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doi:10.2204/iodp.proc.314315316.131.2009

Principal results

Site C0004

Site C0004 (proposed Site NT2-01I) targeted the uppermost 400 m at the seaward edge of the Kumano Basin uplift (outerarc high) where the megasplay fault system branches and approaches the surface. This site penetrated the toe of a thrust wedge in the hanging wall of the megasplay fault system. The scientific objectives of drilling into the shallow portion of the megasplay fault were as follows:

1. Clarify the character and behavior of the shallow portion of the megasplay.
2. Characterize the slip, deformation mechanisms, and evolution in the stable region seaward of the unstable seismogenic fault.
3. Investigate the relationship between fluid behavior, slip, and deformation along and adjacent to the megasplay fault zone.

Four lithologic units were defined at this site during Expedition 316 (Fig. F5). The uppermost unit (lithologic Unit I; 0–78.08 m core depth below seafloor [CSF]) is dominated by greenish gray silty clay with a substantial component of calcareous nannofossils (up to ~25%) and a lesser amount of siliceous biogenic debris (sponge spicules, diatoms, and radiolarians). Unit I is interpreted to have been deposited from the early Pleistocene to the late Pleistocene as a sediment blanket on the upper slope mainly by hemipelagic settling with minor volcanic ash and sand-silt input. An observed gradual increase in carbonate content upsection is consistent with greater carbonate dissolution at depth and implies that the unit was slowly uplifted to its present water depth of 2632 m. At 78.08 m CSF, an angular unconformity separates Units I and II; this unconformity was predicted by seismic reflection and LWD data (Kinoshita et al., 2008) and manifests prominent mineralizations by pyrite and other minerals. A significant age gap is indicated by both paleomagnetic and nannofossil data. Below the unconformity, the age of Unit II (78.06–258.01 m CSF) ranges from late Pliocene to middle Pliocene. The uppermost part of Unit II strata (Subunit IIA) consists mainly of sedimentary breccia interpreted as a mass transport complex (MTC) with silty clay clasts that most likely result from deposition of slumps and mass wasting along an unstable slope. The dominant lithology in Subunit IIB is dark greenish gray silty clay. The low carbonate content of Unit II is consistent with deposition at much greater depths than its present setting, presumably near or below the calcite compensation depth.

Unit III is a middle Pliocene structurally bounded package that hosts the brittle deformation of the splay fault zone. A biostratigraphic age reversal across the boundary between Units II and III suggests that the lithologic change to slightly more calcitic and ash-bearing sediments corresponds to a fault contact. A larger age reversal is found at the lower boundary between Units III and IV at 307.52 m CSF. Shipboard sedimentological observations are not able to resolve whether this Pliocene fault-bounded sedimentary package is more closely related to the overlying slightly older Pliocene prism sediments or the underlying slope basin sediments. Unit IV (307.52 m CSF to the bottom of Hole C0004D at 403 m CSF) is early Pleistocene in age and consists of dark olive-gray silty clay with a moderate amount of calcareous nannofossils and a lesser amount of calcareous and siliceous microfossils. Thin sand and silt beds are common in this unit, particularly in the upper part. It is interpreted to have formed in a lower trench-slope basin, dominated by fine-grained hemipelagic deposition with relatively minor sand input.

Structural observation of the upper slope sediments (Unit I) indicates southeast-dipping beds and normal faults (Fig. F5). Between 100 and 256 m CSF, faults and deformation bands are locally observed within the fragments of drilling-induced breccias, indicating deformation within the accretionary prism. In addition, sediment-filled veins (vein structures) are observed within the fragments, although these are rare. In most cases, faults cut and offset shear zones and have a reverse sense of shear with their displacement less than a few millimeters. A ~60 m thick fractured and brecciated zone is observed between 256 and 316 m CSF; this zone contains two age reversals identified by biostratigraphy. The upper boundary is marked by the abrupt occurrence of brecciated fragments with polished and slickenlined surfaces below 256 m CSF. The lower termination of this zone was defined at 316 m CSF based on a gradual increase of unbroken rock intervals in which subhorizontal bedding and fissility are well preserved.

The seismic reflection profile suggests that the projected depth of the splay fault is ~290 mbsf. Based on recovered cores, fractured rocks and drilling-induced breccias are widely distributed in the fault zone, whereas fault breccia and microbreccia have relatively limited distribution. At 291 m CSF, a 6 cm thick microbreccia is bounded above and below by fault breccia ~50 cm thick, which in turn is bounded above and below by fractured rocks. The localized comminution in microbreccia zones contrasts with distributed deformation in fractured rocks and fault breccia. There is no obvious evidence of fluid-rock interaction (e.g., mineralized veins or alteration) in the fault zone. Microbreccia possibly represents a zone of concentrated shear within the splay fault–related zone. However, the location of microbrecciation does not correlate with the two inversions of biostratigraphic age. The borehole image analysis of the structurally defined fault zone (depth shifted between Holes C0004B and C0004D) shows that electrically conductive fractures are relatively highly concentrated in these intervals. The presence of conductive fractures and/or highly deformed material associated with the concentrated shear may have caused the poor recovery within these intervals. The underthrust slope basin sediments show horizontal to gently dipping bedding and fissility, which is consistent with the bedding dips acquired by the borehole images at this site during Expedition 314 and the seismic reflection profile. Steeply dipping faults are sporadically distributed in the underthrust sediments; their random orientations suggest a lack of tectonic influence associated with plate convergence.

