The Nankai Trough and accretionary prism are situated where the Philippine Sea plate (Shikoku Basin) is subducting beneath the Eurasian plate (Moore et al., 2001; Shipboard Scientific Party, 2001b). Drilling during Leg 190 took place along two transects, the eastern Muroto Transect and the western Ashizuri Transect (Fig. F1). Two reference sites were drilled to provide baseline data for characterizing the sediment prior to subduction: Site 1173 (Muroto Transect) and Site 1177 (Ashizuri Transect). Site 1174 penetrated a frontal protothrust and the décollement ~2 km seaward of ODP Site 808 (Fig. F1).
As demonstrated during ODP Leg 131 (Taira et al., 1992; Pickering et al., 1993; Taira and Ashi, 1993), the stratigraphic section entering Nankai Trough from the central part of the Shikoku Basin is more or less consistent with models of accreting margins (e.g., Piper et al., 1973; Dickinson and Seely, 1979) in that it coarsens and thickens upward as the subducting plate approaches land. At Site 1173 (Fig. F2), the base of the Quaternary trench-wedge facies is 102.14 meters below seafloor (mbsf) and the strata consist of hemipelagic mud interbedded with silt, sandy silt, and silty sand turbidites. The upper Shikoku Basin facies (Pliocene-Quaternary; 102.14-445.92 mbsf) was deposited by hemipelagic settling and ash falls. The lower Shikoku Basin facies (Pliocene-middle Miocene; 445.92-790.3 mbsf) is also hemipelagic but lacks recognizable ash layers. The lower Shikoku Basin facies is underlain by a volcaniclastic facies and basalt (Shipboard Scientific Party, 2001c). These same basic facies relations are present at Site 1174 (Fig. F3), but the Quaternary trench-wedge facies extends to 483.23 mbsf, the upper Shikoku Basin facies extends from 483.23 to 660.99 mbsf, and the lower Shikoku Basin facies extends to 1102.45 mbsf (Shipboard Scientific Party, 2001d).
The Ashizuri Transect is located along the distal flanks of the Shikoku Basin spreading center (Fig. F1). Site 1177 (Fig. F4) differs from stratigraphic sections along the Muroto Transect because it does not show a coarsening-upward trend. The trench-wedge facies was not cored, but the upper Shikoku Basin facies (Pliocene) extends from 300.20 to 401.76 mbsf (Shipboard Scientific Party, 2001e). This unit consists of hemipelagic mud and abundant volcanic ash layers. The lower Shikoku Basin hemipelagic facies (lower Pliocene-upper Miocene; 401.76-449.30 mbsf) comprises hemipelagic mudstone and scattered intervals of siliceous claystone. The lower Shikoku Basin turbidite facies (lower-upper Miocene; 449.30-748.35 mbsf) contains abundant siliciclastic sand layers with rare beds of gravel and mudstone-clast conglomerate. A volcaniclastic-rich facies (lower Miocene) extends to 831.08 mbsf and contains variegated mudstone, thin turbidites, and volcanic ash.
Previous studies of detrital clay minerals in the Nankai Trough and Shikoku Basin include those by Cook et al. (1975), Chamley (1980), Chamley et al. (1986), and Underwood et al. (1993a, 1993b). All of those results show a preponderance of detrital illite and chlorite in the Quaternary trench-wedge facies. Details about the illite (e.g., crystallinity index, polytype, and bo lattice dimension) are consistent with their erosion from low-grade metasedimentary rocks, which are widespread throughout the Outer Zone of Japan (Underwood et al., 1993a). When clay provenance indicators are combined with data from sand petrography and paleoflow indicators (Taira and Niitsuma, 1986; Marsaglia et al., 1992; Pickering et al., 1992; Fergusson, this volume), interpretations suggest that the Quaternary trench wedge originated primarily through axial transport from sources located in the Izu-Honshu collision zone of central Japan (Underwood and Pickering, 1996).
In contrast to the Quaternary trench wedge, strata in the Shikoku Basin (Sites 297, 442, 443, and 444 of the Deep Sea Drilling Project) show progressive enrichment of smectite with increasing depth and age (Cook et al., 1975; Chamley, 1980). Previous workers also demonstrated that smectite increases monotonically in the upper Shikoku Basin facies at Site 808, but Underwood et al. (1993b) were not able to discriminate convincingly between the effects of changing detrital sources and/or transport paths through time, as opposed to the effects of in situ ash-to-smectite and smectite-illite diagenesis. Subsequent work by Masuda et al. (1996) showed that glass shards in interbedded volcanic ash layers alter to dioctahedral smectite (aluminum-rich beidellite) and potassium-rich clinoptilolite. The illite-smectite reaction has been inhibited in discrete beds of volcanic ash (Masuda et al., 1996), but there is clear evidence for progressive smectite-illite alteration and uptake of potassium in the bulk mudstones of Shikoku Basin (Underwood et al., 1993b; Underwood and Pickering, 1996; Masuda et al., 2001). Underwood et al. (1993b) speculated that illite-smectite diagenetic progress at Site 808 is a recent response to rapid sedimentary burial beneath the trench wedge (i.e., in the last 0.5 m.y.), but they had no data from an appropriate reference site to constrain the degree of diagenesis seaward of the trench.