Site C0002 Research Articles

Fault strength, healing and stability in the Nankai Trough accretionary prism off Kii Peninsula, Japan, as illustrated by friction experiments on gouge of a cored sample

In order to investigate fault strength, healing and stability in the Nankai Trough accretionary prism off Kii Peninsula, Japan, we conducted two series of triaxial friction experiments on gouge of a silty-claystone sample cored from 2183.6 mbsf (meters below seafloor) at IODP Site C0002, at confining pressure (Pc), pore-water pressure (PH2O) and temperature (T) conditions simulating those in situ at 1000–6000 mbsf there; rate-stepping tests at axial displacement rates (Vaxial) changed stepwise among 0.1, 1 and 10 μm/s, and slide-hold-slide tests at Vaxial = 1 μm/s with hold time (th) ranging from 10 to 104 s.Experimentally determined steady-state and static friction coefficients, μss and μs, respectively, and the log-linear th dependence of frictional healing, β, exhibit a decrease with simulated depth down to 3000 mbsf at which condition T was 100 °C, followed by an increase toward 6000 mbsf. On the other hand, the rate dependence of μss, a – b, gradually decreases with simulated depth, changing from positive at ≤4000 mbsf through ∼0 at 5000 mbsf to negative at 6000 mbsf at which condition stick slips were observed.Our experimental results suggest the presence of a low fault-strength and weak fault-healing zone at ∼3000 mbsf beneath IODP Site C0002, possibly due to elevated pore pressure induced by smectite dehydration. This zone correlates well with the previously reported low seismic-velocity zone and the source area of very low-frequency earthquakes to the south. Our experimental results also suggest that faulting beneath IODP Site C0002 is stable and aseismic at ≤4000 mbsf, transitional at 5000 mbsf, and potentially unstable and seismic at 6000 mbsf. In fact, stick slips corresponding to seismic faulting were observed at the 6000 mbsf condition. This implies that faulting along the plate-boundary thrust located at ∼5200 mbsf beneath IODP Site C0002 is potentially seismogenic.

Physico-chemical properties of illite in the organic matter (OM)-rich claystone of Nankai Trough contrast with coeval OM-poor claystone: IODP Expedition 348

Naturally occurring microbially induced smectite-to-illite (Sm-I) reaction is of great interest to understand the control for the conventional Sm-I transformation, the source of bioavailable iron (Fe) in sediments, and the evolution of the microbial community in the deep biosphere. Direct comparisons of physico-chemical properties of illite-like phases including collapsed high-charge smectite and transient phase, and mature illite were performed for the organic matter (OM)-rich claystone in contrast with those of coeval OM-poor claystone (2163 m–2217 m below sea floor) buried at Nankai Trough (Integrated Ocean Discovery Program Site C0002). These clay layers are interbedded with 70°–90° dipping angle, such that they have nearly identical burial conditions of temperature, time, and fluid geochemistry except for OM content, one of the significant controls that can fuel heterotrophic microbial Fe (III) reduction. Unlike the abiotic Sm-I reaction, the local variations in the crystal structure, elemental composition, and oxidation states of Fe for the illite and illite-like phases in the OM-rich claystone suggest that illitization is promoted through the reduction of structural Fe (III) in smectite by microbial activity, however the effects of OM cannot be ruled out.

Effective Porosity Profile at IODP Site C0002 in the Heart of the Nankai Accretionary Prism, and Its Use for Predicting In Situ Seismic Velocities

AbstractUsing logging data and samples collected by the four expeditions of the International Ocean Discovery Program NanTroSEIZE drilling campaign that occupied Site C0002 in the Nankai margin located southwest of Japan, we determined effective porosity and sonic velocity profiles down to 3 km below the seafloor in the accretionary prism, above the seismogenic plate interface. We measured cation exchange capacity to differentiate clay‐bound water content from effective porosity, which is representative of compaction. The decrease in effective porosity with depth is fitted with a single exponential decay, which suggests hydrostatic conditions for most of the borehole. The Erickson‐Jarrard template was adapted to account for the actual clay‐bearing nature of the sediments. We used this improved P‐wave velocity–effective porosity relationship to fill shipboard data gaps and predict P‐ and S‐wave velocity profiles at Site C0002.

