T31B-1988
NanTroSEIZE Stage 1 operational review: Operational limits and data quality
Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) Stage 1 is first step in the ambitious,
coordinated multistage, multi-expedition project from the Integrated Ocean Drilling Program (IODP). Through
out the 138 days of continuous operations in three expeditions, two major challenges caused various
problems and delays. The Kuroshio current presented at all sites and more often at 3 knot or greater
velocity, and the difficult borehole conditions in this tectonically active faulting environment obviously caused
substantial loss of operational time, failure of equipments and loss of toolstring. Several other technical
problems, ranging from dynamic positioning system, remotely operated vehicle (ROV) to wait-on-weather,
made additional delays as well. During the course of these expeditions, Chikyu was able to test and refine
operational techniques and structures, and able to pioneer the use of new tools and equipments not only in
the lab area but also at the rig floor coring and logging operations. Those include first-ever and rarely-used
logging-while-drilling (LWD) tools in the scientific drilling history, new coring systems and techniques,
extensive use of X-ray Computed Tomography (XCT) on whole core sections and several others. Expedition
314 collected 4274 m of LWD logs and conducted 2285.5 m of Measurement-While-Drilling (MWD) logs from
pilot holes. Expedition 315 recovered 808 m of core from 1287 m penetration, and Expedition 316 recovered
1340 m of core out of 2103 m penetration. With intention of improving CDEX's expedition planning and
science services in the future, major review work is in progress focusing on operational, technological and
science services. Review on the non-productive time records found to be 23.5% for overall stage 1, which
gradually decreases from 33.7% at the beginning (Exp. 314) to 24.9% during the Exp. 315 and then 11.0%
in the Exp. 316. Review on the coring operations and functioning of coring tools in focus of core quality,
actions are taken in place on fixing problematic core operation instruments and making of new Extended
Punch Coring System (EPCS) which was developed and successfully tested during Expedition 316. Science
services review includes expedition planning and coordination, lab area management and services, and data
quality. Big tasks for the IODPfs Implementing Organization (IO) after the long cruises are quality check and
maintaining large volumes of data, cores, and samples gathered under operational difficulties. Post-cruise
additional data quality check is carrying out by lab specialists for the lab measurement results and as
collaboration work with tools and data specialists from the service company for logging data.
http://www.jamstec.go.jp/chikyu/eng/index.html
T31B-1989
Estimation of gas hydrate saturation with temperature calculated from hydrate threshold at C0002 during IODP NanTroSEIZE Stage 1 expeditions in the Nankai Trough
During the IODP Expedition 314, conducted at Nankai trough accretionary prism, gas hydrate was observed at Site C0002. Gas hydrate beneath seafloor is promising energy source and potentially hazardous material during drilling. The precise estimation of gas hydrate saturation is important, but previous works have not considered the effect of the in-situ temperature. In this study, we propose an estimation method of gas hydrate saturation with temperature calculated from threshold of gas hydrate. Gas hydrate saturation was determined based on the Logging While Drilling (LWD) Expedition 314 data. The gas hydrate bearing zone was located between 218.1 to 400.4 m below seafloor. Archie's relation was used to estimate gas hydrate saturation. This relation requires the porosity, the sea water resistivity and formation resistivity. We determined porosity to be between ~70 to ~30% based on density log. Since the resistivity of sea water is temperature dependent, temperature profile (calculated temperature model) was determined from the thermal conductivity and the temperature at the base of the gas hydrate. In our calculated temperature model, the saturation increases from ~10% at ~220m to ~30% at 400 m below sea floor. Spikes that have a maximum value at 80% at sand layers were observed. We also estimated the gas hydrate saturation from the constant temperature profile in 12°C (temperature constant model). This resulted in almost constant saturation (~15%) with the high saturation spikes. We compared these saturations with the hydrate occupation ratio within sand layers derived from RAB image. The hydrate occupation ratio shows increasing trend with increasing depth, and this trend is similar to the gas hydrate saturation with the calculated temperature model. This result suggests that the temperature profile should be considered to obtain precise gas hydrate saturation. Since the high sedimentation rate can affect thermal condition, we are planning to estimate the influence of the high sedimentation rate on gas hydrate saturation.
T31B-1990
Pore Water Geochemistry of IODP Exp 315 and 316: The NanTroSEIZE Transect
IODP Exp 315 and 316 drilled six sites as part of the first stage of NanTroSEIZE, an international, multi-year endeavor to elucidate earthquake-related processes. These six sites form a transect southeast of the Kumano Basin, Japan with boreholes that penetrated the Kumano forarc basin (C0002), the megasplay fault region (C0001, C0004, and C0008) and the frontal thrust including sediment from the subducting plate (C0006 and C0007). One element of this drilling is to evaluate the relationship between pore fluid behavior and slip and deformation in the crust. To help address this relationship 322 pore water samples were extracted from sediment whole rounds. Each of the whole rounds was scanned (CT) before it was processed within a nitrogen-filled glove bag and squeezed to express the pore fluid. In addition, 15 samples from C0002 underwent the GRIND technique to gather baseline pore water chemical data for future deep drilling where highly indurated sediments likely exist. Each of these pore water samples underwent a range of analytical procedures at sea. Additional procedures were conducted ashore. Combined, these procedures resulted in data for 30 chemical species including the stable isotopic composition of O and H in water. These data provide the most thorough preliminary reports tables in the history of DSDP, ODP and IODP, and ongoing measurements include a range of isotopic (e.g., I, B, Sr, Li, C), ionic (e.g., REE), and organic measurements. We will present all of the data that appear in the preliminary reports including GRIND samples for comparison to squeezed samples. Our presentation will highlight changes in pore water composition along the transect of boreholes, putting individual site-related pore water chemical profiles in a broader context. Pore water profiles in the upper about 30 m of the sediment column are dominated by microbially mediated reactions with a highly defined sulfate–methane transition. Deeper within the sediment the dissociation of gas hydrates and diagenetic reactions dominate the composition of pore waters. On the basis of these pore water chemical analyzes, no clear evidence for present or past fluid flow exists, even along major faults that were bisected during drilling.