Shipboard measurements indicate that porosity decreases slightly, from ~65% at the seafloor to 59% at 78 m CSF (Fig. F5). A discontinuity in porosity at ~78 m CSF is related to the unconformity between the younger less compacted slope sediments and the older more compacted Pliocene prism sediments. Within Unit II, porosity slowly decreases with depth, from ~53% at the top of Unit II to ~49% near the bottom. Within Unit III, a region of faulted and brecciated sediments, porosity is scattered over the range 46%–59% with no clear trend. These perhaps higher-than-expected porosity values may be due to its younger age compared to the lower half of Unit II or pervasive crack/damage porosity within the fault zone. At the thrust fault boundary between Units III and IV, porosity abruptly decreases from ~50% to 43% despite the biostratigraphic evidence that the underlying sediments of Unit IV are significantly younger than those of Unit III.

Within the underthrust sediments, P-wave velocity anisotropy is consistent with dominantly vertical compaction (vertical minimum velocity and high maximum and intermediate velocities within the bedding plane). However, anisotropy data could also be consistent with bed-parallel shear of the underthrust sediments. Electrical conductivity data appear to correlate with P-wave velocity data.

The average thermal conductivity values are 1.02, 1.09, and 1.50 W/(m·K), for Units I, II, and IV, respectively. In situ temperature was measured at four depths between 25.4 and 135.0 m CSF. Equilibrium temperatures plotted as a function of depth are relatively linear and, coupled with the average bottom water temperature, give a least-squares gradient of 52°C/km. A constant conductive heat flow appears to describe the overall thermal structure quite well.

Natural remanent magnetization (NRM) intensity spans more than two orders of magnitude, ranging from 0.02 to 80 mA/m. NRM intensity peaks at 352.7 m CSF, corresponding to silty clay with interbedded volcanic sand. Magnetic susceptibility values are generally ~10 × 10^–3 SI within Units I–III and rapidly decrease and then increase within the breccia observed in Subunit IIA (Fig. F5). These susceptibility variations were verified by further measurements of corresponding discrete samples. Magnetic susceptibility is significantly higher within the underthrust slope deposits of Unit IV, which contain sand and silt layers, than it is in Unit III.

Pore fluid profiles in the upper 30 m of the sediment column are dominated by microbially mediated reactions, and the sulfate–methane transition (SMT) is reached between 16 and 20 m CSF. Below this depth, downhole increases in Cl, Na, and H₄SiO₄ concentrations and decreases in Mg, B, K, and Rb concentrations are interpreted to be caused by alteration of volcanic ash to authigenic clay minerals and zeolites (Fig. F5). Sharp increases in H₄SiO₄, ammonium, and Mn concentrations, as well as decreases in Mg, K, and Rb concentrations, are observed ~8 m above, within, and 29 m below the structurally identified splay fault zone. A decrease in K and Mg data is also evident at 50 m CSF. Given the reactive nature of these ions in pore fluids, these spikes may be caused by diagenesis; postcruise research will examine whether these are diagenetic signatures or related to fluid flow. Methane shows spikes of elevated concentration within Unit III and near the top of Unit IV (Fig. F5); however, core recovery and brecciation may greatly influence observed measured methane concentrations.

Under epifluorescent microscopy, very high numbers of microbial cells were observed in core sediments at Site C0004 (Fig. F5). Cell populations slightly decreased with depth to 150 m CSF and slightly increased at ~280 m CSF in the splay fault zone. Cell populations then sharply decreased below the splay fault zone.

Site C0006

Site C0006 (proposed Site NT1-03B) targeted the main frontal thrust at the seaward edge of the accretionary prism (Fig. F3). The scientific objectives of drilling at Site C0006 were as follows:

1. Clarify the evolution of the frontal thrust from its birth to death.
2. Clarify the function of the frontal thrust with respect to large earthquakes.
3. Evaluate the relationship between fluid behavior, slip, and deformation within this fault zone.

Site C0006 was previously drilled during Expedition 314 (Kinoshita et al., 2008), in which LWD logs were obtained to 885.5 m LWD depth below seafloor (LSF). During Expedition 316, coring was completed to 603 m CSF. Poor hole conditions stopped drilling at this site before the frontal thrust was reached.

Three lithologic units were recognized during examination of Site C0006 cores (Fig. F6). Unit I is Pleistocene–Holocene in age and extends from the seafloor to 27.23 m CSF. Unit I consists of a fining-upward succession of silty clay, sand, silty sand, and rare volcanic ash layers. Deposition is interpreted to have occurred on the lowermost slope above the trench floor by hemipelagic settling, turbidite deposition that decreased through time, and accumulation of a thick ash near the base of Unit I.