Potassium-isotope variations of marine sediments adjacent to the Izu-Bonin Trench and Nankai Trough

Characterizing the K-isotope compositions of marine sediments is a necessary step to using this novel isotope system to investigate K cycling both in the ocean system and in subduction zones. Here we report data for 70+ samples in order to characterize the K-isotope compositions of deep-sea sediments for two drill sites in the western Pacific: (1) IODP Site C0011 adjacent to the Nankai Trough and Southwest Japan arc and (2) Site 1149 adjacent to the Izu-Bonin trench and arc. Variation in δ41K at both sites is mostly subtle, reflecting a consistently strong contribution of K from sources that have δ41K similar to the upper continental crust (UCC). Yet, in detail, the mean δ41K at both sites is about 0.05–0.10 ‰ lower than a recently proposed best estimate for UCC (δ41K = −0.44 ‰). We infer that the slightly low mean δ41K values relative to UCC are likely caused by secondary processes acting on the volcanogenic component of the sediments, though chemical weathering of (non-volcanogenic) terrigenous materials may also play a role. We measured a very low δ41K value (−1.33 ± 0.04 ‰) for one discrete ash layer at Site C0011, so volcanoclastic materials may lower the δ41K values of the bulk sediments, despite only accounting for a small fraction of the K in the sediments. Correlations between δ41K, Cr/Al, and K/Al in the upper sediments of Site 1149 were used to investigate mixing between volcanic ash and terrigenous materials. This approach indicates that mixing of isotopically light volcanic ash (with δ41K about −0.75 ‰) and terrigenous materials (about −0.46 ‰) can account for the observed variation in δ41K values in these sediments. In addition, we observe that at Site C0011 K isotopes exhibit a shift (by ~0.08 ‰) at ~250 m below the seafloor from UCC-like compositions in the upper sediments to slightly lighter compositions in the lower sediments. This shift in δ41K coincides with an abrupt change in certain physical properties (porosity and cementation) of the sediments that may relate to the extent of weathering/degradation of the dispersed volcaniclastics, with more extensively weathered materials present in the lower sediments. Finally, we find evidence for variable admixture of a high δ41K silicic volcanic ash in the lowermost sediments at Site C0011, supporting the view that heterogeneous volcaniclastics can give rise to significant δ41K variations in marine sediments.

Provenance study of the Miocene hemipelagic sediments in the Shikoku Basin and implication for the earlier history of the Kuroshio Current

The Kuroshio Current (KC) is the biggest western boundary current in the northwest Pacific Ocean and plays an essential role in the Cenozoic climate evolution and ocean circulation changes. Paleoceanography of the Shikoku Basin, located downstream of the KC region, is sensitive to the evolution of the KC. In this study, the Miocene hemipelagic sediment supply in the Shikoku Basin has been investigated on sediments from lithologic Unit III of IODP Site C0011 using a combination of rock magnetism, Sr isotope composition, grain size analysis, and calcareous nannofossil study. Integrated results reveal that multidomain (MD) (titano-) magnetite concentration increases in the upper part of Unit III since ~10–11 Ma with elevated 87Sr/86Sr values (0.712–0.714) and fining up sediment grain size. Magnetic parameters, Sr isotope composition, and grain size, as reliable indicators of sediment sources, have been used to discriminate the geological sources of the sediments and attribute the increase in MD (titano-) magnetite concentration to the increasing detrital input from the East China Sea (ECS), which is rich in fine-grained particles and coarse-grained magnetite and have high 87Sr/86Sr ratio (0.712–0.727). Furthermore, the intensified western boundary current later became known as the KC is interpreted as a plausible reason for increasing fine-grained flux from the ECS. Our study provides a much earlier record of the KC at ~10–11 Ma, giving new insight into understanding the Cenozoic ocean circulation changes and climate evolution in the northwest Pacific Ocean.

