Although core recovery at Site 808 was exceptionally good and physical properties and structural observations were unprecedentedly complete, the results yield only a one-dimensional view of the interior of the Nankai prism. Previous drilling provides relatively little constraint on how various stratigraphic parameters, fabrics, structures, physical properties, and geochemistry vary along and across strike or how these variations translate over time. This lack of spatial and temporal control makes it difficult to determine the relations among lithology, deformation, diagenesis, and fluid flow. However, first-order predictions for the distribution of physical properties and structures in two dimensions and the role of fluid pressures in their evolution have been made based on high-quality seismic images, velocities, and the core data from previous drilling. The results of these studies have provided models to test and guided the selection of Leg 190 Nankai Trough drill sites, as well as the associated sampling and analyses. To test this distribution of structures and the role of diagenesis and fluid pressure in its development and to obtain better constraints on physical properties from which these models are derived, four sites were drilled during Leg 190 across strike of the prism. Together with Site 808, these compose the Muroto Transect. Site 1174 (ENT-03A) represents a less-deformed analog to Site 808 and penetrates the incipient thrust fault in the PTZ as well as the thickened sediments in its footwall and the incipient basal décollement. In contrast, drilling at Sites 1175 (ENT-07A), 1176 (ENT-06A), and 1178 (ENT-09A) penetrated highly deformed and evolved portions of the prism. Site 1177 was drilled to link previous sites to the west into the Ashizuri Transect.
Although well-imaged seismically, the nature and evolution of the décollement zone remains poorly understood. In seismic profiles along the Muroto Transect, the décollement is a reverse polarity reflection that extends well seaward of the deformation front; this has been interpreted to indicate (1) the presence of fluids along a high-porosity fault zone or (2) the probable presence of high pore pressures in the fault zone itself or in underthrust sediments (Moore and Shipley, 1993). At Site 808, the décollement is a 20-m-thick zone of intensely fractured sediment, with evidence for shear-induced brecciation, pore collapse, and local phyllosilicate reorientation (Byrne et al., 1993). Sediments from within the décollement have lower porosities than samples from above and below. A subtle mottled texture in some samples led Maltman et al. (1993) to infer localized zones of elevated fluid pressure within the zone. To test these hypotheses of décollement formation and evolution, we sampled the décollement zone at critical points beneath the Nankai prism (Site 808) and PTZ (Site 1174) to document the spatial variations in structure and fluid pressure
The variation of the Cl concentration with depth is of great importance to understanding the hydrogeology and geochemistry of the Nankai Trough Muroto region. Site 808 is characterized by a broad region of Cl concentrations that are lower than seawater (~20% less than seawater) within the Shikoku Basin hemipelagic section (~560-1240 mbsf), with a minimum concentration in the underthrust section at ~1100 mbsf (Kastner et al., 1993). Based on shipboard porosity measurements and XRD estimates of smectite abundance, first-order calculations indicate that ~25% smectite in the incoming section would be required to explain observed pore-water freshening, yet residual smectite abundance is generally <10% at depths corresponding to the low Cl zone (Underwood et al., 1993). Two-dimensional models of smectite dehydration and fluid flow show that neither in situ dehydration nor steady state fluid flow can produce the observed freshening (Saffer and Bekins, 1998). However, it is important to note that these calculations are strongly dependent on porosity and mineralogical data from Site 808 and could change significantly with revised porosity values or additional information about smectite content prior to diagenesis. The chemical and isotopic signatures of the pore fluids suggest contributions from a deep-seated, elevated-temperature (>150°C) fluid source. Some combination of in situ diagenesis and steady state or episodic lateral fluid flow along one or more sediment horizons may be responsible for this low Cl zone. The sites along the Muroto Transect are aimed at understanding the lateral variability of both fluid flow and progressive in situ diagenesis.
Seismic profiles of the Ashizuri and Muroto Transects indicate significant differences in prism architecture, structure, and physical properties in the two locations. These differences are assumed to reflect variances in fluid pressure and/or flow regimes, but to date, the mechanisms responsible for such variability are unknown. Structural differences between the Ashizuri and Muroto regions suggest that there may be significant variation in how deformation is accommodated along the two transects; this contrast in behavior may also shed some light on the hydrologic differences. The taper of the prism toe along the Ashizuri Transect (8°-10°) is greater than that of the Muroto toe (4°-5°), a situation that may arise from a relatively stronger décollement to the west or lower internal sediment strength. A strong prism base might arise from smaller fluid overpressures within the fault zone, consistent with the normal polarity reflection. Alternatively, a difference in strength might be due to a variation in clay mineralogy in the décollement zone. Site 1177 drilled through the upper 300 m of the section previously cored at Site 582, sampled the protodécollement zone, and cored the subducting sediment section to document its lithology, physical properties, and clay mineralogy.