Results from Leg 190 help define several important spatial and temporal differences in the stratigraphy of the Shikoku Basin. In turn, these stratigraphic variations influence patterns of deformation within the Nankai Subduction System. Beginning at the stratigraphic base, a volcaniclastic-rich facies overlies basalt basement at Site 1177 (Ashizuri Transect) (Figs. F8A, F10). This facies differs from basal strata at Sites 808 and 1173 (Muroto Transect) in three important ways. First, the oldest strata at Site 1177 are early Miocene in age, rather than middle Miocene. Second, they contain considerable amounts of siliciclastic silt and sand in addition to volcanic sand and ash. Third, the hemipelagic mudstones contain higher percentages of smectite. The lower Miocene siliciclastic and volcaniclastic beds probably were eroded from the Japan Island arc and the Kyushu-Palau remnant arc. The age variation in basal sediment from west to east can be explained by proximity to the axis of the spreading ridge of Shikoku Basin. The ridge was active between 26 and 15 Ma, and seamount eruptions along the Kinan chain may have continued to 13-12 Ma. Temporal correlation of the Miocene ash beds remains uncertain. Given the existing biostratigraphic resolution, volcaniclastic beds along the Ashizuri Transect appear to be older than the thick rhyolitic tuff deposits that were cored at Site 808, so they may have been erupted from a different source.
Cores from Site 1177 include a package of lower(?) to upper Miocene siliciclastic turbidites with abundant woody organic matter. Hemipelagic mudstone interbeds are enriched locally in expandable clay minerals. Deposition of correlative turbidites at DSDP Site 297 also began during the Miocene but continued into the Pliocene. The Miocene turbidites at Site 1177 were derived from a relatively large land mass, most likely southern Japan, and the dispersal system spread terrigenous sediment over a broad area of Shikoku Basin. During that same time interval, the seaward part of the Muroto Transect area sat above the basement high formed by the Kinan Seamounts. Higher seafloor relief evidently prevented the upslope deposition of turbidites.
Fine-grained hemipelagic deposits of Shikoku Basin display an unexpected characteristic of unusually high magnetic susceptibility. Magnetic susceptibility increases even within mudstone intervals between the Miocene turbidite packets at Site 1177. The mineralogic cause of this magnetic response in Shikoku Basin strata remains unknown.
A facies change from the lower to upper Shikoku Basin is defined at all sites by the absence to presence of ash beds containing recognizable volcanic glass shards. Most of the Pliocene volcanic ash beds probably were derived from the Honshu-Kyushu arc. Particle size, chemical composition, temperature, depth of burial, and time affect ash alteration and preservation. Thus, this unit boundary is time transgressive and sensitive to regional and temporal changes in the margin's thermal structure.
Shikoku Basin strata experienced diachronous burial during the Pliocene and Quaternary beneath an upward-coarsening and upward-thickening wedge of trench turbidites. The Nankai Trench wedge is thinner in the Muroto Transect area (above the basement high), but individual turbidites tend to be thinner and finer grained toward the southwest (DSDP Site 582). Quaternary sedimentation rates at Sites 1173 and 1174 were 600 and 760 m/m.y., respectively, more than one order of magnitude higher than the rate for Miocene turbidites at Site 1177 (35.5 m/m.y.). The main reasons for the higher rates of trench sedimentation include erosion from rapidly uplifting volcanic and metasedimentary terranes in the Honshu-Izu collision zone of central Japan and confinement of most sediment gravity flows to an axial dispersal system.
Sites 1175, 1176 and 1178 are among a handful of DSDP and ODP sites to have penetrated completely through slope sediments into an underlying accretionary prism (Figs. F8B, F11). They are, therefore, important for refining models of trench-slope evolution. The upper facies unit at all three sites fits into conceptual models of a trench-slope basin or slope apron, at least for locations in isolation from influx of coarse terrigenous sediment. Resedimentation of muddy material by submarine slumps and mudflows, perhaps triggered by seismogenic activity, contributed to high rates of sedimentation (200-3000 m/m.y.).