Unit II (27.23–449.67 m CSF) is Pleistocene in age and is interpreted as having been deposited in a trench setting, with increasing proximity to the axial portion of the trench upsection. Subunit IIA (27.23–72.06 m CSF) is dark gray to black fine-grained sand, consisting dominantly of metamorphic and volcanic lithic fragments with secondary quartz and feldspar. Individual sand beds (~1–7 m thick) typically grade into silt and sometimes silty clay with indistinct boundaries between the different lithologies. Subunit IIB (72.06–163.33 m CSF) consists of interbedded fine-grained sand, silty sand, and silty clay in approximately equal abundances. Poor core recovery may indicate substantial loss of the sand units indicated by LWD logs (Kinoshita et al., 2008). Sand layers are typically normally graded with indistinct upper boundaries that grade into silty clay. The silty clay is greenish gray and is only slightly bioturbated or mottled in places. An age repetition zone within Zone NN19 was identified at the lithologic Subunit IIB/IIC boundary and is probably caused by faulting or affected by the presence of large amounts of reworked fossils. The dominant lithology in Subunit IIC (163.33–391.33 m CSF) is greenish gray silty clay, and minor lithologies include normally graded silt, sand, and rare ash. Of the siliciclastic beds, silt is generally more abundant than sand, particularly when compared to Subunit IIA. The dominant lithology of Subunit IID (391.33–449.67 m CSF) is greenish gray silty clay, and minor lithologies include silt and ash. Silt layers are thin-bedded and are relatively rare in this section, becoming absent below 405 m CSF. Sand is completely absent from this subunit, and ash layers are relatively abundant in comparison with overlying units. A short age gap (~0.17 m.y.) was found between 434.71 and 439.50 m CSF. A longer age gap or unconformity (~1 m.y.) was found at the Unit II/III boundary.

Unit III is late Miocene–early Pliocene in age and consists of greenish gray to grayish silty clay with some interbedded volcanic ash, including dolomite- and calcite-cemented ash. Unit III has an overall increased clay content and decreased quartz and feldspar contents compared to the overlying sediments. Unit III was deposited by hemipelagic settling along with accumulation of volcanic ash. The Miocene–early Pliocene age and lithologic content of Unit III are similar to the Shikoku Basin facies documented at ODP Sites 1173 and 1174 in the Muroto transect more than 100 km to the west-southwest along the Nankai Trough (Shipboard Scientific Party, 2001a, 2001b; Moore et al., 2001).

Structural geology observations indicate that most bedding surfaces in Unit I strike north–south to northwest–southeast and dip westward. Between 15 and 35 m CSF, these sediments are affected by normal faults striking north–south to northwest–southeast and dipping steeply with offsets <10 cm. The strikes of bedding or fissility surfaces in the accretionary prism sediments (below 27 m CSF) are variable, and no preferred direction is apparent. Between 28 and 31 m CSF, sediments are cut by two reverse faults showing very small offsets (<1 cm).

A broad fractured/brecciated zone extends from 230 to 545 m CSF (Fig. F6). In this zone, cores are commonly strongly fractured, striated or polished planes are common, and tectonic breccias were identified. In contrast, the sections above or below this fractured and brecciated interval are more coherent and less fractured and generally do not include tectonic breccias. The transition between the fractured/brecciated zone and the underlying sections appears to be sharp. Deformation bands are observed in cores between 300 and 460 m CSF with a clear predominance between 300 and 405 m CSF. Within this section, most deformation bands dip >30° and the sense of shear is reverse. These bands predominantly strike about northeast–southwest and dip either northeastward or southwestward, forming two sets with an apparent conjugate geometry. Given the reverse sense of slip observed along these deformation bands, this geometry suggests a shortening axis that is horizontal and oriented along 138°.

Concentrated deformation zones occur at 235–243, 277–297, 367.5–369.5, 433.75–440, and 533–543 m CSF (Fig. F6). These zones contain greater densities of deformation bands, fractures, and tectonic breccias. The lowermost interval (533–543 m CSF) contains a progression from tectonic breccia to microbreccia and then fault gouge. The sediments recovered in the 545–603 m CSF interval, which consist of bioturbated hemipelagic mud (Unit III), are less fractured or brecciated than observed above. Normal faults predominate in this interval; dip angles of fault planes are scattered between 20° and 88°, but steep angles predominate. No preferred orientation of fault plane directions is recognized.

A notable feature of the porosity profile at Site C0006 is that near-surface (>5 m CSF) porosity values are quite low, averaging ~48% (Fig. F6); this suggests the possibility of erosion of overlying material. From ~5 to 410 m CSF, porosity gradually decreases with depth from ~48% to ~38%. Between ~410 and ~450 m CSF, porosity increases from ~39% to ~49%; the highest porosity values (~50%) are between ~450 and 490 m CSF. This interval includes a deformation zone at 433.75–440 m CSF, raising the possibility that elevated porosity results from microcracks and other fault-related damage within the samples or that sediments in this zone are underconsolidated because of elevated fluid pressures. The observed increase in porosity is also coincident with an increase in clay content, which could contribute to low permeabilities and encourage undercompaction. Alternatively, if the clay contains interlayer water, the apparent porosity change could partially reflect interlayer water that cannot be distinguished from pore water in the moisture and density (MAD) measurements. Porosity values that are corrected postcruise to account for hydrous clay content will shed light on this possibility. From ~475 to 570 m CSF, porosity decreases from ~49% to ~40%, reaching values at 570 m CSF that are in good agreement with the extrapolated porosity trend observed from 5 to ~410 m CSF.

