At Site 1174, laboratory measurements were made to provide a downhole profile of physical properties at a site within the protothrust zone, landward of reference Site 1173 and seaward of Site 808. With the exception of some extremely short (<50 cm), small diameter (<4 cm), and intensely fractured sections, all cores were initially passed through the MST before splitting. Gamma-ray attentuation (GRA) and magnetic susceptibility measurements were taken at 4-cm intervals with 2-s acquisition times for all cores. Natural gamma ray (NGR) was counted every 30 cm for 30-s intervals. Voids and cracking caused by gas expansion were noted in cores between 0 and 67 mbsf and degraded MST measurements. Biscuiting and reduced core diameter in RCB cores also degraded measurements. Data are not available between 74.1 and 143.70 mbsf because this interval was not cored.
Moisture and density samples were selected from undisturbed core at regularly spaced intervals of at least one per section. Additional samples were taken within the décollement zone. Measurements of dry volume and wet and dry mass were uploaded to the ODP (Janus) database and were used to calculate water content, bulk density, grain density, porosity, void ratio, and dry bulk density. During moisture and density measurements for Hole 1174B, a calibration problem was noted for the pycnometer. After recalibration, a linear correction was applied to samples that had been run with the incorrect calibration. This correction affected Cores 190-1174B-1R through 42R and Section 43R-3. The dry volumes and calculated parameters reported in the database have been updated to incorporate this correction.
P-wave velocities were measured on split cores or discrete samples at a frequency of two to three per core. Measurements were taken in three directions when core conditions permitted. Electrical conductivity measurements were taken at a frequency of two to three per core. Raw data and calculated physical properties are available from the Janus database for all MST, moisture and density, velocity, and thermal conductivity measurements (see the "Related Leg Data" contents list). Because electrical conductivity data are not currently available from the Janus database, they are included in Tables T21 and Table T22, respectively.
Sediment bulk density was determined by both the GRA method on unsplit cores and the mass/volume method ("index properties") on discrete samples (see "Physical Properties" in the "Explanatory Notes" chapter). The GRA density data and the bulk densities determined by the mass/volume method show similar downhole trends, but density values from the two methods are significantly different (Fig. F38A, F38B). The GRA density values exhibit considerable scatter at all depths and are generally 0.2 g/cm3 lower than the moisture and density measurements. This may be caused by the small and variable diameter and the biscuited nature of RCB cores.
Grain densities determined from dry mass and volume measurements increase from ~2.64 g/cm3 at ~144 mbsf (start of coring in Hole 1174B) to ~2.79 g/cm3 at ~1000 mbsf (Fig. F38C). A shift occurs between Units III and IV; the average grain density within Unit III is 2.71 g/cm3, whereas the average for Unit IV is 2.77 g/cm3.
The calculated porosity profile is shown in Figure F38D. Porosities from silty clays within lithostratigraphic Unit II (trench-wedge facies) are characterized by a general decrease with depth, from 58%-72% at the seafloor to 36%-42% by 480 mbsf (Fig. F38D). Typically, the lower porosity values represent sands and silty sands. Scatter within the silty clay samples may reflect subtle differences in grain size and composition that were not distinguishable in hand specimen. Porosities decrease slightly from ~38% to 35% at the top of Subunit IIC (trench to basin transition facies), and this transitional unit is not readily distinguished from Unit III (upper Shikoku Basin facies) solely on the basis of moisture and density.
With the exception of two zones of scattered, elevated porosities (at 500-550 and 600-650 mbsf), the shipboard data show essentially constant porosity of 35%-42% throughout lithostratigraphic Unit III (upper Shikoku Basin facies; 480-661 mbsf). This is a significant deviation from both normal compaction trends for silty clays (e.g., Hamilton, 1976; Athy, 1930) and the decrease of porosity with depth observed within Units II and IV above and below (Fig. F38D).
Porosities drop slightly to 34%-40% at the top of lithostratigraphic Unit IV (lower Shikoku Basin facies; 661 to 1102 mbsf). The change in porosity at the boundary between the upper and lower Shikoku Basin facies is similar to the pattern observed at Site 1173. The correlation between the discontinuity in porosity at ~661 mbsf and the boundary between Units III and IV suggests that the character of porosity change with depth is controlled, at least in part, by lithology. Porosities resume a compaction trend from 661 to 807 mbsf, decreasing to 30%-35% by the top of the décollement zone (807.6-840 mbsf; see "Structural Geology"). Within the décollement interval, porosity continues this compaction trend but exhibits somewhat greater scatter (perhaps due to increased sampling). Porosity increases sharply to 33%-38% directly below the décollement zone. In the underthrust section (below 840 mbsf), porosities remain relatively constant to 985 mbsf then decrease slightly with depth from 33%-38% at 985 mbrf to 32%-37% at the bottom of the hole. Overall, the rate of porosity decreases with depth is comparable to the porosity decrease with depth in the underthrust sequence (below 965 mbsf) at Site 808 and the age-equivalent sequence (below 390 mbsf) at Site 1173 (Fig. F39).
