*, of 0.42 below the décollement zone at the latter site (Screaton et al., 2002):
A terrigenous sediment-dominated subduction zone, such as the Nankai margin, should show horizontal shortening within the accretionary prism. The overall geometry of thrust faulting along this margin is apparent in seismic reflection profiles (Bangs et al., 2004; Gulick et al., 2004; Moore et al., 1990, 2001a; Gulick and Bangs, this volume); our view of this primary tectonic style is enhanced by detailed consideration of the information at seismic, logging, and core scales. Working from a seaward to landward direction, seismic profile and borehole data show a transition from extensional or basinal to a compressional stress regime.
The evidence for extensional deformation in the incoming sedimentary section of the Shikoku Basin is apparent at core, logging, and seismic profile scales. At Site 1173, fractures, as seen in cores and logs, are oriented at high angles, show normal displacement where measurable, and have variable strike orientation (Fig. F3) (Shipboard Scientific Party, 2001a, 2002b). Seismic data in this area reveal a pattern of normal faults within the incoming hemipelagic sediment with strikes statistically parallel to the margin and parallel to incoming highs in the oceanic crust (Fig. F4) (Heffernan et al., 2004). The patterns from the seismic data are more systematic than those observed in the borehole resistivity images, perhaps because the seismic data sample a larger sediment volume. To explain the extensional pattern of faulting, Heffernan et al. (2004) favored differential compaction both downdip of the subducting plate and off the flanks of the horsts in the oceanic crust.
The transition from a basinal or extensional regime to a compressional state of stress is obviously manifest at the PTZ, as seen in the seismic data where the seafloor begins to be subtly uplifted ~2 km seaward of the surface trace of the protothrust, indicating initiation of horizontal shortening (Fig. F5). The seaward extension of the décollement reflector also remains bright at this point, suggesting it may be an active surface of overpressuring and perhaps slip (Bangs et al., 2004). Cores from Site 1174 show well-developed deformation bands that indicate a horizontal maximum principal stress parallel to the direction of relative plate motion (Ujiie et al., 2004). A similar stress orientation was observed from analysis of numerous small-scale faults at Site 808 (Leg 131) (Lallemant et al., 1993). Borehole resistivity images at Site 808 show breakouts that indicate the maximum principal stress is oriented at ~N40°W, approximately parallel to the plate convergence direction (Fig. F6) (McNeill et al., 2004). Anisotropic orientation of formation resistivity derived from core sample measurements at Site 1174 indicates consistency with the direction of the plate convergence above the décollement zone (Henry et al., 2003). Thus, the geometry of the large-scale frontal thrusts plus the above-cited studies of cores and logs all indicate a maximum principal stress oriented parallel to the convergence direction (Seno et al., 1993).
Several studies concentrated on the permeability of the Nankai prism sediments (Gamage and Screaton, this volume; Adatia and Maltman, this volume; Bourlange et al., this volume). Other laboratory studies quantified the stress state of the prism sediments (Kopf and Brown, 2003) and the consolidation of the clay-rich sedimentary sections (Karig and Ask, 2003). Sunderland and Morgan (this volume) documented microstructural variations in sediments of the prism toe.
The décollement zone separates the little-deformed underthrust material from the folded and thrusted offscraped and underplated rocks (Fig. F2). The décollement is the plate boundary and, therefore, how it develops and evolves is of significant interest. Near the deformation front of the accretionary prism, the décollement zone is characterized by a strong negative polarity seismic reflection (Bangs et al., 2004; Moore and Shipley, 1993). The décollement zone can be traced seaward as a bright reflector beyond the protothrust zone (Fig. F5). It locally bifurcates into several apparent splays near its seawardmost extent (Adamson, 2004). At Site 1173 the décollement zone, as observed at Sites 1174 and 808, stratigraphically projects into the lower Shikoku Basin deposits at ~400 m subbottom. No striking lithologic change occurs at this depth (Shipboard Scientific Party, 2001a). However, the onset of the smectite to illite phase transition does occur at this level (Steurer and Underwood, this volume a).
At Site 1173, physical property data from both logging and core measurements show minor changes in density and porosity at the depth of the projected décollement (Shipboard Scientific Party, 2001a, 2002b). Compressional velocities from cores and processed LWD data both show a reversal in the normally increasing trend with depth at the interval to which the décollement is projected at Site 1173 (Shipboard Scientific Party, 2001a; Yoneshima et al., 2003). This decrease in velocity may represent an increase in the elastic compressibility and a decrease in the rigidity of the sediment at this depth.
