Sensitivity to a Low-Permeability Barrier

 Previous accretionary complex studies have suggested the idea of a low-

permeability barrier at the decollement as a possible cause of excess pore pressures. For

example, at the Barbados accretionary prism, Bekins et al. (1995) simulated a 15-m thick

low-permeability cap above the top of the decollement based on an anomalously low

permeability value (6.5 x 10 21 m2) measured by Taylor and Leonard (1990). In a more

recent study based on inferred porosity variations obtained from inverted seismic

reflection data at Nankai, Bangs and Gulick (2005) argued that consolidation of the

uppermost lower Shikoku Basin strata forms a barrier blocking the fluid flow from

below. Because the barrier lies just above the projected level of the decollement, they

suggest that higher-porosity, underconsolidated, and overpressured sediment below forms

a surface of potential decollement propagation.

 To test the effects of a low-permeability barrier at the decollement at Nankai, I ran

simulations with a 10-m thick low-permeability barrier above the top of the decollement

zone. This barrier was implemented after the initial sedimentation steps and prior to the

loading by the prism (Table 3-4). As expected, when the low-permeability was added,

the k* values below the barrier abruptly increased to a maximum because the fluids in the

underthrust could not migrate through the barrier fast enough to keep pace with loading.

The simulated k* values at both Sites 1174 and 808 gradually decrease with depth below

the decollement (Figure 3-8).