The Nankai Trough is the subducting plate boundary between the Philippine Sea plate and the Eurasian plate (Fig. F1). The Shikoku Basin is part of the Philippine Sea plate, which is subducting to the northwest under southwest Japan at a rate of 2-4 cm/yr (Karig and Angevine, 1986; Seno, 1977), slightly oblique to the plate margin. Active sediment accretion is presently taking place at the Nankai Trough. The record of present-day accretion extends landward to Shikoku Island, where older (Cretaceous to Tertiary) accretionary prism rocks are exposed (Taira et al., 1988).
The modern episode of subduction is documented by widely distributed volcanic activity, which started at ~8 Ma in southern Kyushu and by 6 Ma in southwest Japan (Kamata and Kodama, 1994). Subduction of the Shikoku Basin crust at the Nankai Trough formed a frontal accretionary prism to the southwest Japan forearc (Nankai Trough accretionary prism) (Taira, Hill, Firth, et al., 1991; Taira et al., 1992; Le Pichon et al., 1987) and shaped the forearc basins (Okamura et al., 1987; Sugiyama, 1994). One of the prominent topographic features in the vicinity of the Leg 190 sites is an embayment of the trench landward slope (Figs. F1, F2). Yamazaki and Okamura (1989) interpreted this embayment as an indentation caused by the collision of seamounts with the prism. Recent seismic reflection work and ocean-bottom seismometer experiments on crustal structure support this interpretation (Park et al., 1999; Kodaira, 2000).