Depth profile of frictional properties in the inner Nankai accretionary prism using cuttings from IODP Site C0002

We conduct frictional experiments using cuttings collected at Nankai Trough IODP Site C0002 over 980.5–3262.5 mbsf (meters below seafloor) depth interval to better understand the frictional properties through the accretionary prism. The experiments are conducted at the in situ effective normal stresses (9–37 MPa) under brine-saturated conditions, and the slip velocity is abruptly changed in a stepwise manner to either of 0.3, 3, or 33 µm/s after the steady-state friction is reached. The friction coefficient (μ) of the cuttings samples ranges from 0.45 to 0.60, with a slight increase in μ with increasing depth, related to decreasing smectite content. The velocity dependence of friction (a − b) is positive at all depths and ranges from 0.001 to 0.006, which indicates a velocity-strengthening behavior; these values are consistent with relatively homogeneous deformation microstructures. The critical slip distance (Dc) ranges from 0.5 to 123 μm, with relatively large values obtained for the smectite-rich samples. The changes in both the friction coefficient and rate- and state-friction parameters are likely associated with mineralogical change and consolidation with increasing depth. Although all of the cuttings samples collected from Site C0002 exhibit a velocity-strengthening behavior, a slight decreasing trend in a − b with increasing depth indicates either a nearly neutral velocity dependence or a possible transition to velocity-weakening behavior at greater depths, which may be attributed to the occurrence of slow earthquakes in the Nankai accretionary prism.

Deformation-enhanced diagenesis and bacterial proliferation in the Nankai accretionary prism

Abstract. Understanding diagenetic reactions in accreted sediments is critical for establishing the balance of fluid sources and sinks in accretionary prisms, which is in turn important for assessing the fluid pressure field and the ability for faults to host seismic slip. For this reason, we studied diagenetic reactions in deformation bands (shear zones and veins) within deep mud sediments from the Nankai accretionary prism (SW Japan) drilled at site C0001 during IODP Expedition 315, by means of microscopic observation, X-ray diffraction, and major- and trace-element analyses. Deformation bands are not only more compacted than the host sediment but are also enriched in framboidal pyrite, as observed under microscopy and confirmed by chalcophile-element enrichments (Fe, S, Cu, As, Sb, Pb). In tandem, one shear zone sample displays a destabilization of smectite or illite–smectite mixed layers and a slight crystallization of illite relative to its sediment matrix, and another sample shows correlated increases in B and Li in shear zones and veins compared to the host sediment, both effects suggesting a transformation of smectite into illite in deformation bands. The two diagenetic reactions of sulfide precipitation and smectite-to-illite transformation are explained by a combined action of sulfate-reducing and methanogen bacteria, which strongly suggests an increased activity of anaerobic microbial communities localized in deformation bands. This local bacterial proliferation was possibly enhanced by the liberation of hydrogen from strained phyllosilicates. We suggest that the proliferation of anoxic bacteria, boosted by deformation, may contribute to the pore water freshening observed at depth in accretionary prisms. Deformation-enhanced metabolic reactions may also explain the illitization observed in major faults of accretionary prisms. Care is therefore needed before interpreting illitization, and other diagenetic reactions as well, as evidence of shear heating, as these might be biogenic instead of thermogenic.

Sediment provenance, routing and tectonic linkages in the Nankai forearc region, Japan