T31B-1991
Bulk and Clay Mineralogy of Mud from Nankai Trough IODP Expeditions 315 and 316
Bulk powder and clay-size fraction X-ray diffraction (XRD) analyses of hemiplegic mud from IODP Expeditions 315 and 316 provide useful constraints on relative mineral abundances. The main bulk minerals include total clay minerals, quartz, plagioclase, and calcite. The calcite content shows significant variation.Values remain low (generally less than 2%) in the accretionary prism. A gradual upward increase in carbonate content within the slope apron (to 46%) is consistent with reduced carbonate dissolution and implies that the seafloor was slowly uplifted to its present water depth above CCD. At the frontal thrust sites, the calcite content is low in the sandy turbidite deposits and the underlying upper Shikoku Basin facies (average 2%). In the Kumano Basin, the basal forearc basin has greater calcite concentration (average 16%) because the depositional environment was starved of significant terrigenous influx. Dilution of biogenic carbonate by high influxes of terrigenous silt and clay began with turbidite influx at ~ 1.6 Ma. The abrupt compositional shift across the basal unconformity is noteworthy. We regard this contact as a manifestation of uplift along a system of splay faults at ~ 5 Ma. The unconformity between slope apron and accretionary prism at Site C0001 is a manifestation of uplift at ~ 3.7 Ma, with a similar shift in composition.The contents of total clay minerals increases with age in the slope and frontal thrust areas. The quartz and feldspar content show roughly a reverse correlation with the clay content. It is noteworthy that the upper Shikoku Basin facies at the frontal thrust sites has more clay (average 63%) compared to the overlying sediments (average 44%).The clay minerals smectite, illite, kaolinite and chlorite make up most of the clay-size fraction. Increasing smectite concentrations with age have been documented by ODP Legs 131, 190, and DSDP Leg 87 in the western Nankai Trough. Thus, smectite dehydration should yield more fluid in the older accretionary prism. Clays may also decrease permeability of the sediments/rocks, which increases the likelihood of overpressured pore fluids. The removal of interlayer water of smectite during drying process also causes artifacts in measurement of porosity.
T31B-1992
New Magnetostratigraphic and Rock Magnetic Results From Sediments and Rocks of IODP Expedition 316
We report new magnetostratigraphic and rock magnetic results from cores recovered from IODP Expedition 316 sites, indicating that sedimentary sequence sampled at the Nankai Trough was formed during multiple polarity epoch and events. Expedition 316 drilling was conducted at 2 sites in the megasplay region (Sites C0004 and C0008) and 2 sites within the frontal thrust region (Sites C0006 and C0007). Stable characteristic remanent magnetization (ChRM) components are observed throughout the majority of the recovered cores, following removal of a low stability drilling-induced remanence. A number of clearly defined magnetic reversals can be discerned on the basis of changes in sign of inclinations of the cores from all four drilled sites. Inclination values of ChRM are moderate downward or upward, consistent with the expected inclination for the site. Rock magnetic study demonstrates that the ChRM components of both polarities are mostly carried by fine grains of magnetite. Parts of the magnetic polarity interval were able to be correlated with biostratigraphic data of the drill sites. The new magnetic results from this study help refine the age history of the sedimentation process and faulting activities of Nankai Trough seismogenic zone.
T31B-1993
Age discrepancy of pore water iodine-129 between forearc and accreted sediments from NanTroSEIZE Sites C0001 and C0002, Nankai Trough, Japan
Integrated Ocean Drilling Program (IODP) Expedition 315, part of the multistage Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), provided an excellent opportunity to compare the fluid regime in the old accreted and overlying young forearc sediments. We cored the Kumano forearc basin sediment downhole to the upper accretionary prism in the Nankai Trough, Japan, with the D/V Chikyu in 2007. The I-129 system, a recently developed cosmogenic isotopic system, has been applied to pore waters collected from two deep cores in order to determine sources for fluids in different parts across the trough. The iodine ages in pore waters collected from the landward site (C0002) are close to 30 Ma throughout the sediment column, suggesting uniform fluid system between the lithologic units. Similar ages are found only in the accretionary prism at the seaward site (C0001), those in the overlying basin sediment are, however, significantly younger, suggesting a limited migration of upwelling deep fluids beyond the lithologic boundary where porosity considerably drops. The results suggest that deep, old sources are responsible for most of the iodine in these cores, but that differences in hydrologic behavior, related to changes in porosity in the basin sediments, prevent deep fluids to migrate to the surface at the seaward site.