Bangs and Gulick (this volume) inverted the seismic reflection data for impedance and inferred porosity variations along one seismic line of the three-dimensional (3-D) seismic volume (Moore et al., 2001a). Based on the observed porosity variations, they argue that consolidation of the uppermost lower Shikoku Basin strata forms a barrier blocking fluid flow from the sediment section below that is consolidating because of loading by the trench turbidite fill. The barrier lies just above the projected level of the décollement, and they believe that higher-porosity, underconsolidated, and overpressured sediment below forms a surface of potential décollement propagation (Fig. F7).
Analysis of the microfabric of the décollement zone (Ujiie et al., 2003) indicates that it initiated in an interval of porous clayey sediment characterized by cementation due to intergranular bonding of authigenic clays. According to these authors, microstructures suggest (1) an initial phase of compactive deformation that involves the destruction of porous cement structure, probably caused by fluid pressure fluctuation, and (2) a later compactive deformation characterized by clay particle rotation and porosity collapse along slip surfaces, resulting in preferred orientation of clay particles. Studies of consolidation characteristics of the underthrust sediment (Morgan and Ask, 2004) also indicate a phase of early cementation and strengthening that may contribute to their mechanical coherency and propensity for underthrusting.
The role of fluid transport and/or high fluid pressure pervades concepts of décollement evolution (e.g., Saffer, 2003). Comparison of compaction curves between the reference Site 1173 and Site 808 at the frontal thrust suggests a fluid pressure, *, of 0.42 below the décollement zone at the latter site (Screaton et al., 2002):
LWD resistivity data (Bourlange et al., 2003) and porosity data in the décollement zone suggest higher fluid pressures and high permeabilities in this fractured interval. Thus, the décollement zone may represent a preferential interval of fluid transport from higher pressure sources.
Based on geochemical data, the décollement zone has been considered as a locus of episodic fluid flow (Kastner et al., 1993; Spivack et al., 2002). However, recent analyses of clay mineral composition and associated modeling suggest that the large negative chloride anomaly in the vicinity of the Nankai décollement zone may be explained by smectite dehydration (Brown et al., 2001; Henry and Bourlange, 2004). The explanation of the low chloride anomaly by clay dehydration does not preclude lateral flow or overpressure within the décollement zone but simply eliminates the use of the chloride anomaly as an indicator of flow.
The preponderance of evidence suggests that the décollement zone develops because of initial overpressuring and perhaps extensional fracturing succeeded by a shear, an idea that emerged from the Leg 131 results (Karig and Morgan, 1994; Morgan and Karig, 1995a). Propagation of the décollement zone in a relatively homogeneous sedimentary section is puzzling. However, the development of a hydrologically resistive barrier caused by dewatering of the uppermost lower Shikoku Basin unit or to intrinsically low vertical permeability provides a reasonable explanation.
The evolution of the décollement zone can be evaluated using information available from the two boreholes that penetrated it and from inferences gained through study of the 3-D seismic volume (Fig. F2). Both Sites 1174 and 808 indicate a significant downsection porosity inversion due to underthrusting of the lower Shikoku Basin deposits at a rate faster than they can dewater, thus resulting in overpressures. This inversion in physical properties produces a strong negative polarity reflection beneath Sites 808 and 1174 (Moore and Shipley, 1993; Bangs et al., 2004). The amplitude of the negative reflection declines in a landward direction so that the reflection between the offscraped and underthrust sediment sections diminishes to very low amplitude. The décollement steps down to the oceanic crust ~40 km landward of the deformation front, thus underplating the remaining sediment on the subducting plate (Bangs et al., 2004; Moore et al., 2001a). The declining amplitude is consistent with dewatering and consolidation of the underthrust section. In addition to escaping through intergranular permeability, the fracture permeability of the thrust faults overlying the décollement zone may act as fluid escape paths (Gulick et al., 2004).
In 2001, multilevel ACORKs were installed in Holes 1173B and 808I during Leg 196. These ACORKs were configured to provide long-term (10 yr or longer) monitoring of in situ fluid pressures at five depth intervals in Hole 1173B, including basement, and six depth intervals in Hole 808I, including the décollement zone. Details of the installations are described in Mikada, Becker, Moore, Klaus, et al. (2002). A significant installation flaw in the ACORK set at 922 meters below seafloor (mbsf) in Hole 808I resulted from the inability to drill the final ~37 m above the décollement zone. This circumstance precluded installation of a bridge plug to make the final seal of the deepest monitoring interval just above the décollement zone.
Since Leg 196, the ACORKs have been revisited annually with ships and submersibles from the Japan Marine Science and Technology Center (JAMSTEC), including the remotely operated vehicle (ROV) Kaiko in 2002 and 2003 and the submersible Shinkai 6500 in 2004. Kaiko operations in 2002 (Mikada et al., 2003) showed that the majority of sampling valves had somehow worked open during the Leg 196 installation operations a year earlier (perhaps caused by current-induced vibrations); the Kaiko was used to close the valves in 2002, but the recovered data were not useful scientifically. The Kaiko revisit in 2003 was intended to collect a year of data and install a bridge plug within the top of casing in Hole 808I, but these objectives were left unfulfilled when the Kaiko vehicle was catastrophically lost at the end of the first dive. In late spring of 2004, two dives with the Shinkai 6500 collected the first useful pressure data from both ACORKs. However, the limited bottom time did not allow installation of the bridge plug in Hole 808I, which must await a future submersible operation.