AbstractUnderstanding sedimentary routing systems at continental margins can be challenging in tectonically active regions such as forearc basins. Here, we combine sand modal composition and detrital zircon U‐Pb data from the Kumano Basin and surrounding Nankai Trough region of Japan to explore sedimentation in an active forearc and its linkages to the regional tectonic history. Our analyses are on Miocene‐to‐Pleistocene samples collected by the Integrated Ocean Drilling Program (IODP) at inner and outer forearc basin sites C0009 and C0002, river samples from onshore Honshu, and published results from other Nankai IODP sites. Results show the sediments of many IODP samples are similar to that of the modern rivers. Data from C0009 demonstrate that the early accretionary prism already contained slope basins fed by sediments similar to those in the modern Kumano River by 7.1 Ma. The more distal C0002 section was predominantly fed by a system similar to the modern Yodo River, located to the west‐southwest. We conclude that the section was therefore tectonically transported, in agreement with the ‘mobile model’ of oblique tectonic convergence across SW Japan during late Miocene time. Deposition resumed after a ca. 5.6–3.8 Ma hiatus, with sediments primarily fed from the paleo‐Kumano River, with some input from the paleo‐Tenryu and Kiso/Nagara rivers located to the northeast. Input from the paleo‐Tenryu and Kiso/Nagara rivers was likely re‐routed towards the trench by ca. 1.3 Ma. Sediments similar to those of the distal Yodo River also make up part of the Kumano basin fill and are best explained by erosion of uplifted older accretionary prism sediments along the Megasplay Fault that bounds the forearc basin. Despite the temporal variability in sedimentation routed to the basin, basin‐fill dynamics appear to have been governed primarily by the critical‐wedge style dynamics driven by variations in frontal accretion and in‐ or out‐of‐sequence thrusting response.

The Impact of Grain Size on the Hydromechanical Behavior of Mudstones

AbstractThe consolidation behavior of mudstone is controlled by the clay fraction and affects a wide range of deformation and fluid transport processes in accretionary wedge systems. I analyze the compression and permeability behavior of six sediment mixtures based on uniaxial resedimentation and constant rate of strain consolidation data. The sediment mixtures are composed of varying proportions of hemipelagic mudstone and silt‐size silica resulting in clay fractions ranging from 56% to 32% by mass. The hemipelagic mudstone is from Site C0011 drilled seaward of the Nankai Trough, offshore Japan, during Integrated Ocean Drilling Program Expedition 322. I show that porosity and compression index consistently decrease and permeability consistently increases with decreasing clay fraction over vertical effective stresses ranging from 0 to 21 MPa. Backscattered electron microscope images reveal that matrix porosity declines and large, jagged pore throats are being preserved in compaction shadows between silt grains as the clay fraction decreases. I compare the behavior of reconstituted samples with that of intact core and field measurements and interpret that increasing creep with depth and different mudstone fabrics explain differences in their compression curves. Finally, I provide empirical compression and permeability models that describe the evolution of porosity (void ratio) and permeability with vertical effective stress and as a function of grain size. Characterizing the in situ hydromechanical properties of subduction inputs is critical in order to relate input sediments to those at frontal thrust regions and predict pore fluid pressures and compaction‐driven fluid sources within the outer part of the accretionary prism.

Mechanical Weakness of the Nankai Accretionary Prism: Insights From Vp Measurements of Drill Cuttings

AbstractThe relationship between P‐wave velocity (Vp) and unconfined compressive strength (C0) can be used to estimate in situ rock strength and stresses at depth in subduction zones. However, few reliable data on the Vp‐C0 relationship in deep accretionary prism sediments are available. Here, we determined the Vp‐C0 relationship down to ∼3.2 km in the Nankai accretionary prism from Vp measurements and indentation tests of drill cuttings retrieved from International Ocean Discovery Program Site C0002. In our results, the Nankai accretionary prism sediments tended to be weaker compared with estimates obtained by using other empirically determined Vp‐C0 relationships. We also examined the relationship between Vp and porosity in the accretionary prism. The Vp‐porosity relationship obtained by using only data of Nankai accretionary prism sediments showed that Vp generally increased with decreasing porosity; this result is consistent with published Vp‐porosity relationships for Nankai Trough sediments. The empirical relationships derived in this study should improve estimates of stress state, pore pressure, and fault strength in the Nankai accretionary prism.