T31B-1994
Experimental characterization of seismic velocities in Nankai Trough samples
We have performed tests to characterize the seismic velocities in samples collected in the Nankai Trough off the Kii Peninsula (Japan) during NanTroSEIZE stage 1. To reproduce in-situ conditions, we determined velocities in samples after applying both lithostatic and fluid pore pressure. The objective of these tests is threefold: First we aim at comparing these velocities, determined on a small scale (a few cm), with larger scale seismic velocities acquired either during logging or through seismic reflection studies. Second, we plan to study the dependence of seismic velocity on applied effective pressure, to estimate the possibility of tracking potential fluid overpressures. Finally, we aim at studying the variations in seismic properties within the prism, using the different cores, which were collected in very distinct domains of the accretionary prism. In particular, logging seismic velocity data acquired during campaign 314 (Tobin et al., 2008) show large variations between Site C0006, located near the frontal thrust and Sites C0001/C0002/C0004, located much landward near the out-of-sequence-thrust: for a given porosity, seismic velocities are much lower near the frontal thrust than in more internal domains of the prism. Starting from this observation, we characterize experimentally the extent of these variations and propose interpretation regarding the consolidation state and microstructure of accretionary prism sediments.
T31B-1995
Iron Sulfides and Sulfur Isotopes in Sediments from the Nankai Trough, Japan
We present first results of the iron and sulfur geochemistry from sediments from the shallow mega splay fault system at the Nankai Trough, collected during the IODP 316 Expedition. The objective of this research is to better understand the impact of dynamic systems on biogeochemical processes as well as microbiological signals in deep subsurface sediments, and to study linkages between the sulfur and iron cycle and the impact on highly dynamic sedimentary systems on these cycles. We measured the concentrations of acid volatile sulfur (AVS) and chromium reducible sulfur (CRS) from sediments at stations C0004 and C0008. The sediments at these stations are characterized by shallow sulfate/methane transition zones where released hydrogen sulfide reacts with reactive iron minerals to form iron sulfides. The alteration of iron oxides and their primary signals is reflected in the AVS and CRS data. We furthermore determined the sulfur isotope composition of AVS, CRS and pore water sulfate. Our data show a high variability, that may be caused by different sulfur sources and biochemical sulfur cycling, which relate to the tectonic and sedimentary complexity of the Nankai Trough.
T31B-1996
Petrophysically Determined Lithofacies at the Nankai Trough Accretionary Prism: NanTroSEIZE, IODP Expedition 314
Characterising the physical properties and identifying boundaries within active accretionary prisms is necessary in understanding their behaviour and recent movement. In such unstable conditions core recovery is not always reliable, especially around fault zones. IODP Expedition 314 was the first stage of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), and used logging while drilling (LWD) technology to record continuous physical property data. We use iterative non-hierarchical cluster analysis (INCA) to quantitatively define the characteristics of the slope sediments and sediments within the accretionary prism at sites C0001 and C0004. A new and detailed log-based lithostratigraphy is developed, and positions of major boundaries, defined by the 3-D seismic profiles and initial interpretation of individual log responses, are refined. The results produce clusters that clearly distinguish the slope sediments and characterise formations within the accretionary prism. Boundaries which correlate to the seismically-defined unconformity between the slope sediments and the accretionary prism, and a boundary within the accretionary prism that corresponds to a mega-splay fault previously unresolved by individual log analysis and borehole images, are identified. Our study demonstrates INCA analysis of LWD data can accurately define boundaries and characterise sediments in an environment where core recovery could be incomplete.
T31B-1997
Quality analysis of sonic logs and comparison to seismic checkshots during@NanTroSEIZE Stage1 LWD
NanTroSEIZE (Nankai Trough Seismogenic Zone Experiment) Stage 1A, the first IODP (Integrated Ocean Drilling Program) expedition by the drilling vessel gChikyuh, was completed in off-shore Kii peninsula, Japan. Natural gamma ray, resistivity, resistivity image, density, porosity, sonic velocity, and seismic checkshot surveys were obtained with various LWD (Logging While Drilling) tools during the Expedition 314. Our target geology was faults and the accretionary prism, therefore the formation was very sticky and fractured. Moreover because the holes were drilled with a riserless system and without mud control, it was very difficult to keep the hole condition stable. The ultrasonic caliper indicates large washouts and the sonic data are noisy in most zones at all sites. The hole diameters are classified into three washout conditions: 9in, 9.5in and larger than 10in. The sonic quality is classified into six conditions: mud slowness, dispersive very slow compressional slowness, good quality, low S/N, no compressional signal and others. The good quality intervals of sonic logging are consistent with the intervals of 9in hole diameter. The low S/N and no compressional signal intervals of sonic logging are consistent with the intervals of larger than 9.5in hole diameter. The sonic quality has good correlation with hole conditions, especially hole diameter. The sonic quality log bring us not only velocity values but also hole conditions and formation information. This is first time in the scientific ocean drilling history to use the LWD seismic checkshot survey tool, which measures seismic signals transmitted by an air gun source at the surface with a downhole tool consisting of a hydrophone and a 3-component geophone. The new technology allows us to monitor partial waveforms and the first arrival time in real time. It assisted in evaluating the data quality and in navigating the drill bit location while drilling. Although the operation faced several tool failures under strong challenges (Kuroshio current and borehole conditions) and the memory data was noisy, the velocity profiles were very consistent with those of 3D surface seismic data and sonic logs. In the low S/N and no compressional signal intervals of sonic logging, the quality was low but the values of velocities are reliable judging from the seismic checkshot results.
