T21F-01
Estimation of strain accumulation around the Kumano Mega-Splay during IODP NanTroSEIZE Stage 1 expeditions in the Nankai Trough
In 2007-2008, the first series of IODP expeditions on Nankai Trough Seismogenic Zone Experiments were completed using the deep-sea drilling vessel 'Chikyu' of JAMSTEC. 8 sites were explored, from the accretionary toe, shallow portion of mega-splay thrust to the Kumano forearc basin of the Nankai Trough off Kii Peninsula. Through borehole breakout images acquired during Exp.314 LWD operation, we found that the direction of maximum horizontal principal stress is parallel or ~10 deg oblique to the direction of plate convergence in the accretionary prism (Sites C0001/C0004/C0006), whereas that in the Kumano forearc basin (Site C0002) is perpendicular to it. 3D seismic record shows clear thrusts and compressive features in the accretionary prism and extentional character in the forearc basin. Fractures identified either in the resistivity image and in the core samples indicate stress condition similar to estimate from borehole breakouts. These are indicative that the stress regime abruptly changes between Sites C0001/C0004 and Site C0002, each ~10 km apart, and that it has been caused by the movement of mega-splay fault that branches further landward of Site C0002 at ~10 km below sea floor, and reaches the seafloor seaward of site C0004. We are not sure whether the extentional feature at site C0002 and around is caused by gravitational instability or by elastic rebound due to uplift of the hanging wall. Here we carried out a simple estimation of strain and stress caused by the movement of mega-splay in order to assess potential stress and fracturing regime around the mega-splay. For simplicity we set the fault straight, because any kink can cause huge amount of stress singularity. Also we assumed a homogeneous elastic media The result is consistent with observation, showing that sigma1 is horizontal and perpendicular to the strike near the exit of the thrust, whereas sigma1 is vertical and sigma3 is parallel to the strike in the forearc basin region. By applying the coulomb criteria of rock failure, the condition of the normal faulting in the Kumano basin can be tested using the above estimates and the frictional coefficient estimated from the dip angle of normal faults. Overburden is calculated from the measured bulk density data; we ignored the effect of pore pressure in this study. Most of Kumano basin hanging wall region is either a potential normal faulting or a potential strike-slip faulting, and among this region only shallower part satisfies the failure criterion (normal/strike-slip faulting). The mega-splay fault system has been active, at least to produce slope failure and extention in its hanging wall side, for the recent few tens of kyears.
T21F-02
Characteristics of the fault rocks at the shallow portion of the megasplay fault system and the frontal thrust in the Nankai accretionary prism off Kumano
IODP Expedition 316 drilled at the shallow portion of the megasplay fault system (Site C0004) and the frontal thrust (Site C0007) in the Nankai accretionary prism off Kumano. At Site C0004, the megasplay fault is an ~60 m thick zone of fractured, brecciated, and microbrecciated mudstone and volcanic ash deposits. At least two biostratigraphic age reversals occur within the megasplay fault zone. Shear displacements along individual fractures and slip surfaces are generally small, and the original layering of mudstone and ash is preserved in the fault zone. Locally, thin (~1-cm-thick) shear zones are developed in the microbreccia. The shear zones are deficient in visible fragments, more dense than the surrounding material, and display fine clay foliations. The crosscutting relationship between the microbreccia and the thin shear zone, as well as the 3D geometry of the cm-thick shear zones observed in X-ray computed tomography images, indicate the recurrence of microbrecciation and localized shearing in the cm-thick zones. At Site C0007, the frontal thrust is defined by an ~50 m thick zone of fractured hemipelagic mudstone and ash, fault breccia, and foliated fault gouge. A significant biostratigraphic age reversal occurs in the lowermost part of the frontal thrust zone where hemipelagic mudstone is cut into fragments that are in 1-10 mm size along an anastomosing network of slip surfaces. Several ~2-mm-thick shear zones are present in this brecciated interval, and cross-cutting relations indicate a recurrence of brecciation and localized shearing along the mm-thick shear zones. Overall, the character of fracturing and brecciation in the megasplay fault and the frontal thrust zones are similar to that in the fault zones in the Nankai accretionary prism off Muroto. However, unlike the fault zones at Muroto, both the megasplay fault and the frontal thrust show clear evidence for repeated localization of displacement to ~2-10-mm-thick shear zones within broader zones undergoing distributed fracture and brecciation.