Electrical conductivity exhibits an overall decrease with depth but increases at ~410 m CSF, mimicking the change in porosity. Electrical conductivity anisotropy values show considerable scatter at depths shallower than 300 m CSF; between 300 and 400 m CSF, the transverse anisotropy decreases with depth. This observation, coupled with the decrease of electrical conductivity magnitude, suggests progressive horizontal compaction. P-wave velocity generally increases with depth through the hole. The anisotropy of P-wave velocity data show a general decrease with depth with two discrete zones of higher transverse anisotropy at ~400 and ~480 m CSF.

Vane shear measurements of shear strength increase rapidly through the uppermost 40 m CSF, below which measured shear strength drops rapidly and is associated with the prevalence of sandy beds, where the measurement technique might not be suitable. The boundary between Subunits IIA and IIB is coincident with an increase in shear strength at 72 m CSF. A similar increase at 110 m CSF does not appear to be concurrent with a significant lithologic change. Shear strength continues to increase to 200 m CSF. Between 200 and 275 m CSF there is an apparent decrease in shear strength.

Thermal conductivity from the seafloor to ~80 m CSF shows a striking positive excursion through Unit I and Subunit IIA that likely reflects higher sand content through this interval. A negative trend in thermal conductivity through Subunit IIA likely reflects a decrease in sand content. Between ~80 and 400 m CSF, thermal conductivity values generally increase and likely reflect decreasing porosity. Thermal conductivity in Unit III increases with depth but is offset to lower values relative to the trend through Subunits IIB and IIC. In situ temperature was measured using both the advanced piston coring temperature tool (APCT3) and the Davis-Villinger Temperature Probe (DVTP). The best fitting thermal gradient through the first six measurements is 27°C/km. This very low thermal gradient gives rise to an anomalously low heat flow value.

Paleomagnetic measurements indicate that the mud in Unit III has the lowest NRM intensity (averaging ~0.1 mA/m) compared to turbiditic Unit II (~12 mA/m) and the nannofossil-bearing mud and sand of Unit I (~2 mA/m). Variations in magnetic susceptibility generally parallel variations in NRM intensity. Magnetic susceptibility values are generally ~19 × 10^–3 SI for sediments in Unit I, >200 × 10^–3 SI for Unit II, and 12 × 10^–3 SI for Unit III (Fig. F6). A few discrete peaks of higher NRM and susceptibility values appear at some depth intervals in Units I and II and can be tied directly to the visible presence of volcanic sand in these regions.

Pore fluid profiles in the upper ~60 m of the sediment column are dominated by microbially mediated reactions, and the SMT is reached at 8–12 m CSF. The relatively shallow SMT at this site suggests an elevated upward methane flux; coupled with anomalously low heat flow, this flux provides ideal conditions for gas hydrate formation in the sediment. Assuming equilibrium with Structure I gas hydrate, the gas hydrate stability field extends from the seafloor to ~800–850 mbsf. Dissociation of disseminated gas hydrates within the sediment pore space during recovery is suggested by pore fluid Cl concentration that gradually declines from 571 to 540 mM at depths of ~100–500 m CSF (Fig. F6). Assuming that all the hydrate dissociates during core recovery and handling, the gradual decrease in pore fluid Cl with depth indicates that gas hydrate concentrations, though overall very low (negligible to <2%), increase with depth. Superimposed on the gradual decline in Cl concentration are three Cl minima at 76, 292, and 570 m CSF that indicate horizons of elevated gas hydrate occurrence; however, the concentration at these depths are still relatively low, comprising less than ~5% of the sediment pore space.

Pore fluid geochemical profiles are relatively smooth through the deformation zone at Site C0006 (Fig. F6). No local maxima or minima are coincident with the faults described at Site C0006. Thus, pore water chemistry provides no indication of fluid flow along these fault zones. From ~450 m CSF to the base of the hole at ~600 m CSF, many of the pore fluid chemical profiles (e.g., Mg, Ca, Li, K, Rb, Cs, B, and Mn) change distinctly. These geochemical variations could reflect diagenetic reactions because of the higher clay content or greater age of pore fluids in the Miocene sediments of Unit III. Another possibility is that these fluids reflect communication with a fluid originating deeper than the base of Hole C0006F.

The depth profile of methane below 14 m CSF decreases throughout the sediment column, except for two peaks at ~100 and 310 m CSF (Fig. F6). These increased methane values could be related to the destabilization of gas hydrates and the consequent release of methane into the sediment. Ethane was detected in low amounts in sediment at depths below 76 m CSF. Higher molecular weight hydrocarbons were not detected at Site C0006. Except for a few enrichments, total organic carbon (TOC) remains low throughout the core (average = 0.46 wt%). Total nitrogen (TN) is highest in Unit III, perhaps because of clay-bound nitrogen substances. The ratio of TOC and TN (C/N) averages ~7.0, indicating that the organic matter at Site C0006 is mainly of marine origin. The concentration of total sulfur (TS) in the sediments is generally low with a mean value of 0.25 wt%.