Thermal conductivity was measured using one of two methods depending on core condition. For shallow, nonindurated samples, a needle probe was inserted into the unsplit core for a full-space conductivity measurement. For samples from below 67 mbsf, insertion of the needle caused fracturing, so a half-space method was used on split cores. Between the mudline and ~815 mbsf, thermal conductivities increase gradually with depth from ~0.74 to 1.8 W/(mˇ°C) (Fig. F40A). Thermal conductivities decrease slightly below the décollement zone at ~840 mbsf, and range from 1.62 to 1.81 W/(mˇ°C) within the underthrust sediments. The change in thermal conductivity across the décollement zone correlates with the abrupt increase in porosity (Fig. F38D). This relationship is expected because the thermal conductivity of sediment grains is higher than that of pore fluid.
Shipboard thermal conductivities and downhole temperature measurements to 65.5 mbsf (see "In Situ Temperature and Pressure Measurements") define a near-surface heat flow of 180 mW/m2. Using this estimated heat flow and measured thermal conductivities, and assuming steady-state vertical conductive heat flow, projected downhole temperatures reach ~110°C at the top of the décollement zone and ~140° at 1111 mbsf (Fig. F40B). These estimates are highly speculative because it is likely that (1) thermal steady-state has not been reached and (2) conductive heat flow is perturbed within the toe of the accretionary complex by fluid flow.
In APC cores from Hole 1174A, P-wave velocities were measured using the P-wave sensors 1 and 2 (PWS1 and PWS2) insertion probe system along (z-axis) and across (y-axis) the core axis, respectively. The PWS3 contact probe system was used to measure P-wave velocities across the liner (x-axis) (Fig. F41A). Because of unfavorable core conditions, measurements in more than one direction could rarely be obtained in the same interval. In RCB cores from Hole 1174B, sample cubes were cut and measurements in all three directions were performed using the PWS3 contact probe system.
The velocity-depth profile at this site displays important deviations from a smooth compaction curve. Between 200 mbsf and the décollement zone, three zones with higher velocities are identified: (1) between 360 and 420 mbsf, near the base of the outer trench wedge (Subunit IIB); (2) ~520 mbsf, near the top of the upper Shikoku Basin facies (Unit III); and (3) ~660 mbsf, across the transition from upper to lower Shikoku Basin facies (Unit III/IV boundary). Of these anomalies, only the deepest one is well defined in Site 808 data, where it is also found at the same Unit III/IV boundary. Surprisingly, the two lower zones correspond to zones of lower, not higher, bulk density (Fig. F41A).
This anomalous behavior also appears on the porosity-velocity crossplot (Fig. F41B) and suggests cementation. This behavior is similar to the relatively high velocities and high porosities in the upper Shikoku Basin facies at reference Site 1173. The two zones at 520 and 660 mbsf still retain unaltered volcanic glass, and there is a correlation between these two zones in high velocity and high silica content in the pore fluid (see "Inorganic Geochemistry"). The upper zone of high velocities (360-420 mbsf) is not ash rich, as it lays within the turbidite wedge, but is characterized by a higher than average cristobalite/quartz ratio (see "Lithostratigraphy"). This observation suggests that (probably biogenic) recrystallization of amorphous silica is taking place in this interval as well. Velocity drops across the décollement zone by ~300 m/s and then increases regularly down to the base of the hole. The velocity decrease across the décollement zone correlates with the increase in porosity over the same interval.
Velocity anisotropy results (Fig. F41C) are similar to those obtained at Site 808. In Unit II, anisotropy averages 3% and decreases very slightly with depth. Varying bedding dips in a 100-m-thick zone around the décollement zone and between 400 and 450 mbsf (see "Structural Geology") may contribute to scatter of both vertical and horizontal anisotropy. Below the décollement zone, anisotropy increases with depth, reaching ~10% at the base of the hole.
Part of the anisotropy in this shallow part is attributed to the well-developed lamination in some samples, which increased attenuation along the core axis and made measurements along this direction more difficult. Lamination tends to decrease downhole, occurs only as occasional bedding-parallel cracks in samples from Units III and IV, and is absent in the zones of abnormally high velocity around 400, 540, and 660 mbsf. The effect of discrete cracks on the velocity measurements is not obvious.
Measurements were made on APC cores with a four-needle, 30-kHz electrode array. On RCB cores, conductivity was measured on the same sample cubes used for P-wave measurements with a 30-kHz two-electrode system.
