ABSTRACT The genesis of the forearc magmatism in southwest Japan at 14 Ma was studied using geologic and seismic observations. Before the magmatism, the Shimanto accretionary complexes were uplifted by 1000–3000 m between 21 and 17 Ma during the opening of the Japan Sea and the Shikoku Basin. Opening of the Japan Sea and the Shikoku Basin terminated at 15 Ma, when the Kinan Seamount Chain on the Shikoku Basin activated. The magmatic products are distributed at segment boundaries and in aseismic areas of the subducting Philippine Sea plate. The segment boundaries are located on syncline and anticline axes of the subducting slab at Kyushu. The magmatic products at Shikoku are distributed at places where olistostromes uplifted between 21 and 17 Ma. Beneath the Kumano volcanic rocks at Kii, a significant discontinuity in the locations of deep earthquakes is observed. These observations indicate that rupturing of the subducting slab by the load of the overriding plate occurred at around 14 Ma. The slab rupturing would have enabled subslab asthenosphere and/or magma to be injected into the plate interface through the tear and cause the forearc magmatism. Since the oceanic plate has a number of preexisting weaknesses, such as fracture zones, slab rupturing could occur more commonly than previously considered. The forearc magmatism caused by slab rupture is an important process associated with the growth of continental crust in subduction zones.

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 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.