well developed throughout this zone and diminishes abruptly at the boundary between this zone and the LTSZ. Site 1178 penetrated the slope sediments and the sediments characterized by the LDRs (Fig. F11).
The structural domains described above show variation along the strike of the prism. Along two parallel transects separated by ~100 km, marked differences in prism architecture and structure are evident. The Ashizuri Transect, which includes Leg 87 sites (Figs. F6, F8, F10), displays a well-developed PTZ, containing a series of subparallel dipping discontinuities of unknown origin. These features are not evident within the Muroto Transect PTZ (Fig. F9). Differences in prism taper and seismic character of the décollement along the two transects suggest that the mechanical behavior of the prism differs along strike and that this variability may result from significant differences in mechanical properties and/or pore pressures at the two locations.
Site 1177 is a seaward reference site in the Ashizuri Transect and was designed to establish a baseline of stratigraphic, geochemical, and physical properties for comparison to sites previously drilled during Legs 31 and 87 (Sites 297, 298, 582, and 583).