Table 4-2. Summary of sedimentation and prism thickening rates calculated from
 biostratigraphy at Site 672.
 Site Unit Thickness Time Vertical loading
 (m) (Ma)
 Initial sedimentation Prism thickening
 rate (m/yr) rate (m/yr)
672 Accreted sediments Present-5 2.60 x 10-3 3.90 x 10-5
 Hemipelagics 173 5-18 1.50 x 10-6
 Decollement 15 18-25 1.70 x 10-6
 Turbidites 285 25-43 1.45 x 10-5
 Hemipelagics 187 43-50 1.70 x 10-5

 According to Bekins et al. (1995), in deeper parts of the accretionary complex, fluid

 released from clay dehydration becomes important and had been used to explain the

 observed low-chloride anomalies at the toe of the prism. Moreover, if the rate of fluid

 released during the dehydration reaction is high enough this mechanism could also

 contribute to the generation of excess pore pressures. However, studies have not shown

 that fluids released during smectite dehydration are large enough to affect pore pressures.

 A fluid flow and budget study by Saffer and Bekins (1998) concluded that dehydration

 fluid sources are 10-1000 times smaller than fluid released from compaction sources and

 thus, calculated pore pressures are largely independent of the clay content. Because the

 model used here only extends to 50 km from the deformation front while based on

 previous studies (e.g., Bekins et al., 1994) the smectite dehydration is important at

 distances >50 km from the deformation front, thus fluid sources from dehydration were

 not included in this study.

 Studies based on fluid budget modeling show that prism fluids do not contribute

 significantly to the decollement flow (Bekins and Screaton, 2006), thus prism fluid

 reaching the underthrust should be even be smaller. However, elevated pore pressures

 within the prism due to sources could inhibit upward migration of fluid from the