T21F-03
Geochemical Constraints on Fluid-Rock Reactions, Fluid Sources, and Flow Pathways Along the NanTroSEIZE Transect; IODP Expeditions 315/316
The aim of the NanTroSEIZE project is to core through fault rocks at a range of pressures and temperatures to determine if systematic, progressive material and state changes control the onset of seismogenic behavior on subduction zone thrust faults. Fluid chemistry is predictably altered with increasing pressure and temperature, and can facilitate the estimation of the temperature and depth of fluid sources, thus constraining the role of in situ diagenetic reactions and/or deeper-sourced fluid advection in producing the observed composition of fluids in fault zones. The end-member composition of any deep-sourced fluid advected within fault zones could provide critical insight into the mineral reactions occurring at depths beyond those penetrated during IODP Expeditions 315/316 and, potentially, within the seismogenic zone. Six sites were cored during IODP Expeditions 315/316. The mega-splay fault was cored at Site C0004, imbricate thrusts associated with the deformation front were drilled at Site C0006, and the deformation front was penetrated at Site C0007. The sediments underthrusting beneath the mega-splay fault were cored at Site C0008, and Site C0001 penetrated a slope basin and terminated before reaching the mega-splay. Site C0002 is the deepest site (~1,000 mbsf) and penetrated the Kumano forearc basin and the uppermost accretionary prism. Preliminary results show that the pore fluid profiles in the upper ~60 m of all sites are dominated by microbially-mediated reactions, and the SMT occurs at depths shallower than 20 mbsf, except for Site C0001 where it is located at ~30 mbsf and Site C0007 where it is deeper than 150 m. At depths greater than 60 mbsf, the pore fluid chemical profiles are predominantly controlled by in situ inorganic diagenetic reactions such as adsorption, ion exchange, and volcanic ash alteration. At Sites C0001, C0002, C0006, C0007, and C0008, there is chemical evidence for discrete intervals of elevated gas hydrate concentrations with pore space occupancies up to 69%; mainly associated with ash layers. Unlike at other subduction zones where there is clear evidence of deep-sourced fluid flow along the decollement and associated thrust faults manifested by elevated Li, B, and C2-C4 concentrations, and depleted Cl and K values (e.g. Muroto Transect, Costa Rica, Barbados), there are only minor variations in the pore fluid geochemical profiles along the mega-splay fault and deformation front, and no variations along the imbricate thrusts at Site C0006. These results suggest there is little contribution of deeper-sourced fluids to the shallow portion of the mega-splay and deformation front at present. There is some evidence for a deep-sourced fluid(s) at Site C0002; Cl concentrations increase linearly to the basal Kumano basin sediments, then remain relatively constant to TD at ~82% of bottom water value most likely as a result of clay dehydration. The pore fluids in the prism also contain elevated concentrations of propane. Collectively, these results suggest fluid migration from a source temperature >50-80 °C. Additional isotopic and elemental analyses are in progress, and the results will provide tighter constraints on the temperature, depth, and mineral reactions at the source of this fluid.
T21F-04
Initial results from the Nankai Trough shallow splay and frontal thrust (IODP Expedition 316): Implications for fluid flow
Integrated Ocean Drilling Program (IODP) Expedition 316 examined the frontal thrust and the shallow portion of the megasplay fault offshore of the Kii peninsula, and was the third drilling expedition of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE). NanTroSEIZE will integrate seafloor observations, drilling, and observatories to investigate the processes controlling slip along subduction zone plate boundary fault systems. Site C0004 examined a shallow portion of the splay fault system where it overrides slope basin sediments. Site C0008, located in the slope basin 1 km seaward of Site C0004, provided a reference site for the footwall sediments. Results of drilling indicate that the footwall sediments have dewatered significantly, suggesting permeable routes for fluid escape. These high-permeability pathways might be provided by coarse-grained layers within the slope sediments. In situ dewatering and multiple fluid escape paths will tend to obscure any geochemical signature of flow from depth. Sites C0006 and C0007 examined the frontal thrust system. Although poorly recovered, coarse-grained trench sediments were sampled within the footwall. These permeable sediments would be expected to allow rapid escape of any fluid pressures due to loading. At both sites, low porosities are observed at shallow depths, suggesting removal of overlying material. This observation is consistent with interpretations that the prism is unstable and currently in a period of collapse. Anomalously low temperatures were measured within boreholes at these sites. One possible explanation for the low temperatures is circulation of seawater along normal faults in the unstable prism.