Preliminary microbial cell abundances were enumerated by visual inspection (Fig. F6). At Site C0006, ~10⁹ cells/cm³ were detected in the upper sedimentary unit above 100 m CSF. The porous sand layers in the upper unit harbor abundant microbial populations, suggesting that the environment is rich in available energy sources and habitable space. The potential energy sources in the sand layers are probably buried consumable organic matter (e.g., organic acids). Below the upper sedimentary unit, the population decreased with increasing depth in the accretionary prism with no or very small proliferation of cell abundance observed at sand layers or lithologic boundaries. Generally, the microbial population in the accretionary prism at Site C0006 was found to be low (10⁷ cells/cm³), suggesting that this accretionary prism may be a harsh and energy-starved habitat for subseafloor life. This is in clear contrast to the accretionary prism at Site C0004, where relatively high biomass is present throughout the cored materials.

Site C0007

Site C0007 (proposed Site NT1-03A) targeted the main frontal thrust at the seaward edge of the accretionary prism (Fig. F3). This site was drilled during Expedition 316 after hole conditions at Site C0006 precluded reaching the frontal thrust.

Four lithologic units were identified at Site C0007 (Fig. F7). The uppermost unit (lithologic Unit I; 0–33.94 m CSF) consists of hemipelagic silty clay with interbedded sand. Deposition of this unit is interpreted to have occurred on the lowermost slope above the trench floor by hemipelagic settling, turbidite deposition, and possibly subsequent soft-sediment slumping on an oversteepened slope.

The top of Unit II is placed at the first occurrence of a thick interval of dark sand. Unit II (33.94–362.26 m CSF) constitutes a coarsening-upward succession from fine-grained mud to sand- and gravel-rich deposits. Unit II is interpreted to have been deposited in a trench setting with increasing proximity to the axial portion of the trench upsection. Repetition of coarser facies, interpreted as axial-channel fill, indicates (1) a repetition of the sequence because of thrust faulting, (2) switching of the position of the trench channel because of advancement of the thrust front, or (3) major slumping of the rapidly steepened frontal slope. Biostratigraphy suggests a possible age reversal below 135 mbsf in Hole C0007C (within Unit II) and a significant age gap between Units II and III.

Unit III (362.26–439.44 m CSF) is a Pliocene succession of green bioturbated fine-grained hemipelagic sediments. Thin (<1 cm) greenish layers, reworked glauconite, and pervasive burrowing suggest that low sedimentation rates attended the deposition of this lowermost mud. Small ash layers and dispersed ash made of clear glass and pumice are observed throughout the section but are especially abundant within the lowermost portion of Unit II and in Unit III. Unit III was deposited by hemipelagic settling with accumulation of volcanic ash; its age and characteristics are similar to the Shikoku Basin facies as documented in the western Nankai Trough (Shipboard Scientific Party, 2001a, 2001b).

Unit IV is of possible Pleistocene age based on its correlation on seismic profiles with the active trench wedge of the Nankai Trough to the southwest of Site C0007. Only 25 cm of dark gray sand and a few drilling-affected mud fragments were recovered. The presence of thick unconsolidated sand is also suggested by the significant increase in drilling penetration rate below 439 m CSF. The sand is fine to medium grained and consists of abundant black lithic fragments, metamorphic rock fragments, ferromagnesian minerals, quartz, feldspar, and opaque grains. Nannofossil dating of Unit IV was not possible, as recovered sediments were either potential fall-in material or a sand slurry that did not yield nannofossils.

Structural geology observations describe slump-related features (e.g., disaggregated or chaotically mixed bedding) in the upper portion of Hole C0007C. Bedding dips between 20° and 70° to the southeast in Unit I, which is in contrast to gently dipping beds in the prism below (Fig. F7). In places, faults offset bedding with normal or reverse sense of shear and displacement of less than a few centimeters, and faults show scattered orientations after paleomagnetic correction. Sandy layers are locally disturbed and form inclusions in the muddy matrix, or in the extreme case, sandy layers are chaotically mixed with the muddy layers.

Bedding and fissility dip gently (0°–30°) throughout Site C0007 prism sediments. Bedding-oblique deformation bands show a conjugate geometry of reverse slip associated with northwest-directed layer-parallel contraction. Two types of healed faults are observed in bioturbated hemipelagic mud: a parallel to bedding type and a type defined by randomly oriented normal faults.

Three fault zones were recognized in cores recovered from Hole C0007D. A fault zone at 237.5–259.3 m CSF (fault Zone 1) is related to a thrust that brings mud onto sand. Fault breccias in this fault zone are 17–19 cm thick and are characterized by 1–10 mm sized angular to subangular fragments. The brecciated fragments are commonly polished and slickenlined and lineations indicate multiple slip directions. Occurrence of three similar ash layers in this fault zone could indicate repetition of the same layer by thrust faulting; this possibility needs to be confirmed by postcruise research.