One of the unexpected discoveries of Leg 190 is the lack of change in lithofacies across the interpreted contact between trench-slope basin and accretionary prism, as deduced from seismic reflection data. Carbonate content within the inferred accretionary prism is depleted relative to slope sediments, which is consistent with deposition of trench sediment below the CCD. Strata throughout the slope to prism transition at Sites 1175 and 1176 include abundant sand turbidites and muddy gravel. One way to explain this paradox would be through frontal offscraping of a trench fan that had been fed by a transverse sediment conduit (e.g., throughgoing submarine canyon). Evidently, coarse siliciclastic material was diverted away from the slope basin relatively early in its development, as canyons were rerouted upslope by uplift and tilting of the prism. This change in the sediment-dispersal network resulted in an overall upward-thinning and upward-fining megasequence within the slope basin. Regardless of the ambiguity in the exact position of its basal unconformity, the slope basin at Sites 1175 and 1176 must have developed within the last 2 m.y. This is a major revision to previous estimates of the accretionary wedge growth and shows that the outer 40 km of the accretionary prism has been added to the Nankai margin over only 2 m.y. This finding has important ramifications for structural and hydrologic models of this margin.
Interpretations of results from Site 1178 are also complicated by biostratigraphy and structural deformation. A fundamental lithotectonic boundary exists between a mud-dominated slope apron and sandy accretionary prism at a major facies break (~200 mbsf). This contact is probably a slump surface. The underlying prism strata probably were accreted starting in the late Miocene, prior to the frontal accretion at Sites 1175 and 1176. It is clear that strata below 200 mbsf display many effects of intense deformation (e.g., fractures, incipient cleavage, and steeply dipping beds). Their facies character also matches that of the axial trench wedge and the outer trench wedge, and there is good sedimentologic evidence for thrust repetition of the axial trench-wedge facies, with slices at 199-411 mbsf and below 564 mbsf. These distinguishing features of both sedimentology and structural geology have important implications for interpretation of kindred rock units in such ancient subduction complexes as the Shimanto Belt of southwest Japan.
Detailed bio- and magnetostratigraphic records were obtained at all sites to aid in correlation between sites. Paleomagnetic results for Site 1173 and 1174 show clear geomagnetic changes from the middle Miocene to Pleistocene (Fig. F34) and can be correlated with the polarity changes at Site 808. The basal age of the trench-wedge turbidites including the transition zone to the upper Shikoku Basin facies is estimated to be ~0.8 Ma. The boundary between the upper and lower Shikoku Basin at Sites 1173 and 1174 is interpreted to be 3-5 Ma. The age of the décollement zone at Site 1174 is estimated to be from 5.894 Ma (beginning of C3A Chron) to 6.935 Ma (termination of C3B Chron, which corresponds to the age of décollement zone at Site 808). At Site 1177 located in the Ashizuri Transect, the lower Shikoku Basin facies extends from the Pliocene to early Miocene, although it is difficult to identify the geomagnetic records because of poor core recovery (Fig. F34). Because Site 1177 was drilled without coring to 300.2 mbsf, correlations could not be made between Site 1177 and Site 582.
Pleistocene to Pliocene paleomagnetic data are similar at Sites 1175 and 1176 and document a few geomagnetic excursions within the Brunhes Chron.
At Sites 1173, 1174, 1177, and 808, the trench-wedge turbidites and the Shikoku Basin show contrasting sedimentation rates in the age-depth plots (Fig. F35). The sedimentation rates of the trench turbidites within the transition zone of each site shows high values, with the highest at Site 808 (204 m/m.y. at Site 1173, 698 m/m.y. at Site 1174, and 842 m/m.y. at Site 808). In contrast, sedimentation rates are slower in the upper and lower Shikoku Basin facies. Rates for the upper Shikoku Basin facies are 77.82 m/m.y. at Site 1173, 78.46 m/m.y. at Site 1174, and 81.62 m/m.y. at Site 808. The sedimentation rate of the upper Shikoku Basin facies at Site 1177, however, is lower (40.37 m/m.y.). Sedimentation rates for the lower Shikoku Basin facies are lower than those of the upper Shikoku Basin facies. The lower Shikoku Basin facies rates are 27.40 m/m.y. at Site 1173, 35.22 m/m.y. at Site 1174, 28.27 m/m.y. at Site 1177, and 31.67 m/m.y. at Site 808.
Magnetic susceptibilities proved a valuable tool for correlation among sites (Fig. F8A) and helped constrain the correlation of the décollement and incipient décollement horizons at Sites 1173, 1174, 808, and 1177.
Leg 190 completed a transect of the basal décollement of the Nankai accretionary prism from an undeformed state at Site 1173 to the well-developed fault zone landward of the deformation front documented at Sites 1174 and 808 (Fig. F36).
