Saffer (2003) integrated measurements of logging-while-drilling (LWD), physical

properties data, and laboratory consolidation tests to compare pore pressure development

and progressive dewatering in the underthrust sediments. Calculations of k* based on

Saffer's (2003) results indicate values of 0.30 and 0.44 at 15 m below the decollement at

Sites 1174 and 808, respectively. These values are slightly lower than the average values

over the entire underthrust profile inferred from porosity data by Screaton et al. (2002).

The estimated k* values calculated from results of Saffer (2003) generally decrease with

depth below the decollement at both Sites 1174 and 808.

 Evidence concerning possible lateral fluid flow is ambiguous. Broad low chloride

anomalies were observed within the lower Shikoku Basin facies (Kastner et al., 1993;

Gieskes et al., 1993; Spivack et al., 2002). Controversial ideas have been suggested as

possible explanations for the observed low chloride anomalies. Kastner et al. (1993) and

Underwood et al. (1993) suggested that smectite dehydration is the most likely

mechanism for the observed low-chloride anomaly. On the basis of geochemical

analyses, Kastner et al (1993) suggested lateral fluid flow along the decollement or along

a deeper conduit within the underthrust sediments while Spivack et al. (2002) suggested

continuous lateral fluid flow between Sites 808 and 1174 within the underthrust

sediments. Brown et al. (2001) suggested that in situ dehydration of-10-15 wt% of

smectite could generate most of the freshening observed at Site 808 and therefore,

provides a possible explanation for pore fluid freshening. However, if the initial amount

of smectite is less than 10-15 wt% then the low-chloride anomaly could reflect the

combined effects of both in situ and lateral flow at depth (Brown et al., 2001). This in

situ dehydration hypothesis has been further supported by a recent study by Henry et al.