INTRODUCTIONProminent among the important scientific problems given high priority in the current Ocean Drilling Program (ODP) JOIDES Long-Range Plan (LRP) (1996) is understanding deformational and fluid-flow processes at convergent plate margins. Objectives include addressing "key questions, such as the distribution of deformation throughout an accretionary prism, the controls on what material is accreted and what is subducted, and the role of fluids and fluid flow in deformation of the prism" and understanding "the fluid-linked diagenetic and tectonic processes in the rapidly deforming geochemical factory of the accretionary wedge." These objectives are not only important scientifically but also have great societal relevance. For example, the deformation associated with plate convergence can be associated with catastrophic earthquakes and the fluid and chemical fluxes may impact global geochemical budgets, including carbon fluxes relevant to climate issues. The process-oriented, interlinked nature of these objectives requires not only detailed spatial and subsurface sampling but also a long-term time-series observational approach in the subseafloor. Thus, these objectives are highly appropriate for the ODP initiative for in situ monitoring of geological processesthe approach to be embodied in Leg 196.
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, including a three-dimensional multichannel seismic (MCS) survey conducted in the summer of 1999 (Fig. 1 and Fig. 2). It thus represents an excellent setting to address LRP objectives for accretionary prism coring, in situ monitoring, and refinement of mechanical and hydrological models. These are precisely the objectives of the two-leg (Legs 190 and 196) integrated program begun in 2000 to "define the interrelationship of the dynamics of deformation and fluid-flow processes in an accretionary prism characterized by thick terrigenous sediments." Coring operations during Leg 190 included coring a transect of sites off the Muroto Peninsula (Fig. 2). Leg 196 will complete the program with logging while drilling (LWD) at three sites along the Muroto Transect plus installation of two advanced circulation obviation retrofit kits (ACORKs) hydrogeological observatories.
Despite the various drilling successes mentioned above, direct measurements of fluid circulation indicators (e.g., pore pressures, flow velocities, in situ geochemical flow proxies) have been difficult to accomplish in an accretionary complex. Nevertheless, the combination of approaches planned for Leg 196LWD plus long-term in situ monitoringshows great promise in addressing these issues. For example, recent LWD data from the Barbados accretionary prism indicated that décollement sediments have near-surface porosity maintained by high pore-fluid pressures (Moore et al., 1995, 1998). CORK pressure data directly documented these high fluid pressures (Becker et al., 1997). In contrast, Leg 190 data shows the décollement at Nankai is marked by low porosity (Fig. 3); what is the role of fluid pressures there?
Background | Table of Contents