At Site 1173, there is little evidence from the structural geology or physical properties for a protodécollement zone (i.e., incipient deformation indicative of a major fault). The stratigraphic equivalent to the Site 1174 décollement interval indicated in Figure F36 is based on correlation of core magnetic susceptibility (Fig. F8). This interval is part of a thicker domain of increased bedding dip but shows no localized increase in observed deformation. However, a marked downhole decrease in P-wave velocity and a slight porosity increase at the top of the interval (~389 mbsf) suggest that a subtle mechanical strength discontinuity could contribute to the localization of the décollement in this interval. Pore fluid chlorinity also shows a small low-chloride excursion above this interval and an abrupt transition to higher values at ~390 mbsf; however, there is no corresponding feature in the Sites 1174 or 808 chlorinity data. It is unknown, of course, whether in the future the décollement actually will propagate along this particular stratigraphic horizon to the position of Site 1173.
The hallmark of the décollement zone at Sites 808 and 1174 is intense brittle deformation, manifested as finely spaced fracturing that breaks the mudstone into millimeter- to centimeter-scale fragments (Fig. F19). The fragments have polished and slickenlined surfaces, showing complex and heterogeneous slip directions, but they do not exhibit obvious internal deformational structures at the core scale. At Site 1174, the upper limit of the décollement zone is marked by a sharp downward increase in the intensity of brecciation, although the lowermost prism section above exhibits distributed fracturing as well. Within the décollement zone, there is a downward increase in intensity of the brecciation, peaking in a 7-m-thick zone of fine comminution of the mudstone just above the very sharp base of the décollement zone (Fig. F20). Within the fault zone are several intervals up to tens of centimeters thick of unbroken mudstone, which are interpreted as intact blocks in a multistranded shear zone.
It is remarkable that the décollement zone at Site 1174 appears to be at least as well developed as it is at Site 808. It is thicker at Site 1174 than it is at Site 808 (32.6 vs. 19.2 m thick, respectively). It is also brecciated to a finer scale, despite the more landward—thus presumably more structurally evolved—position of Site 808. Differences in the observed structures could be explained by differences in core recovery; however, the greater thickness at Site 1174 could not. Notable at both sites is the complete absence of veins, alteration zones, or other evidence of past fluid-rock interaction specific to the décollement.
The development of the décollement and the strain discontinuity across it are clearly exhibited in the core physical properties data. At Site 1174, there is a sharp porosity increase and P-wave velocity decrease immediately below the structurally defined décollement. These same features are even more pronounced at Site 808. Site 808 exhibits evidence of a porosity minimum within the décollement, whereas no clear evidence for such a minimum exists at Site 1174. However, the most prominent feature at both sites is the discontinuity across the base of the zone crossing into the underthrust section. This discontinuity is likely due to a combination of undercompaction of the rapidly loaded underthrust section (e.g., Saffer et al., 2000) and enhanced tectonic compaction of the prism and décollement caused by the imposition of lateral tectonic stress (Morgan and Karig, 1995a).
In summary, the décollement beneath the toe of the Nankai accretionary prism develops from an unremarkable and homogeneous interval of hemipelagic mudstone into a 20- to 32-m-thick zone of intense brittle deformation, the base of which marks a boundary between the distinct physical/mechanical regimes of the prism and the underthrust section. The two drilling penetrations of the fault zone suggest an anastomosing system of discrete brittle shears similar to faults observed in mudrocks on land. Despite a major effort to detect localized fluid flow along the fault, there is no unambiguous evidence for flow of a chemically distinct fluid in the décollement zone along the transect defined by these three sites.
Correlation of the décollement horizon between Muroto and Ashizuri Transects imposes an intriguing question on the localization of the décollement in the lower Shikoku Basin mudstone. Although DSDP/ODP drilling has not penetrated the décollement at the Ashizuri Transect, a clear and continuous seismic reflector allows us to correlate the décollement horizon at the toe region with Site 1177 (Fig. F37). At Site 1177 this reflector is at 420 mbsf and coincides with the identical horizon of the décollement of the Muroto Transect based on chronological and magnetic susceptibility correlations. This raises the important question of why the décollement stays at the same stratigraphic horizon despite a major difference in the thickness of turbidites and the lithology and diagenesis of the Shikoku Basin sediments between the two transects. This question should be addressed by further shore-based study.