Preliminary inspection of the data collected in 2004 shows that, despite the lack of full seal in the décollement interval, the ACORK in Hole 808I captured the signal of a July 2003 solitary pressure wave that seems to have originated in the décollement zone (Fig. F8). Interpretation of this event as well as determination of background in situ fluid pressures is ongoing as this synthesis is being written (Davis et al., submitted [N1]). At this time it should be cautioned that the signal is that of a pressure transient; it does not necessarily imply any significant fluid flow. The fact that signals like these are being recorded demonstrates the long-term usefulness of the ACORK installations despite any original installation flaws.
Core samples obtained at Sites 1173, 1174, 1176, and 1177 were analyzed for microbiology using polymerase chain reaction (PCR) amplification, genetic analyses, and other standard techniques. The counts of bacteria are close to predicted values for deep ocean sediment at Site 1177 and at the shallower depths of Sites 1173 and 1174. However, the total counts of bacteria for depths deeper than 400–500 mbsf at Sites 1173 and 1174 show very low numbers. Sequences of bacteria within the core samples were similar to others retrieved from marine sediment and other anoxic habitats, and so probably represent important indigenous bacteria (Toffin et al., 2004a, 2004b; Webster et al., 2003, 2004; Zink et al., 2003). Analysis suggests limited methanogen diversity with only three gene clusters identified within the Methanosarcinales and Methanobacteriales. The cultivated members of the Methanobacteriales and some of the Methanosarcinales can use H2 and CO2 to produce methane (Newberry et al., 2004). These counts might be influenced by the temperature gradients in the sediments at Sites 1173 and 1174. Members of the genus Thermococcus, which in general live in 48°–73°C environments, are found at Site 1176, where in situ temperature ranges 1°–12°C. The Thermococcus populations thus are presumably not metabolically active in situ. They could represent either inactive relict cells introduced by past hydrothermal activity or cells that were recently introduced through interstitial fluid flow (Kormas et al., 2003).
The Shikoku Basin sedimentary section dewaters as it underthrusts the Nankai accretionary prism. Legs 190 and 196 drilling documented significant along- and across-strike variations in the physical, geochemical, and sedimentologic properties of the subducting section that influence its compaction and dewatering (e.g., Steurer and Underwood, this volume b; Hoffman and Tobin, this volume; Goldberg et al., this volume).
The upper Shikoku Basin facies (Pliocene–Quaternary) drilled at Sites 1173, 1174, and 1177 was deposited by hemipelagic settling and ash falls; the lower Shikoku Basin facies (Pliocene–middle Miocene) is also a dominantly hemipelagic sequence but lacks recognizable ash layers (Shipboard Scientific Party, 2001c). The lower Shikoku Basin section at Site 1177 drilled along the Ashizuri Transect contains abundant siliciclastic sand layers with rare beds of gravel and mudstone-clast conglomerate, whereas the lower Shikoku Basin section off Muroto (Sites 1173 and 1174) does not contain turbidite layers.
Lower–upper Miocene turbidite beds of the lower Shikoku Basin section at Site 1177 were derived from a continental source with exposed plutonic and volcanic rocks, possibly the inner zone of southwest Japan (Fergusson, this volume). Turbidite sand was transported across the trench and at least 600 km out onto the Shikoku Basin plain. The turbidites were interbedded with hemipelagic layers in topographic lows, while topographic highs did not receive any turbidity current deposits and were thus dominated by hemipelagic deposits. Similar compositions are present in the thick sand packages of the lower–upper Miocene accreted trench deposits at Site 1178. The thrust slices penetrated at Site 1178 are 400–600 m thick, indicating that the trench was accumulating large amounts of terrigenous sediment at that time.
Clay mineral data from Sites 1173, 1174, and 1177 show downhole increases in smectite percentage within the Shikoku Basin deposits. Part of this increase is caused by changes in detrital influx, but diagenetic reactions also affect these hemipelagic sequences. Steurer and Underwood (this volume a) conclude that recent episodes of burial near the toe of the Nankai accretionary prism (i.e., by trench-axis sedimentation, tectonic thickening, and frontal thrusting) may have been too fast for the smectite-illite reaction to keep pace. The sedimentation rate of smectite in the Shikoku Basin and Nankai Trough was higher during the Miocene and decreased progressively through the Pliocene and Quaternary. In situ alteration of disseminated volcanic glass added even more authigenic smectite to the clay assemblage as burial depths and temperatures gradually increased. The geothermal gradient at Site 1177 is substantially lower than at Sites 1173 and 1174; consequently, volcanic ash alters to smectite in lower Shikoku Basin deposits but smectite-illite diagenesis has not started along the Ashizuri Transect (Moore et al., 2001b; Wilson et al., this volume; Underwood et al., this volume). The absolute abundance of smectite in mudstone from Site 1177 is sufficient (30–60 wt%) to influence the strata's shear strength and hydrogeology as it subducts along the Ashizuri Transect (Steurer and Underwood, this volume a).
