Fluid Flow Paths and Chemical Gradients
The origin of the Cl concentration depth profile is of great importance to the understanding of the hydrogeochemistry of the Nankai Trough eastern region. Site 808 is characterized by a broad region of lower than seawater Cl concentration (~20% seawater dilution) within the Shikoku Basin hemipelagic section (~560-1240 mbsf), with a minimum concentration in the underthrust section at ~1100 mbsf (Kastner et al., 1993). Preliminary one-dimensional modeling of this profile excludes the possibility of in situ production of water, hence requiring its introduction from elsewhere. The chemical and isotopic signatures of the pore fluids indicate a deep-seated elevated temperature (>150°C) source. It seems that a combination of active or episodic lateral fluid flow along one or more sediment horizons and fluid advection may be responsible for this strikingly broad Cl profile. The sites along the eastern transect will be aimed at understanding lateral variability of fluid flow.
Spatial Distribution and Temporal Progression of Deformation
Although core recovery at Site 808 was exceptional and physical properties and structural observations complete, the results yield only a one-dimensional view of the interior of the Nankai prism. We have almost no constraint on how various fabrics, structures, physical properties, or geochemistry vary along and across strike or how these variations translate over time. This lack of spatial and temporal control makes it nearly impossible to determine the relationships between 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 dispersed core data. The results provide models to test and guide the selection of future drill sites at the Nankai Trough, as well as the associated sampling and analysis. 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, across-strike drill holes are desperately needed. Site ENT-03A should represent a less deformed analog to Site 808, penetrating the incipient thrust fault in the PTZ as well as the vertically thickened sediments in the footwall. Sites ENT-06A and 07A will penetrate a highly deformed and evolved portion of the prism. Site ENT 04A will characterize the intermediate zone in terms of deformation and chemical gradient.
Contrasting Deformational and Fluid Flow Behavior Along Strike
Seismic profiles of the western and eastern transects across the prism indicate significant differences in prism architecture, structure, and physical properties in the two locations. These are assumed to reflect variances in fluid-flow regimes, but, to date, the mechanisms responsible for such variability are unknown. Structural differences between the western and eastern 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 western transect (8°-10°; Fig. 5) is greater than that of the eastern toe (4°-5°; Fig. 3), a situation that may arise from relatively stronger décollement to the west or lower internal sediment strength. A strong décollement might arise from a lack of pressurized fluids within the fault zone, consistent with the normal polarity reflection. Alternatively, this difference in strength might be due to a variation in clay mineralogy in the décollement zone. Site WNT-01A will drill through the upper 300 m of the section previously cored at Site 582 and will core the subducting sediment section to document its clay mineralogy.
The western PTZ also contains an enigmatic series of closely spaced dipping seismic discontinuities, which are absent to the east (Fig. 5). Their origin is unclear, but they may mark zones of concentrated fractures that may serve as dewatering conduits. The presence of such fractures in the PTZ would also point to a more brittle mode of deformation, which can be induced by localized high pore pressures. Drilling through the western PTZ is a secondary objective aimed at understanding these features and their roles in defining the hydrology of the prism toe. A comparison of these structures with observations from the eastern PTZ will elucidate the dissimilarities in internal sediment strength and behavior between the two regions, which is critical to distinguishing the different effects of material properties and fluid pressures in how these sediments deform.
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