X-ray diffraction analyses of the clay-sized fraction of sediments from the Nankai Trough and Shikoku Basin (Sites 1173, 1174, and 1177 of the Ocean Drilling Program) reveal spatial and temporal trends in clay minerals and diagenesis. More detrital smectite was transported into the Shikoku Basin during the early-middle Miocene than what we observe today, and smectite input decreased progressively through the late Miocene and Pliocene. Volcanic ash has been altered to dioctahedral smectite in the upper Shikoku Basin facies at Site 1173; the ash alteration front shifts upsection to the outer trench-wedge facies at Site 1174. At greater depths (lower Shikoku Basin facies), smectite alters to illite/smectite mixed-layer clay, but reaction progress is incomplete. Using ambient geothermal conditions, a kinetic model overpredicts the amount of illite in illite/smectite clays by 15%-20% at Site 1174. Numerical simulations come closer to observations if the concentration of potassium in pore water is reduced or the time of burial is shortened. Model results match X-ray diffraction results fairly well at Site 1173. The geothermal gradient at Site 1177 is substantially lower than at Sites 1173 and 1174; consequently, volcanic ash alters to smectite in lower Shikoku Basin deposits but smectite-illite diagenesis has not started. The absolute abundance of smectite in mudstones from Site 1177 is sufficient (30-60 wt%) to influence the strata's shear strength and hydrogeology as they subduct along the Ashizuri Transect.

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A combination of Deep Sea Drilling Project-Ocean Drilling Program drilling results and site survey data in the Shikoku Basin, Nankai Trough, and Nankai landward slope region provides a unique opportunity to investigate the sedimentary facies evolution in the clastic-dominated accretionary forearc. Here, we consider the facies evolution model based on the drilling results, IZANAGI sidescan images, and seismic reflection profiles.The sedimentary facies model of the Nankai forearc proposed in this paper is composed of two parts: the ocean floor-trenchlower slope sedimentary facies evolution and the upper slope to forearc basin sedimentary facies evolution. The former begins with basal pelagic and hemipelagic mudstones overlain by a coarsening upward sequence of trench turbidites which are, in turn, covered by slope apron slumps and lower slope hemipelagic mudstone. This assemblage is progressively faulted and folded into a consolidated accretionary prism that is then fractured and faulted in the upper slope region. Massive failure of the seafloor in the upper slope region produces olistostrome deposits that contain lithified blocks derived from older accretionary prism dispersed in a mud matrix. The contact between the older accretionary prism and the upper slope olistostrome is a submarine unconformity that is the first stratigraphic evidence for the exhumation of an older prism to the seafloor. The olistostrome beds are then overlain by forearc basin-plain mudstone and turbidites which are progressively covered by coarsening-upward delta-shelf sequences. An application of the forearc facies model to the Shimanto Belt was attempted in order to ensure feasibility of this model. Several characteristic lithologies observed in the Shimanto Belt can be interpreted with this model. The unconformities found in the Shimanto Belt were evaluated to provide an estimate for the exhumation rate of the prism. Comparison with independent measurement of exhumation rate provides a 0.5 to 1 km/m.y. vertical growth of the prism.