The objective of this paper is to document stratigraphic trends in clay mineralogy and clay diagenesis at Sites 1173, 1174, and 1177, which were cored by the Ocean Drilling Program (ODP) in the Nankai Trough (Fig. F1). During studies of marine sedimentation, clay minerals provide important information for determining the detrital origin of sediment (e.g., Hathon and Underwood, 1991; Fagel et al., 1992; Petschick et al., 1996). Detrital chlorite, for example, is a typical product of physical weathering of plutonic and metamorphic rocks, as well as shale; chlorite tends to increase in ocean sediments toward higher latitudes where physical weathering predominates (Biscaye, 1965; Naidu and Mowatt, 1983). Kaolinite is more prevalent in tropical latitudes where chemical weathering is more intense (Biscaye, 1965; Faugeres et al., 1991; Petschick et al., 1996). Detrital illite resists chemical weathering and is common in most continental soils (Biscaye, 1965). Illite and illite/smectite (I/S) mixed-layer clays are common low-temperature diagenetic products in marine basins (Burst, 1969; Perry and Hower, 1970; Hower et al., 1976).
Fagel et al. (2001) showed how subtle differences among minerals belonging to the smectite group could be used to decipher changes in detrital provenance and dispersal route through time. This type of approach is possible because different precursors and weathering processes produce different varieties of smectite. In brief, dioctahedral smectites (beidellite and montmorillonite) are typical products of silicic volcanic sources (Chamley, 1989), although iron montmorillonite can also be produced from biogenic silica (Hein et al., 1979). Hydrothermal alteration of basalt produces dioctahedral nontronite, but nontronite is also found in rocks exposed to greenschist facies metamorphism. Saponite (trioctahedral smectite) and celadonite (dioctahedral silica-rich mica) form during alteration of crystalline basalt and basaltic glass, especially during the early stages of hydrothermal circulation (e.g., Porter et al., 2000). Saponite also results from zeolite facies metamorphism and can be eroded from an island arc. In the late stages of alteration of basaltic glass, saponite evolves to dioctahedral (Mg, Fe2+ rich) smectite (Chamley, 1989).
In addition to their value as provenance indicators, clay minerals exert a significant impact on sediment shear strength. Clay-sized material, especially expandable clay minerals, affects both internal friction and permeability (Olson, 1974; Wang, 1980; Shimamoto and Logan, 1981; Morrow et al., 1984; Logan and Rauenzahn, 1987; Freed and Peacor, 1989a; Mitchell, 1993). Liberation of water from smectite's interlayer site shrinks the original mineral volume by as much as 35% if illitization goes to ~80% completion (Bird, 1984; Bruce, 1984; Colten-Bradley, 1987). Dehydration during the illite-smectite reaction may be a source of low-chloride fluids in accretionary prisms (Kastner et al., 1991). The lower permeability of clay-rich sediments can cause pore fluid pressure to increase, thereby reducing effective normal stress (Moore and Vrolijk, 1992). Compressibility also changes with clay content (Robinson and Allam, 1998). It is important, therefore, to document the original composition of sediment and to show how that composition changes as pressure and temperature increase. To help assess the extent of smectite-illite diagenesis in the frontal Nankai subduction zone, we utilized the kinetic model of Huang et al. (1993) to generate numerical simulations. Understanding how sediment composition evolves in three dimensions at the Nankai prism toe is an important step in predicting how mechanical behavior changes at greater depths along the subduction boundary.