T31B-1998
Seismic Attribute and Logging Data Correlation at IODP NanTroSEIZE Sites C0001, C0002, and C0004
As a part of a Core-Log-Seismic integration study for the NanTroSEIZE project, we compare seismic attributes to logging data at three drill sites in the Nankai trough. Seismic attributes are calculated from seismic traces and often expose new information relative to the conventional seismic section. We calculate several seismic attributes from the high quality 3D seismic volume and correlate them with the logging data. Site C0001 The envelope attribute section shows high amplitude in the slope sediments of Log Unit I. Conversely, Log Unit II is characterized by very low envelope amplitude. The characteristics of gamma ray and ring resistivity log data within Log Units I and II do not exhibit significant difference between these Log Units except for some spikes in the log data in Log Unit II. However the seismic envelope attribute shows a clear difference between these intervals. The dominant frequency attribute also shows remarkable difference between Log Units I and II: ~60 Hz in Log Unit I and ~30 Hz in Log Unit II. Site C0002 A zone of low envelope amplitude is detected in the 3D attribute map just below the Bottom Simulating Reflector (BSR) near Site C0002. In this zone, gamma ray and ring resistivity log data indicate relatively less perturbed values compared to the depths above and below. On the other hand, a zone of high envelope amplitude exists just below the low envelope amplitude zone mentioned above. In this zone, gamma ray and ring resistivity log data are both characterized by increased high frequency content. This zone corresponds to Zone B, where log data suggested the existence of free gas. Dominant Frequency attribute data are correlated with ring resistivity log data. Above the depth of the BSR, we observe two intervals of high dominant frequency (~60Hz) at the depth ranges of 2150-2220 m LSS and 2280-2330 m LSS. At these depths, ring resistivity data frequently show spiky high values. Between these intervals, at the depths of 2220-2280 m LSS, the dominant frequency is lower (~35Hz) and ring resistivity is less spiky. The spiky resistivity signals are correlated with the existence of hydrate at this depth range at site C0002. High dominant frequency may suggest concentration of gas hydrate around this site. Site C0004 The upper part of Site C0004 (Log Units I and II) is a low envelope amplitude zone. This zone includes slope sediments and a wedge shaped thrust package. The gamma ray and resistivity log data indicate remarkable variation within this zone, however the envelope amplitude attribute does not reflect this variation. Log Unit III is characterized by a high envelope amplitude and high dominant frequency (50-60 Hz).
T31B-1999
Interpretation of LWD Resistivity from Nankai Accretionary Wedge in the Light of Clay Physico-Chemical Properties
This first stage (IODP Legs 314, 315 and 316, from September 2007 to February 2008) succeeded in obtaining LWD downhole measurements and cores at several sites along a transect from the subducting plate to the Kumano forearc basin, including the frontal thrust of the wedge, a major splay fault, and the forearc basin and older wedge. The approach we use is to combine LWD resistivity data and properties (physical and physico-chemical) obtained on core samples to infer the water content and the distribution of water between clay minerals, pore water and fractures. Some of the physical property contrasts observed may be explained by variations in sediment compositions: clay vs silt/sand content and variations in the clay mineral composition. The cation exchange capacity (CEC) was measured by exchange with cobaltihexamine and is used as a proxy for the swelling clay content (e.g. smectite) of the sediment. We find that the CEC is larger in accreted sediments (more than 0.2 mol/kg) than in slope sediments (less than 0.2 mol/kg). A decrease in the CEC is also observed at Site C0006, at the toe of the accretionary wedge, over the transition from basin to trench facies. These variations of CEC also correspond to changes in physical properties. At a given depth, lithologies with higher CEC tend to have higher water content. The relationship between resistivity and porosity also changes across lithological transitions. We conclude that in the cored intervals, lithological differences could explain most of the deviations of water content from normal compaction trends. However, the zones of low resistivity observed below 420 m LSF at Site C0001 more likely have a different explanation. At the top of this zone, the comparison of LWD data with core data (resistivity and P-wave velocity) obtained in the lower part of Hole C0001H suggest a contribution of fracture porosity to the in situ properties on the formation.
