INTRODUCTION

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 laboratory.

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. Our approach for drilling at the Nankai margin includes sites for coring, in situ observation, and long term monitoring to (1) constrain prism hydrology, mechanical properties, and deformational styles and (2) test existing models for prism evolution.

Leg 190 was the first of a two-leg program concentrating on coring and sampling a transect of sites across the prism within a three-dimensional (3 D) seismic survey. One additional site was drilled to the west to compare along-strike variations in accretionary processes. Leg 196, in 2001, will use logging-while-drilling technology to collect in situ physical properties data and will also install advanced circulation obviation retrofit kits (Davis et al., 1992) for long-term in situ monitoring of prism processes including pressure, temperature, fluid geochemistry, and strain.