Earth scientists have long recognized the complex interplay of deformational, diagenetic, and hydrologic processes in developing mature mountain belts and have sought to understand the controls on and interactions among these fundamental processes. Accretionary prisms represent unique, accessible natural laboratories for exploring initial mountain building processes. The geometries and structures of accretionary prisms are relatively simple and have been well imaged seismically. Typically, the materials incorporated within prisms are only moderately altered from their original states, so competing active processes can often be isolated, quantified, and reproduced in the lab.

Studies of the processes occurring at convergent plate boundaries have established that fluids play a major role in how prisms and mountain belts evolve (e.g., Carson et al., 1990; Henry et al., 1989). Tectonic stresses lead to the expulsion of intergranular fluids through compaction of unlithified sediments (e.g., tectonic dewatering [Moore et al., 1986] or shear dewatering [Bray and Karig, 1988], whereas thermal alteration of primary minerals produces excess fluids by dehydration (Moore and Vrolijk, 1992). These fluids move through the system under pressure gradients; the paths they take depend on the intergranular and fracture permeability of the rock. In turn, trapped fluids can induce high pore pressures, which may favor brittle deformation and the production of more fractures and discrete faults, thereby enhancing fluid flow. Thermal and chemical changes introduced by fluid flow may lead to mineralogical alteration or growth of authigenic phases. Chemical species transported by fluids may be deposited during flow, mineralizing and sealing fracture conduits, or cementing and strengthening grain contacts. In regions of substantial flow, advective transport may play an important role in chemical and thermal budgets. The magnitude of flow and dissolved mass transport and their effect on the global chemical balance are largely unknown and may rival that of mid-ocean ridges. Lithification of accreted materials through the interaction of physical and chemical processes leads the prism to become rigid enough to accumulate strain energy which is eventually released as mega earthquakes.

The Nankai Trough accretionary prism represents an "end-member" prism accreting a thick terrigenous sediment section in a setting with structural simplicity and unparalleled resolution by seismic and other geophysical techniques. It, thus, represents a superb setting to address ODP's Long Range Plan objectives for accretionary prism coring, in situ monitoring, and refinement of mechanical and hydrological models. In this prospectus, we present a plan for drilling at the Nankai margin that carefully targets sites for coring, in situ observation, and long-term monitoring (1) to constrain and resolve aspects of prism hydrology and mechanics and (2) to test existing models for prism evolution. Two legs are planned: Leg 190, in mid-2000, will focus on coring and sampling the prism along two transects within a three dimensional (3-D) seismic survey (Fig. 1). Leg 196, in 2001, will use logging-while-drilling (LWD) technology to collect in situ physical properties data and install ACORKs (advanced circulation obviation retrofit kits; Davis et al., 1992) for long-term monitoring of prism processes.


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