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Subduction zones are characterized by the world's largest and potentially most catastrophic earthquakes (Kanamori, 1986). Moreover, these plate convergence zones return materials to the Earth's interior with chemical fluxes that may impact global geochemical budgets and influence climate (Plank et al., 1998). Subduction zones also are the location of the incipient stages of formation of convergent and collisional mountain belts (Moores and Twiss, 1995).

Recent studies of the processes occurring at subduction zones have established beyond doubt that fluids play a major role in the physical and chemical evolution of subduction zones and mountain belts (e.g., Carson et al., 1990; Henry et al., 1989; Kastner et al., 1991; Vrolijk et al., 1991; Fisher, 1996). When sediments resting on the incoming or subducting plate meet the overriding plate, they are, in part, scraped off and deformed and, in part, underthrust. During this deformation and burial, tectonic stresses lead to the expulsion of intergranular fluids through compaction (Bray and Karig, 1985), increased temperatures cause mineral dehydration (Moore and Vrolijk, 1992), and biological and thermal processes produce hydrocarbons from organic matter (Suess and Whiticar, 1989). To understand the processes of fluid production, transport, and rock response requires not only detailed spatial and subsurface sampling but also long-term observations in the subseafloor. Accordingly, Ocean Drilling Program (ODP) Leg 196 was designed to measure the in situ physical properties and provide long-term monitoring of the physical and chemical states of the initial deformation zone of the subduction zone off of southwest Japan.

Evaluation of this solid and fluid flow in the subduction system off southwest Japan involves outstanding questions that can be addressed by our logging and monitoring program in combination with the borehole coring results from Legs 131 (Hill, Taira, Firth, et al., 1993) and 190 (Moore, Taira, Klaus, et al., 2001). These questions include

  1. What is the character of the faulted rock and associated fluids of the plate-boundary thrust fault, or décollement?
  2. What is the porosity or fluid content of the incoming sediments, the accretionary prism, and underthrust sediments?
  3. What is the relationship between porosity and seismic velocity?
  4. How and why does pore pressure vary between structural environments?
  5. What changes in physical properties (especially velocity) occur in association with diagenetic alterations of sediments (particularly methane hydrate occurrence)?
  6. What is the relationship between deformational structures at core, log, and seismic scales, and what do they tell us about tectonic processes in the accretionary prism?
Addressing these questions will provide a clearer understanding of the issues of deformation and fluid flow during shallow subduction and the basis for predicting the behavior of these materials at seismogenic depths.

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