The shipboard geochemical data provide insight on the origin of fluids and the depth intervals and paths of possible recent fluid flow. In addition, abiogenic and microbially mediated diagenetic reactions that have modified fluid composition have been characterized and quantified.
The most interesting and pronounced feature of the pore fluid concentration-depth profiles in the Muroto Transect from Site 1173 through Site 1174 to Site 808 is the ~350-m-thick low-Cl zone within the lower Shikoku Basin unit (Fig. F38). It has a clear concentration minimum ~140 m below the décollement (or its equivalent at Site 1173). At Sites 1173 and 1174, this low-Cl zone decreases in intensity gradually upsection to the sediments overlying the upper Shikoku Basin facies. Additionally, the extent of Cl dilution relative to seawater Cl concentration systematically differs among the sites; it has evolved from 8%-9% at reference Site 1173 to 16%-17% at intermediate Site 1174 to 20%-21% at adjacent (<2 km) Site 808. Based on the residual smectite content of the sediment section at Site 808, some of the freshening may not be due to local smectite dehydration but could result from transport of freshened fluids from greater depth. It is important to note, however, that the original smectite concentration and clay-sized fraction are not known. The low-Cl concentrations most likely reflect some combination of (1) in situ clay dehydration and other reactions, (2) the transport of freshened water from dehydration reactions at greater depth, and (3) the uptake of Cl by deep-seated hydrous silicate reactions; for example, serpentine, chlorite, talc, or amphibole incorporate considerable amounts of Cl in their structure. These reactions occur at temperatures of >250° up to ~450°C. Thus, the broad low-Cl zone possibly carries a signal from Cl uptake by high-temperature reactions in the seismogenic zone. The relative contributions of these processes can be resolved by rigorous mass-balance calculations, modeling of the physical and chemical hydrology, shore-based measurements of Cl, Br, and F concentration, and Cl and Br isotope analyses.
The origin of slightly higher Cl concentration within the décollement zone observed at Sites 1174 and 808 is unclear.
Other potential fluid flow horizons characterized by sharp changes in downnhole geochemical profiles are
It is interesting to note that the chemical characteristics of fluids from the protothrust at Site 1174 and the source potentially associated with the OOST fault at Site 1176 are similar to the characteristics of the fluid in the low-Cl zone centered below the décollement. The composition of the fluid along the trench wedge/Shikoku Basin boundary is, however, distinct.
Another distinct characteristic of the Muroto Nankai Transect, not observed at any of the other drilled DSDP and ODP subduction zone sites, is the elevated (up to 10 mM) dissolved sulfate zone found at depth. It is beneath the near-surface sulfate reduction zone, and prevails from the boundary between the upper and lower Shikoku Basin facies to the oceanic basement and probably deeper. The fact that microbial activity has not reduced the dissloved sulfate zone over the past 0.5 m.y. indicates that the amount of labile organic matter available for microbial activity (for sulfate reducers and/or methane oxidizers) above the proto-décollement and décollement zones, where temperatures do not limit bacterial activity, is extremely low. Dissolved sulfate can only be reduced inorganically at temperatures of 250°-300°C, and thus may persist into the seismogenic zone. The presence of dissolved sulfate in an anaerobic environment affects the oxidation state of the system and should influence sediment magnetic properties as well as inorganic reactions with transition metals, such as Fe and Mn.
The dominant diagenetic processes are ash alteration to clays and zeolites and silicate (mostly clay) reactions at the deep water sites and carbonate reactions at the shallow water sites; carbonate diagenesis, however, also occurs at the deep water sites. Opal-A dissolution controls the Si concentration profiles at each of the sites in the top few hundreds of meters, and other silicate reactions control it deeper in the sections.
Although no solid gas hydrate was recovered during Leg 190, their presence was documented indirectly. Both temperature measurements of cores on the catwalk and pore fluid Cl concentrations indicate the existence of gas hydrates at two slope sites, Site 1176 and Site 1178. Gas hydrate dissociates upon recovery because it is unstable at ambient temperature and pressure. Recovery of solid hydrate is unlikely unless it is extremely abundant.
