(Table T1) Mineral composition of oriented clay aggregates from ODP Leg 190 sites

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.

Annotated record of the detailed examination of Mn deposits from DSDP Site 32, Leg 5 (Cores 5-32-13, 5-32-14)

Site 32 was proposed by the Pacific Advisory Panel at a location over a strong positive magnetic anomaly (Number 13 on the Pittman-Heirtzler scale, 38 million years) where samples of the basement and the basal sediment would be of value in testing hypotheses for origin of the linear magnetic anomalies from this part of the Pacific. Comparison of this site, south of the Pioneer Fracture Zone, with later sites north of the Fracture Zone would be the basis for evaluating the discontinuity formed by the Pioneer.

Element analyses of DSDP/ODP Hole 504B basalt and breccia (Table 2)

Geochemical well logs were used to measure the dry weight percent oxide abundances of Si, Al, Ca, Mg, Fe, Ti, and K and the elemental abundances of Gd, S, Th, and U at 0.15-m intervals throughout the basement section of Hole 504B. These geochemical data are used to estimate the integrated chemical exchange resulting from hydrothermal alteration of the oceanic crust that has occurred over the last 5.9 Ma. A large increase in Si in the transition zone between pillows and dikes (Layers 2B and 2C) indicates that mixing of hot, upwelling hydrothermal fluids with cold, downwelling seawater occurred in the past at a permeability discontinuity at this level in the crust, even though the low-to-high permeability boundary in Hole 504B is now 500 m shallower (at the Layer 2A/2B boundary). The observations of extensive Ca loss and Mg gain agree with chemical exchanges recorded in the laboratory in experiments on the reactions that occur between basalt and seawater at high temperatures. The K budget requires significant addition to Layer 2A from both high-temperature depletion in Layers 2B and 2C and low-temperature alteration by seawater. Integrated water/rock ratios are derived for the mass of seawater required to add enriched elements and for the mass of hydrothermal fluid required to remove depleted elements in the crust at Hole 504B.