A fault zone at 341.5–362.3 m CSF (fault Zone 2) is located where the lithology changes from mud (Unit II) to bioturbated hemipelagic mud (Unit III). The location of this zone is comparable to the interval where a subhorizontal strong reflector is recognized in a nearby seismic line (Fig. F3) and an age gap is noted by the biostratigraphy. This fault zone consists mostly of fractured rocks characterized by fragmentation along sets of polished and striated surfaces. Concentration of deformation at the bottom of the fault zone is indicated by occurrence of breccia and fault gouge. Fault breccia includes subangular to rounded fragments of ~1 mm to 2 cm in size and a random texture. Fault breccia is replaced by fault gouge at 362.1 m CSF. This is marked by a decrease in size and volume fraction of fragments to <2 mm and 30%, respectively. Fault gouge is weakly foliated and the angle between a foliation defined by alignment of fragments and the horizontal plane is 38°. Deformation bands are concentrated above fault Zone 2 and band kinematics show reverse slip associated with northwest-directed contraction. Deformation bands are absent below fault Zone 2, but healed faults are well developed and show normal slip consistent with vertical compaction of sediments during burial. The changes in deformation style and kinematics of structures across the fault zone, as well as the asymmetric distribution of fractured rock, suggest that the major slip zone is located at the bottom of the fault zone, possibly represented by the fault breccia/​fault gouge zone.

Fault Zone 3 is located at the basal part of the prism above the frontal thrust that juxtaposes the hemipelagic mud (Unit III) above and sand-dominated sediments (Unit IV) below. This zone is marked by a heterogeneous distribution of fractures, commonly with polished and slickenlined surfaces and brecciation. A foliated fault gouge was observed at 418.83–418.94 m CSF with a shear sense consistent with thrust faulting. The lowermost part of the fault zone at 438.28–438.57 m CSF is intensely brecciated into fragments ~1–10 mm in size. This 29 cm thick breccia shows a foliated aspect from an anastomosing network of polished and striated surfaces. At the base of this zone, a 2 mm thick dark layer sharply separates intensely brecciated hemipelagic mud above from unbroken hemipelagic mud and ash below. There is a biostratigraphic age reversal across the lowermost part of the fault zone (Fig. F7). These features indicate that the thin dark layer most likely represents extreme localization of slip associated with thrust faulting. Below this interval recovery was poor; however, additional thrust faulting is inferred from the transition to unconsolidated axial trench deposits (Unit IV).

Porosity decreases rapidly in the uppermost 34 m at this site (Fig. F7). From 34 to 320 m CSF, porosity decreases more gradually from ~48% to ~39% at 320 m CSF, increases from ~39% to ~50% at 400 m CSF, and then decreases to the bottom of the hole. Porosity data reveal no clear discontinuities at depths corresponding to possible faults or at lithologic boundaries within that depth range. The zone of highest porosity (~50%) between ~360 and ~400 m CSF overlies fault Zone 3. Lithologic analyses reveal that Unit III exhibits an overall increase in clay content, which could lead to spuriously high values of porosity because the drying process during MAD analysis removes interlayer water from smectite. Alternatively, elevated porosity in this zone could indicate microcracks and other fault-related damage or that these sediments are underconsolidated (and therefore overpressured), which may have localized shear deformation in this interval and would be consistent with the probable lower permeability associated with higher clay contents. Clays may also decrease permeability of the sediments/​rocks in this interval, leading to overpressurization of pore fluids.

P-wave velocity values of discrete samples range between 1800 and 2000 m/s. Velocity generally increases to 360 m CSF (Unit II/III boundary) and then decreases to ~440 m CSF. It is possible that the apparent decrease below 360 m CSF is the combination of a P-wave velocity offset (about –100 m/s) because of lithologic changes or thrust faults observed at ~430–440 m CSF. Electrical conductivity data appear to be inversely correlated to P-wave velocity data. Shear strength increases rapidly from the seafloor to 35 m CSF. Shear strength through this interval increases at a much faster rate than at similar depths at Sites C0004 and C0006. Below 35 m CSF, shear strength shows cycles of decreases and increases until it is too consolidated for measurement below 91 m CSF.

Thermal conductivity ranges from 0.94 to 1.58 W/(m·K). Thermal conductivity increases rapidly from the seafloor to ~50 m CSF, likely reflecting the decrease in porosity with depth and relatively high sand content. A negative trend in thermal conductivity occurs between ~50 and 75 m CSF, likely reflecting a decrease in sand content. Between ~80 and 400 m CSF, thermal conductivity values fall within a restricted range of values between 1.2 and 1.4 W/(m·K). In situ temperature was measured using the both the APCT3 and sediment temperature tool. The average apparent bottom water temperature is 1.65°C. Equilibrium temperatures as a function of depth are quite linear. The best fitting thermal gradient through the first six measurements is 42°C/km. The least-squares fit to the temperature as a function of thermal resistance indicates a heat flow of 53 mW/m² and a bottom water temperature of 2.0°C. The computed heat flow is anomalously low with respect to other values of heat flow in the vicinity of the Kii transect (Yamano et al., 2003), although it is higher than measured at Site C0006.

In the nannofossil-bearing mud and sand cores in Unit I, both the intensity of NRM and magnetic susceptibility increase steadily downhole (Fig. F7). Numerous volcanic ash and sand layers in Unit II have relatively high concentrations of magnetic minerals, causing relatively high NRM intensity (average 15 mA/m) and magnetic susceptibility (~190 × 10^–3 SI units). The mud of Unit III has the lowest NRM intensity (mean ~0.25 mA/m) and magnetic susceptibility (mean ~9 × 10^–3 SI units) at Site C0007. Poor recovery of underthrust trench wedge type sand and rocks in Unit IV limits paleomagnetic work, although a few pass-through magnetic susceptibility measurements indicate relatively high susceptibility values.