Geochemical Variation of Miocene Basalts within Shikoku Basin: Magma Source Compositions and Geodynamic Implications

Shikoku Basin is unique as being located within a trench-ridge-trench triple junction. Here, we report mineral compositions, major, trace-element, and Sr-Nd-Pb isotopic compositions of bulk-rocks from Sites C0012 (>18.9 Ma) and 1173 (13–15 Ma) of the Shikoku Basin. Samples from Sites C0012 and 1173 are tholeiitic in composition and display relative depletion in light rare earth elements (REEs) and enrichment in heavy REEs, generally similar to normal mid-ocean ridge basalts (N-MORB). Specifically, Site C0012 samples display more pronounced positive anomalies in Rb, Ba, K, Pb and Sr, and negative anomalies in Th, U, Nb, and Ta, as well as negative Nb relative to La and Th. Site 1173 basalts have relatively uniform Sr-Nd-Pb isotopic compositions, close to the end member of depleted mantle, while Site C0012 samples show slightly enriched Sr-Nd-Pb isotopic signature, indicating a possible involvement of enriched mantle 1 (EM1) and EM2 sources, which could be attributed to the metasomatism of the fluids released from the dehydrated subduction slab, but with the little involvement of subducted slab-derived sedimentary component. Additionally, the Shikoku Basin record the formation of the back-arc basin was a mantle conversion process from an island arc to a typical MORB. The formation of the Shikoku Basin is different from that of the adjacent Japan Sea and Parece Vela Basin, mainly in terms of the metasomatized subduction-related components, the nature of mantle source, and partial melting processes.

Thermal Conductivity Profile in the Nankai Accretionary Prism at IODP NanTroSEIZE Site C0002: Estimations From High‐Pressure Experiments Using Input Site Sediments

AbstractDepth profiles of sediment thermal conductivity are required for understanding the thermal structure in active seismogenic zones. During the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), a scientific drilling project of the International Ocean Discovery Program (IODP), a borehole was penetrated to a depth of 3,262.5 m below seafloor (mbsf) at Site C0002. Because core samples obtained from below ~1,100 mbsf in an accretionary prism are limited, a thermal conductivity profile over such depths usually determined by laboratory measurements using core samples is not available. To obtain the thermal conductivity profile at Site C0002, we used core samples collected from sediments that overlay the incoming subducting oceanic basement at Nankai Trough Seismogenic Zone Experiment Site C0012, which can be considered to have the same mineral composition as the accretional prism at Site C0002. The thermal conductivity of the C0012 core samples was measured at high pressure to simulate subduction by reducing the sample porosity. We measured the thermal conductivity of six core samples from 144–518 mbsf at Site C0012 up to a maximum effective pressure of ~50 MPa, corresponding to depths greater than ~4 km below seafloor. We obtained an empirical relation between thermal conductivity λBulk in Wm‐1K‐1 and fractional porosity ϕ for the Nankai Trough accretionary prism as λBulk = exp(−1.09ϕ + 0.977). Based on porosity data measured using core/cuttings samples and data derived from P wave velocity logs, we estimate two consistent and complete thermal conductivity profiles down to ~3 km below seafloor in the Nankai Trough accretionary prism. These profiles are consistent with the existing thermal conductivity data measured using limited core samples.

Friction experiments under in-situ stress reveal unexpected velocity-weakening in Nankai accretionary prism samples