T21F-05
Lithostratigraphy and Methane Hydrate Occurrence Estimated from Logging-While-Drilling at IODP Site C0002, Nankai Trough
During Integrated Ocean Drilling Program (IODP) Expedition 314 at Nankai Trough, located on the sudbuction margin offshore Kii Peninsula, five sites were drilled and logged by logging-while-drilling (LWD). A 935-m-thick basin fill was penetrated and logged at Site C0002 in Kumano Forearc Basin. The sediments consist of repeated turbidite sequences intercalated with background hemipelagic mud. A gas hydrate- bearing zone was inferred in the interval of 218-401 mbsf above a bottom-simulating reflector (BSR) by indirect evidences of electrical resistivities and P-wave velocities. More than sixty sharp resistivity spikes were identified and most of them are developed at the lower portion of sand layers. Occurrence of the hydrate layers localizes near the base of the BSR and gradually decreases upward. Hydrate occupation ratio within sand layers also decrease upward from 90% to 20% in the gas hydrate-bearing zone. Below the BSR, a number of turbidite sequences were identified and the thickest turbidite zone occurs at 482-547 mbsf as possible gas/fluid conduits. Distribution of turbidite sequence pays an essential role as potential conduits and containers for gas hydrate generation in Kumano Basin. Detailed observations of the LWD logs demonstrate a possible lithological control on gas migration and systematic distribution of gas hydrates in turbidite- dominant marginal basins.
T21F-06
Development of forearc basin - splay fault system inferred from shipboard results of IODP Expedition 315, Nankai Trough
IODP Expedition 315, entitled Megasplay Riser Pilot, was conducted from November 16 to December 18, 2007 near the seaward limit of the rupture zone of the 1944 Tonankai earthquake in the central Nankai Trough as one of the Stage 1 expeditions of the NanTroSEIZE (Nankai Trough Seismogenic Zone Experiment). Primary objectives of our expedition include acquisition of geotechnical information needed for well planning of future deep riser holes as well as scientific objectives for each drilling site. Site C0001 is located at the small bench on the hanging-wall of the megasplay fault and the footwall of the subsidiary fault. Seismic profiles show the slope basin with 200 m thick series of layered reflectors above the more transparent unit. The coring revealed that the slope basin was composed mainly of Quaternary to late Pliocene silty clay and clayey silt with numerous intercalations of volcanic ash layers. The bottom of the basin is composed of a thick sand layer which overlays the late Pliocene to late Miocene accretionary prism unit. Site C0002, located at the southern margin of the forearc basin, penetrated thick Quaternary alternation of fine-grained sandstone and mudstone (834 m thick) and basal Pliocene mudstone, and cored the late Miocene accretionary prism rock. The basal mudstone is characterized by low sedimentation rate with partly erosional condition, glauconite concentration and pervasive vein structure (mud-filled vein) suggesting sediment-starved conditions such as slope apron. The both sites are located at critical positions to understand splay fault activity and its implication for forearc basin development. End of thick sand layer deposition at Site C0001 and sedimentary environmental change at Site C0002, from condensed mud to forearc basin with rapid sedimentation at about 1.6 Ma, denote a timing of outer ridge building probably caused by splay fault activity.