Pore water analyses indicate that sulfate reduction is relatively rapid in the upper ~20 m of the sediment section of Hole C0007C; however, sulfate never reaches depletion and remains ~8–14 mM to the bottom of the hole. In Hole C0007D, sulfate was totally depleted at the start of coring (175 m CSF) with only localized horizons of elevated sulfate. In Hole C0007D, there is evidence of gas hydrate occurrence at ~325 m CSF where Cl values are ~10% less than modern seawater and dilution is also observed in the other major ions (Fig. F7).

Within Unit III, Ca, H₄SiO₄, Li, and Mn concentrations increase and Cl, alkalinity, Ba, K, and B concentrations decrease. The maxima and minima coincide with the deepest of three fault zones observed in cores from this site (400–420 m CSF). Pore fluid concentration reverses below 420 mbsf in the lowermost 1–3 samples at this site. Pore water chemical changes within Unit III could result from diagenetic changes associated with sediment composition or reflect communication with fluid of a different composition below.

In Hole C0007C, methane concentration increases with depth but remains low (<50 centration increases from the beginning of coring at 175 m CSF to a maximum of 5.3 mM at ~230 m CSF and decreases below 230 m CSF (Fig. F7). Further light hydrocarbons were only detected in one sample in Hole C0007C, and ethane is very low in Hole C0007D. The methane to ethane ratio indicates a biogenic origin. The amount of calcite is generally very low; the highest values in Hole C0007C (up to ~5 wt%) are observed in Unit I sediments and at ~82 m CSF. At this depth, the highest TOC content (~0.7 wt%) was found for this site. A slight enrichment of TS was found in Unit I at ~6 m CSF. In Hole C0007D, calcite concentration increases with depth and shows higher amounts below 300 m CSF. The TOC content remains low throughout the sediment column and the C/N ratio indicates marine origin of the organic matter. Except for a peak at ~190 m CSF, TS is low and only increases slightly below ~360 m CSF in Unit III.

Based on fluorescent microscopic observation, the sediments at Site C0007 consistently harbor ~10⁸ cells/cm³ (Fig. F7). In the sand layers of the upper sedimentary units, very bright fluorescent signals of SYBR Green I–stained cells were observed, indicating that metabolic activities of microbes are generally high.

Site C0008

Site C0008 was proposed as contingency Site NT2-10A in an addendum to the Expedition 316 Scientific Prospectus (Kimura et al., 2007b). This site examined the basin seaward of the splay fault penetrated at Site C0004. Site C0008 is located ~1 km seaward of Site C0004 (Fig. F4). This site was selected during Expedition 316 because of its good fit within the available time window and the scientific objectives of Expedition 316. Recovered material from this basin will help assess the timing and relative age of past fault motions via identification of provenance and age of basin material. Ages across any unconformities between newly deposited sediments and older uplifted fault blocks will constrain the deformation history. The basin material will provide reference properties for the section overrun by the splay fault. Shipboard and shore-based study of these properties will constrain the fluid flow and consolidation response of the footwall to splay fault movement. In situ temperature measurements were also collected to assess the thermal gradient and heat flow in this area for comparison with the temperatures recorded at Site C0004.

Two lithologic units were identified at Site C0008 (Fig. F8). The uppermost unit (lithologic Unit I) consists of a 272 m (in Hole C0008A) succession of hemipelagic silty clay with thin sand beds and volcanic ash layers. In addition to the discrete ash layers, volcanic glass and pumice are disseminated as a significant component within the sediments. At the base of Unit I, a 40 m section of clayey gravel containing rounded clasts of mudstone and pumice constitutes Subunit IB. This subunit is interpreted as a MTC accumulated in the lower slope basin, possibly during an early stage of basin formation. The Pleistocene/​Pliocene boundary is found within Subunit IB. Unit II includes ~57 m of sand-rich sediment for which there was very limited recovery. This sand, along with a minor gravel component, contains a diverse detrital grain assemblage that includes clasts of sedimentary, metasedimentary, plutonic, and volcanic rocks.

Structural observations of the two holes drilled at Site C0008 indicate that sediments are nondeformed to weakly deformed. The main structural features consist of subhorizontal bedding and normal faults (Fig. F8). Deformation bands and sediment-filled veins were not observed. Normal faults do not show any preferred orientation, suggesting that they reflect vertical compaction. A high concentration of normal faults is found in Hole C0008C between 35 and 80 m CSF, and a 5 cm thick gently dipping shear zone was observed by computed tomography (CT) scan image analysis at ~41 m CSF. These features could correspond to the discontinuity in bedding dip that is apparent in seismic reflection profiles.

