SCIENTIFIC OBJECTIVESStructural and Hydrologic Evolution of the Décollement Zone
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 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). 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, 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). 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.
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.
A high-resolution MCS survey conducted in 1999 enabled interpretation of several structural zones within the Nankai Trough accretionary prism, as shown in Figure 5. These are described more fully in the Leg 190 Prospectus (http://www-odp.tamu.edu/publications/prosp/190_prs/190toc.html) and Preliminary Report (http://www-odp.tamu.edu/publications/prelim/190_prel/190toc.html). The most important structural zones for Leg 196 operations include the following:
Nankai Trough Axis Zone. Prior drilling results indicate that the stratigraphy of the trench floor is composed of the following lithologic units in descending order: trench turbidites (Holocene to Pleistocene), turbidite-hemipelagite transition (Pleistocene), hemipelagite with tephra layers (early Pleistocene to late Pliocene), massive hemipelagite (mid-Pliocene to mid-Miocene), acidic volcaniclastics (15 Ma), and pillow basalts (16 Ma).
Protothrust Zone (PTZ). This area represents a zone of incipient deformation and initial development of the décollement within the massive hemipelagic unit. Above the décollement, the sediment thickness increases landward, probably because of tectonic deformation with the development of small faults and ductile strain (Morgan and Karig, 1995a, 1995b).
Imbricate Thrust Zone (ITZ). Landward of the PTZ, a zone of well-developed seaward-vergent imbricate thrusts can be recognized. The thrusts are sigmoidal in cross section with a mean angle of about 30° and typical thrust spacing of 0.5 km. The seaward edge of the ITZ marks the deformation front. Two sites cored the frontal part of the imbricated thrust zone: Deep Sea Drilling Project (DSDP) Site 583 west of the Muroto Transect and ODP Site 808, which was incorporated into the Leg 190 Muroto Transect.
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