T31B-2000
X-ray Computed Tomography Images of the Nankai Trough Seismological Experiment (NanTroSEIZE) Cores : Results From IODP Expeditions 315 and 316 and Comparison With log Density
In this paper, we present a procedure from which information contained in the 3D X-ray computed tomography (CT)images can be quantitatively extracted and transformed into very-high resolution core logs and core image logs including (1) the radial and angular distributions of density values, (2) the histogram of distribution of density and related statistical parameters, and (3) the volume, the average density and the mass contribution of voids and vugs, and the damaged and non-damaged core material fractions. In turn, these quantitative outputs (1) allow the recognition of bedding and sedimentary features, as well as natural and coring-induced fractures, (2) provide a high-resolution bulk density core log, and (3) provide quantitative estimates of core voids and core damaged zones that can further be used to characterize core quality and core disturbance. The procedure is illustrated on core data collected during IODP Expeditions 315 and 316 (Nankai Trough Seismological Experiment – Stage 1, Japan). Focusing on density estimates, XR-CT derived core bulk density is compared to Gamma-ray attenuation density measured on whole round core, moisture and density bulk density data from selected samples and LWD density log collected at similar sites during Expedition 314.
T31B-2001
Rare Earth Element and Trace Metal Composition of Pore Fluids in the Nankai Trough as Relates to the Lithostratigraphy and Deep Biosphere.
One of the primary scientific objectives of the NanTroSEIZE expeditions is to test the hypothesis that the earthquake cycle results in systematic changes in the chemical alteration of pore fluids and sediments through which they flow (Tobin and Kinoshita, 2006). Trace elements, particularly the rare earth elements (REE) are sensitive to subtle changes in temperature, pressure and host lithologies, making them excellent monitors of fluid movement along fault zones. However, because of the difficulty in properly extracting and analyzing REE, compounded with limited sample volume that is typically recovered from squeezing deep sediments, REE are poorly constrained in many sub-surface environments. We measured REE, Y and Cd in pore fluids recovered from six sites that were drilled during IODP Exp 315 and 316, using a modified extraction-HR-ICP-MS technique for low sample volumes (5 ml). These extractions were conducted by an automated system utilizing an 8-hydroxy-quinoline column that selectively immobilizes transition metals and REEs while allowing alkali and alkali earth metals and anions to pass through the column. Results of these analyses indicate that concentrations of REE vary by an order of magnitude throughout the sediment column at each site, one of which (C0002) sampled sediments at depths of 1043 meters below the seafloor. These results suggest that variations in pore fluid compositions result from diagenetic reactions with differing lithologies. On the basis of the REE data, there is no indication of present- day fluid flow and no indication of a deep-sourced fluid even though the sediment section bisected a variety of faults including splay faults and the decollement. There are, however, variations in REE concentrations associated with changes in redox conditions caused by microbial activity in the upper 30-50 m of the sediment column.
T31B-2002
Structural styles based on LWD data at IODP Exp. 314 and its comparison to seismic expressions
At four sites of IODP Expedition 314, off SW Japan, the borehole resistivity images were acquired as one of the Logging-While-Drilling (LWD) data set. We analyzed these images to extract orientations of faults, fractures and bedding planes, as well as borehole breakouts. Based on these structural features, the drilling intervals were classified into three domains at accretionary prism regions; a) shallow layers gentry dipping to SE, b) the intermediate interval consists of deformed and highly fractured layers, and c) deep layers gentry dipping to NW. The intermediate interval includes a number of steeply inclined fractures that dip to NW or SE and bedding planes gentry dipping to NW or SE. These suggest that this domain may be intensively deformed. Other two domains include just a few fractures and their structural styles maybe simple. These domains correspond well with continuous reflections of slope sediments, weak reflection patterns of deformed hanging-wall and continuous reflections of little deformed footwall on seismic profiles. The seismic reflection of the intermediate interval is quite weak, but the bedding planes on the borehole wall show very good agreement with the weak reflections on the profile. This suggests that our image interpretation is accurate enough to correlate the logging results with seismic profiles. The resistivity images thus can be used as a bridge to compare different scales of deformation architecture from core to seismic.
T31B-2003
Properties of anisotropy of magnetic susceptibility in slope sediments, Kumano Trough, Site IODP C0001, Expedition 315
IODP Expedition 315 cored at a slope apron in Kumano Trough (Site C0001). Seismic image of Site C0001 indicates a stratified upper sequence and underlying transparent sequence. A strong reflector is identified between both sequences around 200 m below seafloor. A total of 458-m sediment was recovered from C0001. The strong reflector turned out thick sand layers. The stratigraphic results revealed that the slope basin is composed mainly of Quaternary to late Pliocene fine sediment (Unit I). The bottom of Unit I is composed of a thick sand layer which overlays the late Pliocene to late Miocene accretionary prism unit (Unit II). It was suggested that there is a large time break around the unit boundary. Porosity profile documented a gradual decreasing pattern in Unit I with depth. However it shows an offset sharply in the top of Unit II to higher values and then decreases with depth. This unique pattern of physical properties certainly denotes some important change in the evolution of the slope apron around C0001. Sediment magnetic fabric senses changes that relate to the sedimentary and tectonic environment. Anisotropy of magnetic susceptibility (AMS) on samples from C0001 was measured to detect the environmental condition where sedimentary sequences were deposited. Anisotropy parameters, which were calculated from measured data, were used to express each magnetic fabric shape. L representing lineation of AMS shows a constant and very low value through Units I and II. On the other hand, parameter profiles of anisotropy degree P' and foliation F indicate downward increasing patterns in Unit I, and show sudden drops around the boundary between Units I and II, then reveal relatively constant and low values through Unit II. Downward increasing P' and F with a constant L in Unit I can be simply interpreted as foliated fabric developing with depth due to compaction. However constant values of P' and F in Unit II suggest that the environment where the magnetic fabric developed was not governed by vertical compaction. Some tectonic alteration of anisotropy and foliation of magnetic fabric might take place after original deposition of Unit II and before beginning of Unit I deposition.
