LEG 190 SCIENTIFIC OBJECTIVES
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 of these studies provide 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, a transect of sites was drilled during Leg 190 across strike of the prism. 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 vertically thickened sediments in the footwall. Drilling at Sites 1175 (ENT-07A), 1176 (ENT-06A), and 1178 (ENT-09A) penetrated a highly deformed and evolved portion of the prism.
Structural and Hydrologic Evolution of the Décollement Zone
The nature of the décollement zone along the transect remains a big puzzle. Seismic profiles across the transect represent the décollement as a reverse polarity reflection that extends well in front of the deformation front; this has been interpreted to indicate (1) the presence of fluids along a high-porosity fault zone and (2) probably the presence of high pore pressures (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 much 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. The normal polarity seismic reflection marking the décollement beneath the prism toe along the Ashizuri Transect was sampled at Site 1177. 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 to test these hypotheses of décollement formation and evolution.
Chemical Gradients and Fluid Flow Paths
The origin of the Cl concentration depth profile is of great importance to the understanding of the hydrogeochemistry 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 (~5601240 mbsf), with a minimum concentration in the underthrust section at ~1100 mbsf (Kastner et al., 1993). Some of the shipboard scientists believe the preliminary one-dimensional modeling of this profile excludes the possibility of in situ production of water, hence requiring its introduction from elsewhere. In addition, 2-D 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). It is important to note that these calculations are strongly dependent on porosity and mineralogical data from Site 808 and may change significantly with revised porosity values or additional information about smectite content. The chemical and isotopic signatures of the pore fluids suggest 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 striking Cl zone. The sites along the Muroto Transect are aimed at understanding the lateral variability of fluid flow.
Contrasting Stratigraphic and Deformational Framework along
Seismic profiles of the Ashizuri and Muroto 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 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 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 1177 drilled through the upper 300 m of the section previously cored at Site 582 and cored the subducting sediment section to document its clay mineralogy.
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