Sites 1176 and 1178 are within the stability field of seawater-methane hydrate from the seafloor to the BSR. Because methane is the dominant gas in the sediments at these sites, any gas hydrate present should be primarily methane hydrate, as it is found at nonthermogenic oceanic sites. Formation of methane hydrate is a highly exothermic reaction; its decomposition consumes much heat and cools the cores. At Site 1176, temperatures 4°-5°C colder than background temperatures between ~220 and 240 mbsf were measured in two cores (190-1176A-25X and 26X). Because of poor core recovery no data exist between 240 and 320 mbsf. Pore fluid Cl concentrations suggest minor dilution of Cl by ~1% beyond dilution by other processes.
At Site 1178, gas hydrate appears to be considerably more abundant. Based on pore fluid Cl concentrations, methane hydrate (inferred from gas composition) is present between ~120 and 400 mbsf, with the highest concentrations between 150 and 200 mbsf. The lowest catwalk core temperature of -0.5°C was measured at ~200 mbsf in Core 190-1178A-23X. Temperatures colder than background by 4°-6°C were measured in several cores, mostly between 150 and 200 mbsf.
At Site 1178, the Cl concentration-depth profile has a steep, continuous trend of freshening between 90 and 200 mbsf with two intense Cl minima. The first is between 170 and 185 mbsf. The second minimum with the lowest Cl value of 524 mM compared with that of bottom-water value of 557 mM was measured in Core 190-1178A-23X, which also had the -0.5°C catwalk temperature. This corresponds to >6% dilution by methane hydrate decomposition. The background dilution throughout the 150-200 mbsf interval is 3%. Between 200 and 400 mbsf, Cl concentrations continue to gradually decrease with depth from 545 mM to a minimum of 517 mM at the BSR depth (~420 mbsf), which corresponds with >7% dilution. Superimposed on the background Cl dilution profile are numerous smaller Cl minima. This suggests that throughout the section, from 90 to 400 mbsf, disseminated gas hydrate is present and is responsible for the background 3%-4% Cl dilution and that specific sediment horizons, probably the coarsest grained ones, have higher hydrate concentration, equivalent to 6%-7% Cl dilution.
Cl concentrations sharply decrease below the BSR depth and reach a minimum of 470 mM, almost a 6% dilution, centered around 500 mbsf. The origin of this low-Cl zone is as yet unclear. It may represent a more hydrate-rich young paleo-BSR, which has not had enough time to diffuse. Higher concentrations of methane at this depth are consistent with this hypothesis.
The trench-wedge facies thickens substantially from the basin to the trench. This rapid sedimentation may affect the pore pressures and the compaction state of the underlying sediment. Within the trench-wedge facies, porosities exhibit high scatter, probably because of lithologic variability. In general, porosities decrease with depth within this section at Sites 1173 and 1174 but show no distinct trend at Site 808. Some of the difference in the porosity trend may be attributed to offset along the frontal thrust at Site 808, which would disturb the pre-existing porosity profile.
At Site 1177, the lowermost ~100 m of the upper Shikoku Basin facies exhibits nearly constant porosities of 60%-65%, whereas the P-wave velocities increase slightly with depth (Fig. F39). At Site 1173, porosities increase slightly with depth from 57%-65% at ~102 mbsf to 62%-69% at ~340 mbsf. These values are surprisingly high for a burial depth of 300-400 m, and the porosity within the upper Shikoku Basin facies at both reference sites deviates significantly from normal compaction trends for silty clays. Velocities at Site 1173 remain relatively constant to ~240 mbsf and increase below this, despite the increasing porosity. This behavior suggests cementation. At Sites 808 and 1174, a slight porosity increase with depth is observed in this unit but is less distinct than at Site 1173. Porosities within the upper Shikoku Basin facies at Sites 1174 and 808 range from ~35% to 45%. The difference in porosity values between the reference sites and those in the deformed wedge imply that either compaction, collapse, and dewatering of the sediments has occurred during accretion or the sites within the accretionary wedge have a different diagenetic, cementation, and burial history than the current reference sites. High-velocity layers occur near the top and bottom of the upper Shikoku Basin facies, which is otherwise characterized by gradually increasing velocities with depth.