T31B-2004
Structural evolution in accretionary prism toe revealed by magnetic fabric analysis: NanTroSEIZE Expedition 316, Sites C0006 and C0007
This study presents magnetic fabric analysis to examine internal structure of the accretionary wedge, especially at the toe of the prism in the Nankai Trough, off Japan. Two sites (C0006 and C0007) were drilled as a part of Integrated Ocean Drilling Project Expedition 316, which penetrated the sediment section including in-sequence thrusts and the frontal thrust. Measurement of anisotropy of magnetic susceptibility (AMS) provides insight into recorded strain during sedimentary and tectonic processes. Results from the upper part of the wedge show sedimentary acquired compaction fabric in general. In the lower part, AMS fabrics occasionally rotate almost ninety degree and suggest horizontal compression. In contrast, magnetic fabric did not show any correspondence to in-sequence thrusts or minor faults, which implies that those faults have developed with concentrated shear deformation without disturbing surrounding sediments. Dense sampling from two adjacent drilling sites clearly figured out a change in strain field which is reported by previous ocean drilling studies. Based on the results, we propose a model of structural evolution at the toe of the prism. Plunging sediments induce horizontal stress in the lower part of the wedge, which reduces the effective stress and forms high pore pressure anomaly and fracture zone. The frontal thrust is bended geometrically and terminates its activity in response to increase of friction that triggers initiation of the next- generation frontal thrust. The upper part of the wedge tilts accordingly that results in unstable slope of the wedge surface.
T31B-2005
Subhorizontal Extension of the Upper Plate at NantroSEIZE Sites C0001 and C0002
IODP Expedition 315 targeted the hangingwall of an out-of-sequence thrust fault that is thought to rupture during great earthquakes, and to be tsunamigenic. More than 300 faults observed in core from sites C0001 and C0002 provide constraints on the direction and sense of slip in the forearc basin fill/slope cover, and in the underlying accretionary prism. Paleomagnetic data allow ~28% of the fault kinematic data to be corrected to geographic coordinates. The inversion of these data for partial stress tensors indicates that in the cover sediments, deformation is dominated by normal faulting with maximum principal stress (Sigma 1) oriented subvertically. In the slope cover at site C0001 most of these faults are consistent with the minimum stress axis (Sigma 3) oriented NE-SW, approximately normal to the plate convergence direction. These faults record Quaternary trench-parallel stretching that may be associated with recent transfer of material to the accretionary wedge, or alternatively, with a releasing stepover in a forearc sliver bounding strike-slip fault. In the basin fill at Site C0002 normal faulting is associated with Sigma 3 oriented NW-SE, nearly normal to the Nankai Trough. These faults reflect Quaternary trench normal stretching likely associated with gravitational spreading of the forearc basin. Faults in cores retrieved from within the accretionary prism at both sites include abundant normal faults, and fewer reverse faults and strike-slip faults. Cross-cutting relations suggest that normal faulting postdates reverse and strike-slip faulting. At C0001 and C0002 the reverse and strike-slip faults suggest approximately trench-normal, subhorizontal Sigma 1 orientations. These faults appear to reflect convergence-related stresses. The normal faults at Site C0001 are best fit by a shallow NE- plunging Sigma 3, thus trench subparallel stretching similar to that recorded in the overlying cover sediments. At Site C0002 two stress configurations with steep Sigma 1 orientations appear necessary to account for the normal fault data. One subset of faults is best fit by a WNW-ESE trending Sigma 3, and another subset is best fit by a NNE-SSW trending Sigma 3. Cross-cutting relations suggest that the latter faults, which accommodate trench subparallel stretching, are younger than the former. In total, the fault kinematic data corrected to geographic coordinates provide constraints on 3-dimensional stress geometries in the hangingwall of an important out of sequence thrust. The data suggest that the accretionary wedge and overlying sediments at C0001 and C0002 record a sequence of stress configurations that culminates in trench subparallel stretching. This sequence may reflect the seismic cycle near the boundary between the inner and outer wedges as defined by Wang and Hu (JGR, 2006).
