ABSTRACT The Shikoku Basin is a back-arc basin located offshore southwest Japan. Sediments within the basin make up a key part of the subduction inputs to the Nankai Trough. A 19 m.y. history of sedimentation has been documented at Sites C0011 and C0012 of the Integrated Ocean Drilling Program (Kumano transect) and Sites 1173 and 1177 of the Ocean Drilling Program (Muroto and Ashizuri transects, respectively). This paper focuses on three noteworthy aspects of that history: (1) the onset of substantial pyroclastic influx, which shifted significantly along the strike length of the margin, from 3.3–3.9 Ma at Sites 1177 and 1173 to 7.6–7.8 Ma at Sites C0011 and C0012; (2) transport of sand by sediment gravity flows, which resulted in three discrete sand bodies during the Miocene (Kyushu, Daiichi Zenisu, and Daini Zenisu submarine fans); and (3) clay mineral assemblages within hemipelagic mudstones, which show systematic reduction of 3 wt% detrital smectite per 1 m.y. decrease in age. Collectively, these temporal and spatial adjustments of lithofacies and sediment composition have important implications for downdip and along-strike projections of frictional, geotechnical, and hydrogeological properties as strata enter the Nankai subduction zone. The stratigraphic positions of smectite-rich Miocene mudstones, for example, should match up with increases in the volume of fluid production by clay dehydration during subduction. The higher-permeability sand bodies (Kyushu and Zenisu submarine fans) should act as preferred conduits for focused fluid flow. The potential for buildup of fluid overpressures should increase above and laterally adjacent to stratigraphic pinch-outs of sand bodies, especially where the aquifers are inclined or confined between basement highs. These three-dimensional complexities set the Nankai-Shikoku system apart from other subduction zones (e.g., Japan Trench, Costa Rica) where inputs consist of comparatively homogeneous pelagic and hemipelagic deposits.

ABSTRACT Knowledge of rock thermal conductivity is necessary to understand the thermal structure in active seismogenic zones such as the Nankai Trough subduction zone, SW Japan. To estimate in situ thermal conductivity at the oceanic crust surface in the seismogenic zone, we measured the thermal conductivity of a basaltic basement core sample retrieved from subducting oceanic basement at the Nankai Trough Seismogenic Zone Experiment input site C0012 under high temperature (maximum 160 °C) and high pressure (maximum effective pressure 100 MPa), respectively. These conditions correspond to the in situ temperature and pressure at the oceanic crust surface in the updip limit of the Nankai seismogenic zone (~7 km below the seafloor). Thermal conductivity of the oceanic basalt is both temperature and pressure dependent. In contrast to other rock types such as sandstone and granite, for which thermal conductivity decreases with increasing temperature, the thermal conductivity of the oceanic basalt increased with increasing ambient temperature. The thermal conductivity of the basalt also increased with increasing effective pressure; however, the rate of increase was much lower than that for other rocks. These new temperature and pressure effect data for oceanic crust basalt fill a gap in the research. The estimated thermal conductivity of the basalt at in situ temperature and pressure conditions was less than ~2 W m –1 K –1 , although deformation and alteration associated with subduction could decrease pore spaces in the basalt, leading to enhanced thermal conductivity. This value is significantly lower than that typically assumed for thermal structure simulations in the Nankai subduction zone.

ABSTRACT To accurately estimate the coseismic rupture area in a Nankai megathrust earthquake and predict seismic and tsunami hazards, various three-dimensional models of the subducting plate geometry and simple seismic velocity models of the subduction zone in SW Japan have been proposed. However, to ensure consistency among studies, more realistic and reliable standard models must be developed. Here, we use wide-angle ocean-bottom seismographic survey results to develop models of the three-dimensional geometry of the subducting plate and of the three-dimensional P-wave velocity structure around the Nankai Trough. We confirmed the reliability of the proposed models by comparing theoretical first arrivals, calculated from two-dimensional structure models sampled from the three-dimensional model along seismic profile lines, with observed traveltime data. The proposed models are the first to be visible and to attempt to represent the actual seismic velocity structures around the entire Nankai Trough in SW Japan. Although the spatial validity of the three-dimensional velocity structure model could not be strictly evaluated, we confirmed that the differences between hypocenter parameters determined from previously published seismic tomography results and those obtained by using our three-dimensional structure model were sufficiently small (latitude and longitude within ±0.1° and depth within approximately ±5 km). Therefore, our three-dimensional structure model is suitable for use as an initial model for hypocenter determination.

ABSTRACT In subduction zones, fluid-pressure generation in the underthrusting section is of particular interest because it governs the effective stress conditions of the footwall lining the plate interface. Only a few studies have systematically explored the role of lithological heterogeneity of underthrust sediment on the resulting fluid pressure and its distribution. We used a coupled loading and pore-pressure dissipation model with a new compilation of sand properties to investigate the role of such heterogeneity on the drainage state beneath the plate interface in the western Nankai subduction zone offshore Japan, where the incoming sediment sequence hosts numerous sand layers with a total thickness of up to ~210 m within a matrix of hemipelagic mud. Our results show that sand layers act as important conduits for both pressure translation and solute transport from greater depth to the trench and seaward. The simulated pore pressure is mainly controlled by aggregate sand-layer transmissivity, and to second order by sand-layer depth, which affects the ability of fluids to access permeable sands from the surrounding less-permeable mudstone matrix. Modeled sand permeability in the outer subduction system is in the range of previous estimates for décollement zone permeability (10 –13 to 10 –16 m 2 ) and evolves to approximately three orders of magnitude lower permeability in the inner subduction system. The enhanced drainage leads to 15% lower excess pore pressures in models with sands than without sands. Thus, differences in the lithostratigraphy of the subducting sediment should have implications for the mechanical behavior along the Nankai subduction system.

ABSTRACT A 200-m-thick, near-vertical, middle Miocene (ca. 14 Ma), gabbroic sheeted intrusion in the Muroto area of the Shimanto accretionary complex of southwest Japan yields anisotropy of magnetic susceptibility (AMS) showing a magnetic foliation for the minimum axis (K min ) oblique (by ~70°) to the perpendicular of the intrusive contact. Assuming the K min axis represents the paleovertical axis, these data suggest that the gabbroic sheet was not intruded into the host sediments horizontally. Paleomagnetic measurements of the gabbroic intrusion show an in situ mean direction of reversed polarity (declination/inclination [Dec/Inc] = 287°/–65°, α 95 = 3°) that is considerably different from the expected, reversed-polarity dipole-field direction of this region (Dec/Inc = 0°/–56°). A structural analysis combining the paleomagnetic and AMS data led to the determination of a unique pole of rotation, around which the dike can be back-rotated to its initial orientation. The magnitude of rotation necessary for the in situ paleomagnetic direction to be back-rotated to the expected direction is ~60°, which is consistent with the rotation required for the K min axis to be vertical. This consistency can be regarded as independent support for our interpretation of the AMS results and the reliability of the paleomagnetic data. Consequently, we propose that the Muroto gabbro was intruded when the paleo–trench-fill sediments had been tilted landward by ~20°, presumably by accretion, and that the gabbro might have been intruded as a sill-like sheet along a structurally weak zone, possibly part of the frontal thrust plane in the Shimanto accretionary prism.