CLAY-MINERAL PROVENANCE, SEDIMENT DISPERSAL PATTERNS, AND MUDROCK DIAGENESIS IN THE NANKAI ACCRETIONARY PRISM, SOUTHWEST JAPAN

Offscraped strata within the toe of Nankai accretionary prism display an overall facies pattern of thickening and coarsening upward. Detrita l clay minerals within the Quaternary trench-wedge facies are dominate d by illite; chlorite is the second-most abundant clay mineral, follow ed by smectite. Relative mineral percentages change only modestly with depth. The hemipelagic clay-mineral population is virtually identical to clays washed from turbidite matrix, and different size fractions ( <2 mu m and 2-6 mu m) show nominal amounts of mineral partitioning. Sm ectite content increases beneath the trench-wedge deposits, where the upper subunit of the Shikoku Basin stratigraphy (late Pliocene and ear ly Pleistocene) includes abundance of volcanic ash. Syneruptive, subae rial chemical weathering of volcanic source rocks, together with irt s itu alteration of disseminated glass shards, caused the increase in sm ectite. Smectite begins a monotonic transformation to illite/smectite (I/S) mixed-layer clay at similar to 555 mbsf and an estimated tempera ture of similar to 65 degrees C. Ordered (R = 1) I/S interlayering fir st appears at similar to 1220 mbsf (<2 mu m size fraction) and similar to 1100 mbsf (<0.2 mu m size fraction). The illitization gradient coi ncides with a reduction in pore-water chlorinity, but depth-related ch anges in bulk mudstone geochemistry (K2O, Rb) are subtle. The absolute abundances of discrete smectite and I/S appear to be insufficient to account for the magnitude of pore-water dilution via in situ dehydrati on reactions. Instead, pore water probably was transported to Site 808 , either from sources located deeper in the accretionary prism, where bulk mudstone porosities are lower, or from strike-parallel sources wh ere mudstones originally deposited in the Shikoku Basin might contain higher percentages of smectite.