T31B-2006
Possible Strain Partitioning Between the Kumano Forearc Basin and the Slope of the Nankai Trough Accretionary Prism
A 12 km wide, 56 km long, three-dimensional (3-D) seismic volume acquired over the Nankai Trough offshore the Kii Peninsula, Japan images the Nankai accretionary prism, forearc basin and the subducting Philippine Sea Plate. We have analyzed an unusual, trench-parallel ~1200 m deep depression (a "notch") along the seaward edge of the Kumano forearc basin, just landward of the shallowest branch of the previously- mapped splay-fault system. The shape of this feature varies along strike, from a single, steep-walled, ~3.5 km wide notch in the northeast, to a broader, ~6 km wide zone with several shallower linear bathymetric lows in the southwest. We have mapped the area below the notch and found both vertical faults and faults which dip toward the central axis of the depression. Some dipping faults appear to have normal offset, consistent with the formation of a bathymetric low. Some of these dipping faults may join the central vertical fault(s) at depth, creating apparent flower structures. Offset on the vertical faults is more difficult to determine, but the dip and along-strike geometry of these faults makes predominantly normal or thrust motion unlikely. We conclude, therefore, that the notch feature is the bathymetric expression of a transtensional fault system. Possible causes for such a system in the forearc include variations in splay fault geometry and strain partitioning. By considering only the along-strike variability of the mapped splay fault, we were unable to explain a transform feature at the scale of the notch. Strike-slip faulting at the seaward edge of forearc basins is also observed in Sumatra and is there attributed to strain partitioning due to oblique convergence. The wedge and décollment strength variations which control the location of the forearc basins may therefore play a role in the position where the along-strike component of deformation is localized. While the obliquity of convergence in the Nankai trough is comparatively small (13-30 degrees), we believe it is still significant enough to account for the formation of the observed notch.
T31B-2007
Frictional Properties of Sediments from Nankai Trough IODP Expedition 316: Results from an Intermediate-Velocity Test
Shear deformation experiments were performed on discrete core samples from NanTroSEIZE drilling sites C0004 and C0006 (Exp.316), at normal stresses of 1.0 to 5.0 MPa and at slip rates from 0.0026 to 26 mm/s, with a rotary-shear, intermediate- to high-velocity friction apparatus at Kyoto University. To be used in the experiments, collected discrete samples were disaggregated, oven dried at 90 ° C for 24 hours and then sieved to eliminate clasts larger than about 0.1 mm. The experimental fault is composed of thin layer (< 1.0 mm) of the disaggregated materials, which is put between a pair of 24.8 mm diameter granite cylinders. A Teflon ring surrounds the fault in order to avoid a leak of the sheared material during the experiment. Our preliminary results show that the level of friction recorded for the tested samples are generally low, from about 0.1 to 0.3, over the range of experimental conditions used in this study. The velocity dependence of friction of the tested samples is complex and it varies for different samples. For example, sample from C0004D-26R-1 at 260.5 mbsf exhibits strong velocity strengthening behavior at slower velocities (0.0026 to 2.6 mm/s) and almost no or a weak velocity weakening behavior at the fastest velocities tested in this study (2.6 to 26.0 mm/s). In contrast, sample from C0006E-31X-4 at 232.7 mbsf exhibits a weak velocity weakening behavior to a step change in loading velocity at velocities from 0.0026 to 0.26 mm/s, with a transition to a subtle dependence of friction at velocities from 0.26 to 26 mm/s. Such transitions in velocity dependence with the increase of slip rate might affect the mode of sliding of faults within a shallow portion of the Nankai subduction zone. Additional work to examine associations between the velocity dependence and the composition of the tested materials over a wide range of experimental conditions is needed.
T31B-2008
Slope Failures and Subsequent Mass-Movement Deposition Related to Mega Splay Fault Activity: Results From Drilling the Nankai Accretionary Prism During IODP NanTroSEIZE Expedition 316
Expedition 316 of the Integrated Ocean Drilling Program (IODP) coring the Nankai Subduction zone offshore SW Japan yielded discoveries about the early stage of mega splay fault development, related gravitational slope-failure processes and subsequent mass-movement deposition in a slope basin seaward of the mega splay fault. Late Pliocene to early Pleistocene mass transport deposits (MTD's) in the lowermost part of the slope-basin stratigraphic succession (Site C0008) are characterized by poorly sorted, subangular to rounded mudclasts floating in a sandy, clayey silt matrix. Systematic comparison between clasts and matrix on the basis of lithofacies, physical property, biostratigraphy and clay mineralogy reveal that the clasts were soft or only slightly indurated during emplacement and that they derived from a internal local source (i.e no systematic differences in the lithofacies character between clasts, matrix and hemipelagic background deposits of the slope basin or on the slope apron). Furthermore, shipboard biostratigraphic and magnetostratigarphic data show that the time period of MTD deposition is time-correlative with a prominent stratigraphic hiatus between Pleistocene slope sediments overlying the accretionary prism in the hanging wall block of the mega splay fault (Site C0004). Core observations and X-CT images show mineralization along this stratigraphic unconformity suggesting seafloor exposure. Hence, the angular unconformity is interpreted to represent a buried landslide scar. Our data thus support the hypothesis that the MTD are the depositional products of gravitational slope failures in unconsolidated slope apron deposits overlying the hanging wall block of the approaching mega-splay fault. To further investigate the relation between mega splay activity and gravitational slope failure processes through time, we discuss our findings with respect to a conceptual splay fault development model, constructed on the basis of seismic reflection data, seismic-to-core correlation and shipboard age constraints. Our study suggests that gravitational slope failures in the hanging wall block were most active during an early stage of thrust fault development. This may be related to slope angle readjustments along the newly formed wedge and/or enhanced fault activity as suggested by higher fault displacement rates between ~2.5 and 1.24 Ma.
