Three sites were cored on the landward slope of the Nankai margin of southwest Japan during Leg 190 of the Ocean Drilling Program. Sites 1175 and 1176 are located in a trench-slope basin that was constructed during the early Pleistocene (~1 Ma) by frontal offscraping of coarse-grained trench-wedge deposits. Rapid uplift elevated the substrate above the calcite compensation depth and rerouted a transverse canyon-channel system that had delivered most of the trench sediment during the late Pliocene (1.06-1.95 Ma). The basin's depth is now ~3000 to 3020 m below sea level. Clay-sized detritus (<2 µm) did not change significantly in composition during the transition from trench-floor to slope-basin environment. Relative mineral abundances for the two slope-basin sites average 36-37 wt% illite, 25 wt% smectite, 22-24 wt% chlorite, and 15-16 wt% quartz. Site 1178 is located higher up the landward slope at a water depth of 1741 m, ~70 km from the present-day deformation front. There is a pronounced discontinuity ~200 m below seafloor between muddy slope-apron deposits (Quaternary-late Miocene) and sandier trench-wedge deposits (late Miocene; 6.8-9.63 Ma). Clay minerals change downsection from an illite-chlorite assemblage (similar to Sites 1175 and 1176) to one that contains substantial amounts of smectite (average = 45 wt% of the clay-sized fraction; maximum = 76 wt%). Mixing in the water column homogenizes fine-grained suspended sediment eroded from the Izu-Bonin volcanic arc, the Izu-Honshu collision zone, and the Outer Zone of Kyushu and Shikoku, but the spatial balance among those contributors has shifted through time. Closure of the Central America Seaway at ~3 Ma was particularly important because it triggered intensification of the Kuroshio Current. With stronger and deeper flow of surface water toward the northeast, the flux of smectite from the Izu-Bonin volcanic arc was dampened and more detrital illite and chlorite were transported into the Shikoku-Nankai system from the Outer Zone of Japan.
Sediment depth is given in mbsf. Total clay minerals in bulk mudstone from Shipboard Scientific Party (2001c, doi:10.2973/odp.proc.ir.190.106.2001; 2001d, doi:10.2973/odp.proc.ir.190.107.2001; 2001e, doi:10.2973/odp.proc.ir.190.109.2001).
Supplement to: Underwood, Michael B; Steurer, Joan (2003): Composition and sources of clay from the trench slope and shallow accretionary prism of Nankai Trough. In: Mikada, H; Moore, GF; Taira, A; Becker, K; Moore, JC; Klaus, A (eds.) Proceedings of the Ocean Drilling Program, Scientific Results, College Station, TX (Ocean Drilling Program), 190/196, 1-28
