SUMMARY OF SCIENTIFIC RESULTS
Changes in Physical Properties
The trench-wedge facies thickens substantially from the basin to the trench. This rapid sedimentation may affect the pore pressures and the state of compaction in 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.
Upper Shikoku Basin Facies
At Site 1177, the lowermost ~100 m of the upper Shikoku Basin facies exhibits nearly constant porosities of 60%65%, whereas the velocities increase slightly with depth (Fig. 39). 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 300400 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 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 and cementation history than the current reference sites. High-velocity layers occur near the top and bottom of the upper Shikoku Basin facies, which otherwise appears to be gradually increasing with depth.
Lower Shikoku Basin Facies
Along the Muroto Transect (Sites 1173, 1174, and 808), porosities within the lower Shikoku Basin facies decrease with depth and follow a normal compaction trend. 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 (400449 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 both 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 excess pore pressures 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) are also important. At Site
1173 the stratigraphic equivalent interval of the décollement zone (~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.
Mass Accumulation Rates
Comparison of the thickness of lithostratigraphic units between sites appeared problematic because of the apparent diachronism of the lithostratigraphic boundaries and of the lateral changes of porosity. To help tackle this problem, computations of solid thickness were performed and are presented here. The solid thickness is computed by integrating the solid volume fraction (1 porosity) from the base of the sedimentary column using the moisture and density data. The solid thickness thus represents the thickness of the sediment after vertical compaction to 0 porosity. 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 40 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 40. The upper/lower Shikoku Basin facies boundary moves between
~2.25 Ma at Site 1174 and ~3 Ma at Site 1173 to >4 Ma at Site 1177. The
base of the trench wedge is younger at Site 1173 than at Sites 808 and
The solid thickness the sequence below the stratigraphic level of the décollement displays some lateral variability, which could be attributed to lateral variations of the solid 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 very similar 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 remark may be done for the accumulation rates between 1.8 Ma (Pliocene/Pleistocene boundary) and 4 Ma: 29.7 m/yr at Site 808, 29.9 m/yr at Site 1174, 28 m/yr at Site 1173, and 22 m/yr (based on paleomagnetism) at Site 1177. In contrast with these two sequences, which do not show evidence for tectonic thickening within the precision of the age determinations, 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 also appears that the frontal thrust throw (150 m of vertical throw, corresponding to 80 m of solid thickness where it was drilled) accounts well for the change of thickness of the trench wedge between Site 808 and Site 1174.
Organic Geochemistry and Hydrocarbon Sources
Multiple sources (biogenic, thermogenic, and catagenic) and production mechanisms for the hydrocarbons encountered during Leg 190 were identified by plotting the methane (C1), Bernard ratios (C1/C2+C3) and sulfate profiles for each Site (Fig. 41). At Reference Site 1173, the hydrocarbon profile is dominated by biogenic methane down to 260 mbsf, followed by a shift to a mixture of thermally produced methane and lighter hydrocarbons around 300 mbsf that dominates the composition of gases down to 724 mbsf. The marked shift in hydrocarbon type is likely due to an increase in sulfate concentration below ~400 mbsf that inhibits biogenic production of methane. The hydrocarbons detected in sediments at Site 1174 are indicative of a biogenic source coupled with a thermogenic component down to the décollement (~807.6 mbsf), increasing in concentration with temperature and depth. Like Site 1173, as sulfate increases with depth, methane production decreases (low parts per million). The hydrocarbons below 807.6 mbsf are the products of both rapid thermal maturation of immature organic matter present in the sediments (thermogenic) and the thermal cracking of more mature organic matter or "kerogen" (catagenic).
