Geochemical measurements may indicate lateral fluid flow through the sediments in the Leg 196 area, although its presence and magnitude is controversial. Specifically the evidence of décollement zone-hosted fluid flow is equivocal (Shipboard Scientific Party, 1991; Kastner et al., 1993). Landward of the deformation front, the décollement zone steps down through a water-rich massive hemipelagite below, suggesting that the décollement zone could be progressively dewatering there, with the fluid possibly being expelled seaward up the décollement zone (Park et al., 2000). The broad minimum in the pore water chloride profile obtained during Legs 131 and 190 at Sites 808 and 1174, extending from right above to well below the décollement zone, was probably produced by a combination of in situ clay dehydration and vertical fluid migration, possibly with a component of lateral migration (Kastner et al., 1993; Shipboard Scientific Party, 2001b).
Although the role of fluid is thought to be a key in the study of seismogenic zones (Hickman et al., 1995). The magnitude and location of active fluid flow in this accretionary prism and the potential linkage to the Nankai seismogenic zone are not clearly defined. Our desire to investigate this system motivated the deployment of long-term hydrogeological and geochemical monitoring systems or Advanced CORKs (ACORKs) during Leg 196. By sampling particular stratigraphic and structural intervals, the ACORKs will constrain fluid pressures and permeability and provide a time series of the fluid flow regime at the toe of the Nankai accretionary prism. Thus, our results may provide a better understanding of the linkage of near-surface conditions to those in the seismogenic zone, especially in the case of an earthquake rupturing the deeper levels of the décollement zone.