Deposition of the upper part of the Miocene turbidite succession at Site 1177 was synchronous with deposition of the uplifted and deformed axial and outer trench wedge facies at Site 1178 (Fig. F1) (Shipboard Scientific Party, 2001c). Sand beds at both sites have low quartz content but contain a significant component of sedimentary and metamorphic rock fragments (Fergusson, this volume) that were derived mainly from older accretionary complexes (e.g., Sambagawa and Shimanto Belts) of southwest Japan (Outer Belt) (Fig. F9). These sand beds contrast with those of the modern trench deposits that are derived from the Izu collision zone of eastern Honshu (Taira and Niitsuma, 1986).
Trench slope basin sand beds drilled at Sites 1175 and 1176 contain abundant sedimentary lithic fragments as well as radiolarian chert fragments, indicating that they were derived from erosion of the Shimanto accretionary complex on Shikoku Island (Fergusson, this volume). The eroded material was delivered to the slope basins via submarine canyons that incised the landward trench slope southeast of Shikoku Island (Underwood et al., 2003; Underwood and Steurer, this volume).
Organic geochemical studies have been performed on the clay-rich slope sediments by Yamamoto (this volume) and Suzuki et al. (this volume).
Although definitive proof is lacking, the lines of evidence listed by Taira (2001) indicate that the present phase of north-northwest–trending subduction of the Philippine Sea plate started at ~8 Ma. As discussed above, topographic lows in the Shikoku Basin were filled by a turbidite sequence derived from the Outer Belt of Shikoku (Fig. F9). In contrast, around the Kinan Seamount chain, the higher seamount peaks stood above the cloud of fine-grained turbidite sedimentation, although topographic highs with lower relief were also buried by turbidites and fine-grained hemipelagic sediments. Accretion of these turbidite units at the Muroto Transect by 4 Ma produced an accretionary complex mainly composed of turbidites derived from the Outer Belt (Site 1178). By 2 Ma, further accretion of these turbidite units (Sites 1175 and 1176) and indentation of the accretionary prism by a seamount reshaped the morphology of the landward slope. At the same time, in the Izu collision zone that marks the eastern end of the Nankai Trough, active tectonism produced mountain-to-trough sediment transport, leading to domination by axial channel depositional systems in the Nankai Trough. From ~1 Ma to the present (Fig. F9), accretion of the axial trench wedge and underthrusting of the hemipelagic cap of the Kinan Seamounts in the embayment produced by seamount subduction dominated the present-day tectono-sedimentary framework of the toe part of the Muroto Transect (Sites 808, 1174, and 1173).
One of the more significant discoveries during Leg 190 is the age of accreted trench turbidites at Sites 1175 and 1176. Underwood et al. (2003) interpret contact relations between trench slope sediments and underlying accreted trench deposits to indicate that the youngest accreted turbidites are only ~1 Ma. These sites are located 40 km landward of the frontal thrust. If we assume steady-state seaward growth of the prism over the past 1 m.y., the lateral growth rate is 40 km/m.y. Even if the actual slope-to-prism contact is deeper (and thus somewhat older), the rate of tectonic accretion remains impressive. In comparison, the turbidite-rich Middle America accretionary prism off the coast of Mexico has grown ~23 km in width during the past 10 m.y. (Moore et al., 1982) and the eastern Aleutian accretionary prism has grown 20 km in 3 m.y. (von Huene et al., 1998).
Analyses of core and logging data collected in a series of drill holes in the sediments of the Shikoku Basin, Nankai Trough, and flanking accretionary wedge, interpreted along with seismic reflection data, reveal details of deformation and fluid flow within the Nankai subduction zone. LWD acquired spectacular physical property data and images of unconsolidated sediments in spite of unstable hole conditions. Two holes along the Muroto Transect were instrumented with ACORKs for long-term monitoring of fluid pressure in and around the décollement zone.
Major results include the following:
The results of Legs 190 and 196 clearly demonstrate that fluid has played a major role in the formation and evolution of the Nankai accretionary prism. Because deep-sourced fluids are critical to understanding seismogenic processes, it is important to obtain results from long-term fluid pressure measurements. Further understanding the Nankai seismogenic zone requires deeper drilling into faults active during past great earthquakes.