Site 1175 is characterized by very young sediments and immature organic matter as observed by the production of biogenic methane throughout the hole. As at Site 1175, biogenic methane dominates the first 300 mbsf of Hole 1176A; however, over the last 100 m there is an abrupt shift into the thermogenic zone. Interestingly, the C1/(C2+C3) ratio indicates that no mixing has occurred; therefore, the hydrocarbons detected below 300 mbsf are a separate component. The very low thermal gradient at Site 1176 (~25°C at total depth; 401.6 mbsf), coupled with high sulfate concentrations and the predominance of C2 over C1 indicate that thermogenic hydrocarbons migrated into the lower 100 m of Hole 1177 from a deeper source rather than forming in situ (diagenesis) at low temperature. The hydrocarbon profile for Site 1177, drilled on the Ashizuri Transect, differs significantly from those sites drilled on the Muroto Transect, with low concentrations of methane between 304 and 363 mbsf followed by an abrupt drop in methane concentration (a few parts per million) over the next 270 m of hole. The low abundance of methane below 400 mbsf is the result of an unusually high sulfate content that is controlling methanogenesis or microbial mediation of methane production. The relatively low concentrations of organic matter available in these sediments may be preventing sulfate reducers from consuming the excess sulfate keeping concentrations at high levels. A sharp increase from the low parts per million level to ~1600 ppm was measured in the last 90 m to total depth at 830.3 m, coincident with the transition from the Shikoku turbidite facies (Unit III) to the volcaniclastic facies (Unit IV). As at Site 1176, there is an abrupt shift of the C1/(C2+C3) ratio to the mixing zone with both ethane (C2) and biogenic methane occurring between 750 and 830 mbsf. The temperatures within the mixing zone at Site 1177 are in the range of thermogenic in situ production of the C2 lighter hydrocarbons.
Microbial Activity and Biogeochemistry
Bacteria are responsible for shaping many of the chemical profiles within deep marine sediments through their metabolic activity. The distribution of the microbial community was characterized and their impact on deep marine sediments was investigated in cores from the Nankai Trough (Fig. 42). The apparent environmental controls on bacterial distribution varied among sites from physical (temperature) to geochemical. Overall, the total numbers of bacteria at these sites was either at the low end, or below the range predicted by a general model constructed from bacterial distributions in deep marine sediments at previous ODP sites (Parkes et al. 1994). This is consistent with observations made at a previous accretionary prism siteCascadia margin.
The abundance of bacteria at Site 1173 appears to be primarily controlled
by a steep thermal gradient. The profile agrees with the model for the upper
250 m, with a significant increase in numbers between 43 and 80 mbsf
associated with high concentrations of methane and total organic carbon. At
250 and 460 mbsf the temperature boundaries for mesophiles/thermophiles
and thermophiles/hyperthermophiles, respectively, were crossed. At the
upper boundary populations decreased overall, and at the second boundary
there was a single significant increase at ~85°C. Bacteria were not observed
in the five deeper, and warmer, samples at this site (536673 mbsf).
Overprinting this temperature control is a significant correlation between
bacterial numbers and in situ methane concentration between the near
surface below the sulfate reduction zone and ~400 mbsf. At 400 mbsf
(~76°C), rising concentrations of both methane and sulfate suggest that
bacteria are no longer controlling sediment geochemistry and that
temperature effects dominate.
Bacterial abundance at Site 1174 falls within predicted values from the
surface to ~370 mbsf, apart from two significant excursions in the upper
layers associated with sandy cores. This site is also characterized by a
steep temperature gradient, and at 370 mbsf estimated temperature is
~50°C. From this depth, bacterial populations rapidly decline to not
detectable at 575 mbsf. Bacteria were not observed in the 15 samples
collected from 623 to 1091 mbsf with the exception of two samples (779
and 796 mbsf) located just above the décollement zone. At these depths,
with an estimated temperature of 90°C, bacterial abundance reappears
within the envelope predicted by the model. Bacterial populations were not
correlated with any of the in situ chemistry investigated and appear to be
regulated solely by temperature at this site.
At Site 1177 bacterial abundance appears to be determined by sulfate. Coring began at 300 mbsf, where bacterial abundance was relatively low and remained so until 380 mbsf. Abundance in deeper samples (380675 mbsf) steadily increased and was within predicted values from below 400 mbsf. This increase in bacterial numbers strongly correlated with increases in IW sulfate concentration that were unexpectedly present, at near-seawater concentrations, between 400 and 700 mbsf. Bacteria were also present at lower and decreasing concentrations between 725 and 811 mbsf despite methane concentrations increasing significantly below 740 mbsf. Overall low bacterial populations and high sulfate concentrations are probably attributable to very low total organic carbon concentrations restricting bacterial sulfate reduction. Why populations do not react to increasing concentrations of methane below 740 mbsf remains unclear as the temperature gradient at this site was shallow with the hole estimated at <40°C throughout.
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