offshore of Peru (Langseth and Silver, 1996). One possible explanation for this low heat

flow is the rapid sedimentation at Peru, which may prevent equilibrium (Moore and

Silver, 2004). Similarly, heat flow measurements at Cocos Plate off the Nicoya

Peninsula have revealed significantly low heat flow values (Langseth and Silver, 1996).

It has been suggested that hydrothermal cooling of the oceanic basement occurs from the

flow of seawater into the upper crust of the subducting plate (Langseth and Silver, 1996).

 Furthermore, evidence for possible fluid migration comes from observed low

chloride anomalies (concentrations less than seawater), found in many modern prism

sediments (Kastner et al., 1991). One of the widely used explanations for the observed

low chloride anomalies is the dehydration reaction of smectite to illite. Smectite is a

common and abundant type of clay found in subduction zones (Moore and Vrolijk,

1992). The dehydration of smectite to illite is a kinetic reaction that depends on

temperature and time (Elliott et al., 1991). The reaction takes place in temperatures

above 60C and the amount of water released during the reaction is estimated to be 20%

by weight (Bekins et al., 1995). Smectite is replaced by fluid plus illite, which has

greater volume than smectite, thus resulting in an increase in pore pressures (Bekins et

al., 1995). At Barbados, low-chloride anomalies were observed along the decollement

(Kastner et al., 1993) and it is inferred that pore fluids generated from smectite

dehydration migrate toward the toe of the prism mainly through faults or fractures

lowering the chloride concentration at the toe of the prism (Bekins et al., 1995). A broad

low chloride anomaly was also observed above and below the decollement at the Nankai

accretionary complex. Because the smectite contents at the Nankai sites are low, it has

been inferred that freshening of pore fluids may be related to the in situ dehydration