The tectonic history of Japan over the last 20 m.y. is complicated and involves the development of a major collision zone in central Honshu between the Honshu arc and the Izu-Bonin island arc (Taira et al., 1989). Japan was part of the Asian mainland prior to the major spreading in the Japan Sea at 15-18 Ma (Tamaki et al., 1992; Jolivet et al., 1994; Lee et al., 1999). Subduction was active throughout the Cretaceous and early Tertiary in southwest Japan with development of a subduction complex (Shimanto Belt) that includes rocks as young as early Miocene on Cape Muroto (Taira et al., 1988). The Nankai Trough accretionary prism continues to form by the subduction of the Philippine Sea plate as it converges under southwest Japan (Eurasian plate) in a northwest direction at a rate estimated at ~40 mm/yr (Seno et al., 1993). The Neogene history of subduction along the Nankai Trough is far from clear; for example, Niitsuma (1988) suggested that subduction has been active since at least 7 Ma, whereas Maruyama et al. (1997) portrayed subduction as continuous throughout the Neogene. Island arc collision between the Izu-Bonin and Honshu arcs began at ~12 Ma and was episodic with four major accretion events at ~12, 7-9, 3-5, and ~1 Ma (Amano, 1991; cf. Niitsuma, 1989). The last of these events is responsible for the influx of sediment along the Nankai Trough as seen in the upper Pleistocene sediments at Sites 1173, 1174, and 808. Uplift of the Tertiary and older subduction complexes in southwest Japan were inferred by fission track ages to occur at ~10 Ma in Shikoku and earlier at ~15 Ma on the Kii Peninsula (Hasebe et al., 1993; Tagami et al., 1995).
The subducting Philippine Sea plate underlies the Nankai Trough and consists of 15- to 26-m.y.-old igneous basaltic basement. Outboard of the Nankai Trough in the Shikoku Basin, eruptions continued up to ~12 Ma on the Kinan Seamounts (Fig. F1) (Kobayashi et al., 1995). Miocene-Pleistocene mud and turbidites of the Shikoku Basin succession overlies the basaltic basement.
At Site 1177, the lower Shikoku Basin succession consists of a lower Miocene silicic volcaniclastic unit overlain by a lower-upper Miocene siliciclastic turbidite unit (Shipboard Scientific Party, 2001b). Neither of these units is present in the Muroto Transect. At Site 297, located ~90 km south of Site 1177, Unit 4 consists of early Pliocene turbidites similar in composition to those at Site 1177 but apparently younger (Shipboard Scientific Party, 1975b, 2001b). At Site 808 (Leg 131) the Shikoku Basin succession has a basal volcaniclastic unit with abundant thick-bedded silicic sandy to muddy volcaniclastic layers derived from the ~14-Ma silicic volcanism and plutonic activity that affected the southeastern zones of southwest Japan (Shipboard Scientific Party, 1991). The upper Shikoku Basin succession at Site 1177 and in the Muroto Transect has a lower unit of silty clay(stone) and an upper unit of silty clay with interbedded ash layers (Moore, Taira, Klaus, et al., 2001). The boundary between these units is diagenetically controlled by the breakdown of ash layers in the lower unit.
The Nankai Trough is partly filled by a wedge of trench turbidites that thicken toward the deformation front at the base of the accretionary prism (Fig. F2). At Sites 808, 1174, and 1173 these turbidites are of late Pleistocene age (<1 Ma) (Moore, Taira, Klaus, et al., 2001), whereas farther west at Site 298, early Pleistocene thin-bedded turbidites were also recorded (Shipboard Scientific Party, 1975a). Mafic intermediate volcanic detritus, quartz, and feldspar with a minor component of sedimentary and metamorphic debris dominate these sands and were derived from the island arc collisional zone in central Honshu (Taira and Niitsuma, 1986; Underwood et al., 1993).
Three Leg 190 sites are located on the lower trench slope (Sites 1175, 1176, and 1178) in the northwestern part of the Muroto Transect (Fig. F2). Sites 1175 and 1176 penetrated a trench-slope basin and encountered an upper unit of hemipelagic mud and ash layers with many contorted and slumped horizons that reflect downslope gravitational movement of unconsolidated materials (Moore, Taira, Klaus, et al., 2001). Unit II at both sites consists of hemipelagic mud, sandy mud to muddy sand, ash layers, and rare silt to sand turbidites. At both sites the lowest unit is a slope to underlying accreted trench fill of late early Pliocene to early Pleistocene age. In contrast to the upper Pleistocene trench fill at Site 1174, fine-grained sedimentary fragments, including chert, dominate the trench fill sandstone, pebbly mudstone, and gravel at Sites 1175 and 1176. At Site 1178 an upper unit of slope mud, ash, and sand is underlain by accreted upper Miocene trench fill of the Nankai Trough accretionary prism (Moore, Taira, Klaus, et al., 2001). Given the relatively young ages and proximity to land it is clear that the Japanese Islands are the source of sand and gravel encountered in Leg 190 cores.