T21F-07
Episodic sediment disturbance on the mega-splay fault at Tonankai earthquake area, Nankai trough, Japan
The cores at hanging wall (C0004) and footwall (C0008) of mega splay fault are obtained in the Stage 1A of the Nankai Trough Seismogenic Zone Experiment during IODP Expeditions 316 (Kimura et al., 2008). The mega splay fault develops in the area where the coseismic slip (Ichinose et al., 2003) and tsunami source (Baba and Cumming, 2005) are reconstructed for the 1944 Tonankai earthquake. The X-ray computed tomography (X-CT) makes 3D image depending on X-ray attenuation coefficient that is a function of chemical composition and density of the target material. Whereas optical observation my not resolve small differences, emphasizing by the CT-scanner enables to find small difference in sediment composition and structures. The CT image has been taken for all cores during preliminary core processing. Episodic event deposits are found at uppermost core of hanging wall (C0004). The core sample is composed of un-consolidated silt clay and looks homogeneous in optical observation. The X-CT can show complex structure indicating some horizontal layers, scattered mud clasts and mud-filling burrows. The X-CT 3D observation can distinguish a mud clasts and mud-filling burrow. Because tubular burrow is formed by mud- eating worm or mollusks, the cross section area is constant along long axis. On the other hand, the cross section area changes along long axis in distorted shape mud clast. The mud clasts are intercalated between horizontal layers, indicating episodic event. At least five events are observed. The boundary of some mud- clasts are dim, may have suffered diffusion, and this implies very short transportation or in-situ formation. Thus, repeated mud clast deposit was not found at footwall of the fault (C0008). Strong seismic ground motion can shake surface soft sediment, and sediment suspensions were often observed after large earthquakes (Thunell et al., 1999; Itou et al., 2000; Seeber et al., 2007). The difference of occurrence of sediment disturbance between C0004 and C0008 can be explained by hanging wall effect of seismic ground motion. It is considered that episodic sediment disturbance at C0004 was formed by coseismic rupture of mega splay fault.
T21F-08 INVITED
Kumano Forearc Basin Uplift and Megasplay Fault Development: 3D Seismic Images of Nankai Margin off Kii Peninsula, Japan
Off southern Honshu Island, Japan, a 12 by 56 km, 3D seismic reflection survey images parts of the region known as the Kumano area of the Nankai Trough. This region is where the Nankai Trough Seismogenic Zone Experiment is taking place as part of the Integrated Ocean Drilling Program and has thus far been the target of Expeditions 314, 315, and 316. A structural feature of the Kumano part of the margin is a high-seismic amplitude, regionally continuous splay fault system that acts as an out-of-sequence thrust and may be a pathway for updip seismic rupture propagation. Of seismic hazards interest, the shallower pathway for rupture propagation presented by the splay fault has been suggested to result in efficient delivery of high- angle slip close to the seafloor and thus the splay fault may be tsunamigenic. The Kumano forearc basin lies above the Kumano splay fault system and thus records its affect on margin development. Within the seaward portion of the Kumano Basin that is imaged by the 3D volume, we interpret 12 seismic sequences. In decreasing age, these sequences record a history of infilling and faulting as follows: 1) Sequences 12-9 were deposited in a shelf-edge depocenter that was deepest to the northeast due to a pre-existing structural high to the southwest. 2) From Sequence 8 time to Sequence 6 time, significant uplift occurred such that the primary depocenter shifted progressively landward. Northeast striking normal faults formed in the sediments above the uplifted southwestern structural high. 3) Since the deposition of Sequence 5, uplift has waned and the basin has attempted to re-equilibrate depositionally by filling in the lows and structurally through significant extension that continues today. The resulting normal faults are spaced <250 m apart, strike on average east-northeast, and are present up to ~20 km landward into the basin. The locus of uplift was limited to the outer basin, which was progressively thrust/displaced seaward relative to the inner basin as evidenced by the presence of graben structures at the landward limit of the tilted strata. The lateral extent of the tilted part of the Kumano basin overlies the previously reported high-amplitude part of the mega-splay reflection suggested to have originated from underthrusting of fluid rich sediments. We suggest that the uplift event recorded in the forearc basin corresponded to the formation of the mega-splay fault system and through integration of the sequence stratigraphy with drilling results we can place bounds on the timing of its formation.