The Nankai Trough hosts diverse fault slip modes, ranging from slow slip events to megathrust earthquake ruptures. We performed laboratory friction experiments on samples collected by the Integrated Ocean Drilling Program offshore Kii Peninsula, Japan. This study systematically investigates the effect of effective normal stress on frictional strength and the velocity-dependence of friction for natural fault zone and wall rock samples collected from depths of 270 to ∼450 meters below seafloor (mbsf), and over a range of shearing velocity spanning from 0.01–30 μm/s. In addition, cohesive strength was determined before and after each velocity step experiment while the sample was unloaded. We tested both powdered and intact specimens at estimated in-situ effective stresses, as well as at higher stresses representing deeper portions of the megasplay fault (Sites C0004, C0010) and the frontal thrust zone (Sites C0007, C0006). The apparent coefficient of sliding friction μs varies between 0.22–0.53 and correlates inversely with clay content. Direct measurements of cohesive strength show that on average 11% of the residual shear strength, and up to ∼30% for some specimens, can be attributed to cohesion. Friction coefficient slightly decreases as a function of increasing effective normal stress, attributed to a decreasing proportion of cohesive strength. The lowest μs values are observed for samples from the frontal thrust zone Site C0007 and the megasplay fault Site C0010. All samples show a combination of velocity-strengthening and velocity-weakening behavior, but intact samples tested under in-situ effective normal stress and low shearing velocities (<∼1 μm/s) exhibit consistently large velocity-weakening frictional behavior. The observed velocity-weakening behavior is related to the induration state of the material, which affects the real area of contact along shear surfaces. This is supported by direct measurements of cohesive strength, showing that higher cohesion values in intact samples correspond with a more pronounced evolutionary effect in velocity step tests that, in turn, map to lower values of the rate-dependent friction parameter a−b. We propose that fault zones in the Nankai subduction zone are likely to be velocity-weakening, and thus potentially able to host the nucleation of unstable slip, from seismogenic depths to the seafloor. We also find that testing disaggregated fault zone samples and employing effective stresses exceeding those in-situ lead to overestimation of a−b, emphasizing the importance of testing intact samples under in-situ conditions in laboratory friction studies.

Simultaneous estimation of in situ porosity and thermal structure from core sample measurements and resistivity log data at Nankai accretionary prism

The shallow accretionary prism of the Nankai Trough is a location where both large interplate earthquakes and slow earthquakes occur. Since the physical properties of sedimentary materials are important topics for understanding the structure of the prism, numerous ocean drilling expeditions have been conducted in that region to obtain logging data and core samples. Although the physical properties of the obtained samples are normally measured onboard immediately after coring, estimations of in situ physical properties are difficult because of differences in laboratory and in situ physical conditions. Herein, we propose a new method for estimating in situ porosity from downhole electrical resistivity log data that evaluates in situ porosity and thermal structure simultaneously using correlations between the porosity and resistivity, and between the porosity and thermal conductivity that were established based on laboratory measurements. When constructing physical property correlations, X-ray computed tomography data play an important role in estimating the porosity of samples from which resistivity or thermal conductivity were measured. To validate our method, we compared the estimation with density log data collected at Site C0002 and found that the estimated in situ porosity shows good agreement with the in situ porosity converted from density log data. A comparison with porosity measured onboard for core and cutting samples showed that they are consistent with each other. With this new method, continuous distributions of in situ porosity and thermal structure can be estimated simultaneously based on resistivity log data and heat flow, which are basic quantities acquired during ocean drilling science expeditions.

Strength Profile of the Inner Nankai Accretionary Prism at IODP Site C0002

AbstractThe strength of accretionary prisms is important for understanding their formation and the generation of earthquakes along the underlying plate boundary faults. We establish a continuous depth profile of rock strength and possible ranges of the minimum and maximum horizontal stresses (Shmin and SHmax) based on indentation tests on cuttings samples at the Integrated Ocean Drilling Program (IODP) Site C0002 in the Nankai accretionary prism. The rock strength (i.e., differential stress) increases with depth from 5.1 and 11.0 MPa at 975 meters below the seafloor (mbsf) to 22.8 and 59.6 MPa at 3005 mbsf in normal and reverse faulting regimes, respectively. The minimum value of Shmin and the maximum value of SHmax range from 33.2 and 49.4 MPa at 975 mbsf to 57.1 and 139.6 MPa at 3005 mbsf, respectively. Our estimated strength, in combination with Coulomb wedge model, may improve understanding of the evolution of accretionary prisms.