Along the Muroto Transect (Sites 1173, 1174, and 808), porosities within the lower Shikoku Basin facies decrease with depth and follow a compaction trend typical of fine-grained marine sediments. At Site 1173, porosities within this unit decrease from ~50% at the top to ~36% at its base. At Sites 1174 and 808, porosities decrease from 34%-40% to 30%-35%, with a sharp offset to greater porosity across the décollement. At Site 1177, the lower Shikoku Basin facies includes a thick turbidite sequence that does not correlate with the stratigraphy observed along the Muroto Transect. Porosities within the upper hemipelagic portion of the lower Shikoku Basin facies at Site 1177 (400-449 mbsf) decrease with depth from 60%-65% to 46%-54%. The porosity decrease within the lower Shikoku Basin sequence from Site 1173 to Sites 1174 and 808 may be explained by compaction and dewatering of these sediments with progressive burial. Alternatively, the lower Shikoku Basin sediments at Sites 1174 and 808 may have initially had lower porosities than Site 1173 because of factors such as greater overburden or lithologic differences.
At Sites 808 and 1174, porosities increase sharply across the décollement zone, whereas velocities decrease. This probably reflects a combination of (1) rapid, partially undrained burial of the underthrust sequence resulting in underconsolidation and (2) higher mean stress and tectonic compaction of the accreted sediments. At Site 1174, porosities directly below the décollement zone are slightly lower than at Site 808. This observation suggests that simple progressive compaction of underthrust sediments may not adequately explain the porosity-depth trends and that other factors (such as initial sediment thickness and heterogeneity in mechanical strength) are also important. At Site 1173 the stratigraphic equivalent interval of the décollement zone (~390-420 mbsf) corresponds to the base of an anomalous zone in which velocities decrease with depth. A similar, considerably smaller amplitude velocity excursion correlates with the stratigraphic equivalent of the décollement at Site 1177 (~430 mbsf).
In accordance with the steadily decreasing porosities below the décollement zone, velocities generally increase with depth at Sites 808 and 1174. Horizontal velocities (Vx and Vy) increase more rapidly with depth below the décollement than vertical velocities (Vz). Increasing velocity anisotropy with depth suggests vertical compaction of the sediments.
Comparison of the thickness of lithostratigraphic units between sites appeared problematic because of the apparent diachronism of the lithostratigraphic boundaries and the lateral changes in porosity. To address this problem, we computed solid thicknesses for each unit. The solid thickness is computed by integrating the solid volume fraction (1-porosity) upward from the base of the sedimentary column using the moisture and density data. By comparing solid thickness of stratigraphic units between sites, we can account for differences in consolidation history. The solid thickness is preserved during vertical compaction but is increased by horizontal tectonic shortening, regardless of whether it occurs by ductile strain or by thrusting. Figure F40 shows the biostratigraphic and paleomagnetic ages at Sites 808, 1174, 1173, and 1177 as a function of the solid thickness. The derivative of these curves represents the solid mass accumulation rate.
The diachronism of the lithostratigraphic boundaries appears clearly on Figure F40. The upper/lower Shikoku Basin facies boundary occurs at ~2.25 Ma at Site 1174, ~3 Ma at Site 1173, and >4 Ma at Site 1177. The base of the trench wedge is younger at Site 1173 than at Sites 808 and 1174.
The solid thickness of the sequence below the stratigraphic level of the décollement displays some lateral variability, which could be attributed to lateral variations in mass accumulation rates during sedimentation. This variability is most important in the part of the basin older than 11 Ma. Accumulation rates obtained by linear regression on all age data between 7 and 11 Ma are comparable at all sites: 14.9 m/m.y. at Site 1173, 16 m/m.y. at Site 1174, 13.7 m/m.y. at Site 808, and 16.3 m/m.y. at Site 1177. The same is true of accumulation rates between 1.8 Ma (Pliocene/Pleistocene boundary) and 4 Ma: 29.7 m/m.y. at Site 808, 29.9 m/m.y. at Site 1174, 28 m/m.y. at Site 1173, and 22 m/m.y. (based on paleomagnetism) at Site 1177. In contrast to these two sequences, the interval immediately surrounding the décollement varies considerably in thickness. The solid thickness of the 4- to 7-Ma interval increases from ~15 m at Site 1173 to ~30 m at Site 1174, and to 35 m at Site 808. Note that the décollement varies in stratigraphic age by ~1 m.y. between Site 808 and Site 1174 but stays within the lower part of this thickened interval. Note that the décollement on the Ashizuri Transect also lies within the same age interval. It appears that throw on the frontal thrust (150 m, corresponding to 80 m of solid thickness where it was drilled) accounts for the change of thickness of the trench wedge between Site 808 and Site 1174.