Electrical conductivity and formation factor (see "Physical Properties" in the "Explanatory Notes" chapter) measured on sample cubes are given in Table T22. For the needle-probe measurements, only the apparent formation factor is given. Needle-probe measurements for Hole 1174A yield formation factors mostly between 2.5 and 6 (Fig. F42A). As at Site 1173, coarser lithologies, which include silty and sandy turbidites, appear more resistive, and formation factor measured perpendicular to the core axis (x- and y-direction) is lower than that along the core axis. Data acquired in the horizontal plane closely follow the variation in porosity and display reversals of the compaction trend with distinctive decreases of the formation factor at ~520 mbsf, 600-630 mbsf, and across the décollement zone (Fig. F42B). Formation factor measured along the z-axis displays stronger variations than the horizontal components at the two upper reversals (Fig. F42C). These reversals correlate with the zones of high porosity and anomalous P-wave velocity identified earlier.
Conductivity anisotropy (Fig. F42C) is generally higher than at the reference site. Conductivity anisotropy generally increases with depth and appears higher beneath the décollement zone than above. The anomalous high-porosity and high-velocity zones around 520 and 660 mbsf have a lower anisotropy than the formation above and below. Samples from coherent fragments within the décollement zone all have low apparent vertical anisotropy. Variations of bedding dip may not entirely explain this feature as bedding dip is <20° at most of the intervals sampled within the décollement zone (see "Structural Geology").
Although conductivity is theoretically less sensitive than P-wave velocity to cracks orthogonal to the direction of the measurement, bedding-parallel lamination in the upper part of Hole 1174B appeared to influence conductivity along the core axis. Conductivity along the z-axis increased by 5% or more in laminated samples when a pressure of ~2-3 bars was applied on the sample by pressing the electrode by hand. Measurements along the x- and y-axes were not as pressure sensitive. The conductivity increase from applying the same hand pressure was typically <2% for measurements along the x- and y-axes, as well as for measurements made along the z-axis on samples without lamination. Small gas bubbles were also observed escaping from the larger cracks when pressure was applied. Thus, we suspect that partial desaturation of these cracks contributed to the anomalous increase of conductivity across the cracks in the most laminated samples. This effect can probably be neglected below 500 mbsf. To limit this effect, measurements below 730 mbsf in Hole 1174B (and at later sites) were made applying a pressure of ~1 bar on the sample with a 10-lb weight.
Volumetric magnetic susceptibilities were measured in all recovered cores from Site 1174 (Fig. F43). Uncorrected values of magnetic susceptibility from the Janus database were used. Large magnetic susceptibility values and large scatter between 0 and 280 mbsf are correlated with the occurrence of sand-rich turbidites. Less scatter occurs below 280 mbsf, and susceptibility reaches a minimum between 570 and 640 mbsf. Below 640 mbsf, magnetic susceptibility increases to 820 mbsf. The hemipelagic mudstones between 820 and 950 mbsf are characterized by three peaks of up to 200 × 10-5 SI. These peaks can be correlated with magnetic susceptibility peaks at Sites 1173 and 808. Magnetic susceptibility data show a slight decrease from 950 to 1100 mbsf.
As seen at Site 1173, porosities within the trench-wedge facies (Unit II at Site 1174, Unit I at Site 1173) are characterized by high variability and a general decrease with depth. From the top of Subunit IIC and throughout the upper Shikoku Basin facies, porosities remain nearly constant with depth. These constant porosities deviate from a typical compaction profile for silty clays. The porosities within Unit III at Site 1174 are ~20% less than porosities within this unit at Site 1173 and show considerably smaller deviation from a normal compaction trend than observed at Site 1173. At the boundary between the upper and lower Shikoku Basin facies (Units III and IV), porosity decreases slightly and resumes a trend of gradually decreasing porosity with depth. An increase in porosity across the base of the décollement zone is accompanied by decreases in thermal conductivity, velocity, and formation factor.
The physical properties of sediments drilled at Site 1174 are generally consistent with results from Site 1173 and previous DSDP and ODP sites in the region (Sites 582 and 808). Site 582, located in the Nankai Trough southwest of Site 1174, penetrated the outer trench-wedge and the upper Shikoku Basin facies. Data from Site 582 show the same pattern of decreasing porosity with depth in the trench wedge (Kagami et al., 1986). However, a change in the porosity-depth profile is noted at the top of the upper Shikoku Basin facies at Sites 1173 and 582, whereas a change occurs at the top of the trench to basin transition facies at Site 1174.
Site 808, located ~2 km landward of Site 1174 and ~3 km landward of the deformation front, penetrated the outer trench-wedge and the upper and lower Shikoku Basin facies (Taira, Hill, Firth, et al., 1991). At both Sites 1174 and 808, a slight decrease in porosity was observed at the top of the lower Shikoku Basin facies and porosities increase abruptly across the décollement zone. The porosity increase across the décollement zone at Site 1174 is 2%-4%, whereas the porosity shift at Site 808 is 5%-6%. Porosities within the underthrust sediments at Site 1174 decrease more slowly with depth than at Site 808, reaching 34%-36% at the base of Unit IV, compared to 31%-33% at Site 808 (Fig. F39). This difference in porosity of the underthrust section may reflect progressive compaction and dewatering between the two sites. Alternatively, it may be a consequence of slight variations in initial porosity, cementation, or loading history of the two sites.