Observed stress state for the IODP Site C0002 and implication to the stress field of the Nankai Trough subduction zone

Conducting a stress state analysis is one of major challenges for evaluating earth crust and formation conditions. Stress magnitude is especially essential to comprehend stress heterogeneity, significant structural anisotropy, and/or pre-existing fracture systems. Stress state is influenced by rock strength, structural properties, and near-field principal stresses, which impact borehole integrity during drilling. Factors that we can measure through Logging While Drilling (LWD) and Measure while Drilling (MWD) to better understand borehole conditions, include mud weight, overburden weight and velocity. The LWD resistivity images in Holes C0002A and C0002F of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) indicate that most of the drillings in stable environments. However, the bottom hole assembly became stuck around 3 km depth in Hole C0002P and did not reach the mega-thrust fault. We evaluated the stress state for the Site C0002 by the geomechanical model. With the breakout width and rock strength estimation, we constrain the stress profiles in the drilled depth. Our analysis inferred that the instability of Hole C0002P had been caused by an overpressure drilling fluid state and low horizontal principal stress. Furthermore, we developed an optimally oriented plane (OOP) model to exam the stress state in the Site C0002 from Nankai accretionary prism to the Nankai Trough subduction zone. The normal faulting stress regime modeled from OOP is consistent with the geomechanical model in Site C0002. The horizontal principal stresses magnitude appears to less than the overburden weight above this subduction zone.

OH defects in quartz as a provenance tool: Application to fluvial and deep marine sediments from SW Japan

Hydrogen-related point defects (OH defects) in the crystal lattice of detrital, monocrystalline quartz have been proposed as an indicator to distinguish different sediment sources, making it a potential tool for provenance analysis. This study tests this novel technique on samples from SW Japan by comparing the results of OH defect analyses with those of “classical” provenance studies. OH defects of 188 detrital, monocrystalline quartz grains from five rivers (Fuji, Tenryu, Nagara, Yoshino, and Shimanto) and four offshore wells drilled in the Nankai Trough area (ODP site 1177; IODP sites C0002, C0007, C0012) are characterized using Fourier-transform infrared (FTIR) spectroscopy. Considerable differences in OH defect concentrations are identified between individual river samples. A general increase of values from east to west is observed. Possible links to the respective source geology are established. Very low OH defect contents (<1 wt. ppm water) of sediment from the Fuji and Tenryu rivers may be a result of high deformation and exhumation in the Izu-Honshu collision zone. Moderate to high OH defect contents of Nagara and Yoshino river samples (up to 50 wt. ppm water) are similar to the values of the global average in continental crust, possibly due to a mixing of various source lithologies. High and very diverse OH defect contents of Shimanto River sediments (up to 211 wt. ppm water) are likely an effect of extensive sediment reworking and mixing in the Nankai accretionary wedge. The OH defect signal is shown to be traceable through the offshore sedimentary record. In the western Shikoku Basin (ODP site 1177), turbidite deposits at 15 Ma and 7 Ma show a very similar signal as recent Shimanto river sediment, whereas eastern Shikoku Basin sediments (IODP Site C0012), at 15 Ma and 6 Ma strongly resemble present day Nagara and Yoshino river sediments. Sediments deposited in the Nankai trench wedge at 1 Ma (IODP Site C0007) indicate a shift towards lower OH defect concentrations. This may be due to increased, trench parallel sediment flux from the low-OH defect Fuji and Tenryu river sources, as proposed by previous studies. This study presents the first OH defect dataset in detrital quartz from rivers and offshore drill sites in a subduction zone environment, introducing OH defects as a new useful tool for sediment provenance analysis in active convergent margin settings.

Vertical profiles of arsenic and arsenic species transformations in deep-sea sediment, Nankai Trough, offshore Japan

Concentrations of arsenic (As) and its chemical forms were determined on deep-sea sediments drilled at three sites of Nankai Trough, off the Kii Peninsula, Japan. Those sediments were analyzed to document the behavior of As in relation to methane hydrate formation and the deep biosphere.The analytical results showed the total As concentration of interstitial water (IW) and squeezed cake (SC) ranged from 0.9 to 380 ppb and from 3 to 14 ppm (average, 6.4 ppm), respectively. The sediments from Site C0002, of which sediment column was the longest down to 2200 m below the seafloor (mbsf) among the studied three drilling sites, were analyzed for the host phase transformation of As. The total concentration of As of IW and SC from 200 to 500 mbsf, where methane hydrate zone was included, was higher than those from the uppermost 200 m. Concentration of As was ultimately controlled by pH. Also, organoarsenicals, such as methylarsonic acid (MMA) and arsenobetaine (AsB), were detected in the sediment column, implying that these organoarsenicals appeared in relation to the in situ microbial activities. These observations suggest that As becomes mobilized directly or indirectly as a result of microbial activity in deep-sea sediments.