T31B-2009
An Attempt to Estimate the Total Fracture Surface Energy of the Frontal Thrust
Quantifying the energy budget of an earthquake is a key to understand the physics of earthquake faulting. Recent geological observations of fault zone rocks have been used to estimate the fracture surface energy produced of earthquakes, although there remain discussions about the difference between geologically estimated fracture surface energy and gseismologicalh fracture energy. In Nankai Trough, very low frequency earthquakes have been discovered within the accretionary prism (Ito and Obara, 2006). NanTroSEIZE provides the only possibility to collect the material associated with low frequency earthquakes because the hypocenters of other low frequency earthquakes, slow slips and non-volcanic tremors are located deep within the prism. From this point of view, we present an attempt to measure the total fracture surface area of the frontal thrust to estimate total fracture energy associated with thrust motion. At site C0007 Hole D of expedition 316, most of displacement due to plate convergence is accommodated by the deformation within the frontal thrust zone. The frontal thrust and related thrusts consist of Fault zone 1 (237.5-259.3mbsf), Fault zone 2 (341.5- 362.3mbsf), and Fault zone 3 (398.5-446.0mbsf). Fault zone 1 contains 5.02 m-thickness of fractured rocks, and 0.36 m-thickness of fault breccias (with 14.782 m-thickness of no core recovery); Fault zone 2 contains 7.325 m-thickness of fractured rocks, 0.15 m-thickness of fault breccias, and 0.01 m-thickness of fault gouge (with 6.285 m-thickness of no recovery); Fault zone 3, the main frontal thrust, contains 12.64 m-thickness of fractured rocks, 5.485 m-thickness of fault breccias, and 0.18 m-thickness of fault gouge (with 22.3 m- thickness of no recovery). Based on this classification of fault rocks, and assuming the distribution of fault rock types in the intervals of no recovery are the same as in the recovered part, we have begun quantification of fracture surface area of for each fault rock type using microscopic observation of petrographic sections. Preliminary analysis indicates the total surface area of fractured rocks does not appear to be very large because of the lack of very fine grained material produced through fracture and crushing. In our presentation, we show the methods and tentative results of surface area estimation of the fractured rocks and fault breccias.
T31B-2010
Deformation-induced dehydration structures in the Nankai accretionary prism
This study investigates the chemical changes caused by deformation in the hanging wall of a major, probably seismogenic thrust fault in the Kumano forearc basin, Nankai Trough. In cores from IODP Expedition 315 (site C0001), the clay sediments display numerous deformation structures including tilted beddings, decimeter scale faults and shear zones with normal or thrust offsets, and clusters of parallel curviplanar veins interpreted as earthquake-induced dewatering structures. Curviplanar veins are often observed to merge into small oblique shear zones with millimeter offsets, or to branch on larger shear zones with a ~30° angle. This suggests that some shear zones may form by the coalescence of veins. Curviplanar veins and shear zones appear darker than the surrounding clay at the macroscopic observation scale, and brighter and therefore denser under CT-scan imaging. At the micro-scale, clay has a preferred crystallographic orientation in the deformation structures and no preferred orientation outside. Electron probe micro-analysis reveals that the dark material has a higher sum of major elements (65-80 wt%), i.e. a lower volatile content (assumed to be mostly water) than the host sediment (50-60 wt%). All the major elements are equally enriched in proportion to the volatile depletion. Mass balance calculation indicates that a 20-30 wt% water loss is required to account for chemical change in the deformation microstructures. The water loss may be due to clay dehydration or to pore collapse. Shear zones are equally dehydrated as the curviplanar veins from the mass balance standpoint. In 1 m3 of sediment, a deformed volume of 1 % should produce about 6.2 L of water. Given the low permeability of the sediment, dehydration may increase the pore pressure and enhance further deformation. Deformation localization would be self-sustained by fluid overpressure, suggesting that dewatering veins may evolve into larger deformation structures after an earthquake.
T31B-2011
Structural and Seafloor Morphological Evidence for Collapse of the Nankai Trough Accretionary Prism
Recent models for the evolution and structural architecture of accretionary prisms have suggested that the overall behavior of these systems is governed by the dynamic relationship between the geometry of the basal thrusts and the shape of the seafloor, coupled with the state of the basal thrust (slipping, creeping, or locked), the mechanical characteristics of the prism rocks, and the structure of the internal parts of the prism. In the NanTroSEIZE study area, the seafloor along the outer edge of the accretionary prism adjacent to the trench (the prism toe) exhibits a number of features indicative of gravitational collapse, including several listric headwall scarps and associated debris-slide and landslide deposits. Cores collected from the toe of the prism during IODP Expedition 316 contain a wide array of structures, including several populations of faults exhibiting orientations consistent with the geometry of the inferred headwall scarps. Borehole temperature measurements collected near the toe of the prism (and near areas affected by these inferred submarine slides) give rise to anomalously low heat flow values. Taken together, these data suggest that the toe of the Nankai Accretionary Prism offshore the Kii Peninsula, Japan, is currently in a state of collapse. The expression of this mechanical state is found in the along-trench succession of landslide andother gravitational collapse features, the arcuate or listric fault systems that form the detachment surface for these slides, and the inferred infiltration of seawater into the prism via these fault systems, resulting in drastic (and ongoing) cooling of the prism toe by advecting seawater.