Two microfossil groups, calcareous nannofossils and radiolarians, from core catcher samples were analyzed for biostratigraphy at Site C0008. Moderately preserved and abundant calcareous nannofossils and radiolarians were seen in samples from the upper part of the sequences in Holes C0008A and C0008C, whereas they are relatively poorly preserved in the lower part of the sequences in both holes. For sediments recovered in Hole C0008A, calcareous nannofossil zones from Pleistocene Zone NN21 to Pliocene Zone NN16 were recognized and Pleistocene radiolarian Botryostrobus aquilonaris and Encyrtidium matuyamai zones were determined. Between 11.92 and 21.31 m CSF, a transition from Zones NN21 to NN19 is observed, suggesting low sedimentation rates or an age gap. Both calcareous nannofossils and radiolarians suggest an age reversal at ~90 m CSF; postcruise work will evaluate whether this reversal is an artifact of reworking. No reliable ages were obtained below 282.35 m CSF. A sample at 329.36 m CSF indicates a Miocene Zone NN11 age; however, because this was an isolated clast, it is not clear if this is representative of the formation.

Paleomagnetic studies in Hole C0008A indicate that NRM averages 10 mA/m between 0 and 45 mbsf, 39 mA/m between 40 and 150 mbsf, and 207 mA/m between 160 and 270 mbsf. Magnetic susceptibility and NRM intensity variations through sedimentary units are closely correlated. Two sharp increases in magnetic susceptibility are present at ~45 and 165 m CSF. Interestingly, the stable inclinations also switch polarity from normal to reversed at ~165 m CSF, suggesting that characteristic susceptibility and NRM intensity response might be useful for identifying geomagnetic event boundaries. Variations in magnetic susceptibility are probably caused by variations in the magnetic mineral type or in the content of magnetic minerals in the observed volcanic ashes and silty clay with depth. Parts of the magnetic polarity interval were able to be correlated with biostratigraphic data to refine the age history at the site.

In Hole C0008A, porosity from discrete samples decreases with depth from ~65% at the surface to 50% at 270 m CSF (Fig. F8). This is mirrored by an increasing trend of bulk density values. Bulk density increases with depth from ~1.60 g/cm³ at the surface to ~1.87 g/cm³ at 270 m CSF. Grain density is almost constant at ~2.69 g/cm³. Thermal conductivity values increase slightly with depth but are scattered because of gas expansion cracks in much of the core. Heat flow at Site C0008 is low compared to near-surface data but agrees well with heat flow at Site C0004. Heat flow estimates in Holes C0008A and C0008C yield similar results. Thermal conductivity values associated with Holes C0008A and C0008C vary between 0.7 and 1.2 W/(m·K) and show a great deal of scatter, possibly because of gas expansion cracks. In situ temperature was measured using the APCT3 in Holes C0008A and C0008C and supplemented by the sediment temperature tool in Hole C0008A. The best fitting thermal gradients are 51°C/km and 57°C/km in Holes C0008A and C0008C, respectively.

One of the main features in pore water chemical profiles at Site C0008 is the evidence for gas hydrate occurrence. In Hole C0008A, gas hydrate is evidenced by two sharp and local Cl minima detected at ~120 and 136 m CSF, in which Cl values were ~2%–3% less than modern seawater. These local minima coincide with concentration minima in Na, Sr, PO₄, and salinity, and the minimum at 136 m CSF is very near a maximum in methane concentration (at ~140 m CSF). After thermal infrared camera scanning in the cutting area showed negative temperature anomalies, we were able to sample gas hydrate–bearing horizons in Hole C0008C. Hole C0008C shows seven pronounced negative Cl concentration excursions between ~70 and 170 m CSF. The lowest Cl concentration was observed at 149.8 m CSF and was half modern seawater values. These Cl anomalies are accompanied by minima in the concentration-depth profiles of other chemical species. These coincident anomalies were predominantly associated with ash or coarse sand layers and indicate horizons of high gas hydrate concentration in Hole C0008C.

Inorganic geochemical data suggest that the SMT is reached at 6–10 m CSF in Hole C0008A and between 4 and 6 m CSF in Hole C0008C. The SMT depth at this site is much shallower than the other sites drilled during Expedition 316. Methane concentration increases sharply from the SMT zone and reaches 8.5 mM at ~15.3 m CSF in Hole C0008A. Below this depth, methane decreases to ~2 mM at 60 m CSF and remains at this concentration to the base of the sediment column in Hole C0008A, except for a localized enrichment at ~140 m CSF. At this depth, methane concentration reaches 15 mM, indicating the presence of gas hydrates. The C₁/C₂ ratio increases in the uppermost 15 m and then decreases to 372 at ~160 m CSF, but no further light hydrocarbons were detected.

Calcium carbon content is relatively high in the upper 50 m (with concentration as high as 22.3 wt%) and decreases in the lower portion of the site. Total carbon and TN concentration remains low throughout the site and shows a strong positive correlation. The C/N ratio remains generally low, indicating marine origin of the organic matter. TS content is rather high in the upper 50 m and in the lower ~60 m of the sediment column in Hole C0008A, correlating with the occurrence of iron sulfides.

Microorganisms inhabiting marine subsurface sediments play significant roles for biogeochemical cycling of one-carbon compounds, such as methane production and consumption. At Site C0008, more than 10⁸ cells/cm³ were observed throughout the sediment cores from Holes C0008A and C0008C. Microbial communities are mainly composed of very small coccoids and short rods. Cell abundances in Hole C0008C were slightly higher than those in Hole C0008A, suggesting that the flux of nutrient and energy substrates, as well as the occurrence of methane hydrates, controls the population size and activities of subseafloor microbial life.

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