Spatiotemporal Characterization of Smectite‐to‐Illite Diagenesis in the Nankai Trough Accretionary Prism Revealed by Samples From 3 km Below Seafloor

AbstractThe up‐dip limit of seismogenesis in subduction zone forearcs depends on the lithological composition of the incoming sediment and its subsequent modification during compaction and diagenesis. Here we present results of a multimethodological approach to characterize the smectite‐to‐illite diagenesis in the accretionary prism of the Nankai Trough subduction zone offshore SW Japan. Our X‐ray diffraction analysis, scanning electron microscopy, and transmission electron microscopy clay mineralogical analysis reveal that advanced states of smectite‐to‐illlite diagenesis occur in samples recovered from down to 3 km subseafloor at Integrated Ocean Drilling Program Site C0002, which sampled the inner accretionary prism. Our temperature‐ and time‐dependent reaction kinetics models require elevated temperatures in the prism to explain the illitization, which is consistent with revised thermal models based on recent plate reconstructions. Biostratigraphic data suggest that the inner prism sediment was deposited during a period of slow or inactive subduction and buried in the accretionary prism after the Philippine Sea plate resumed subduction. Rapid burial in the past 6 Ma led to the formation of two authigenic smectite‐illite phases consistent with a broad 1–9.3 Ma age determined by K‐Ar dating. The low K+ smectite‐illite reflects the long‐term burial history, whereas the K+‐rich smectite‐illite was inherited from the younger accretion event. Our study predicts the illitization of smectite approaches completion at ~5 km below the seafloor in the hanging wall of the plate boundary and the megasplay fault zone, respectively, which coincides with the proposed seaward extent of coseismic slip in the 1944 Mw = 8.1 Tonankai earthquake along those faults.

Geometrical dependence on the stress and slip tendency acting on the subduction megathrust of the Nankai seismogenic zone off Kumano

We assess the dependence of megathrust geometry near the updip edge of the Nankai seismogenic zone according to the slip tendency of the megathrust based on a reprocessed 3D PSDM seismic volume image and International Ocean Discovery Program (IODP) NanTroSEIZE drilling data off the coast of the Kii Peninsula, central Japan. The plate boundary fault surface is manually picked from the 3D seismic volume and is divided into three groups: low dip (10°–15°) trending NW to N in the NE region, intermediate dip (~ 25°) trending north in the western region, and high dip (30°–40°) trending NW in the SE corner. We then calculate the overburden (Sv) by converting the modeled 3D velocity to bulk density. Here, Sv ranges from 100 MPa near the SW edge to 160 MPa on the NE corner. In order to derive horizontal principal stresses (SH and Sh) and the pore fluid pressure (Pp), we assign the ratio of horizontal-to-vertical principal stresses, r (= SH/Sv), and the ratio of pore fluid pressure-to-vertical stress, λ (= Pp/Sv) based on IODP drilling data. The directions of SH and the slip on the fault are set parallel to the plate convergence vector (N55W). Assuming a triaxial condition (SH > Sh = Sv), the slip tendency (Ts) can be calculated from the dip angle and dip azimuth of the fault surface, r, and λ. We found that Ts is sensitive to the variation in fault geometry. For r = 1.2 and λ = 0.7 (normalized pore pressure ratio λ* = 0.25), Ts is low (~ 0.1) in the low-angle dip region and higher (> 0.2) in the high-angle dip region. This suggests that the high-angle fault is optimally oriented under this condition. Low Ts in the low-angle dip region would correspond to a weaker region due to a low frictional strength, assuming that the fault surface should slip simultaneously. Using the high pore pressure ratio (λ* ~ 0.85) beneath the fault zone, the fault beneath IODP Site C0002 is very close to failure if r > 1.2.