Detailed mesoscopic and microscopic analyses of the two lower slope sites drilled in the Costa Rica subduction complex have revealed three distinct structural/hydrological regimes. The sedimentary prism records fluctuations in the maximum principal stress from vertical to near horizontal because of variations in the fluid pressure. Earlier structures, such as kink bands and vertical deformation bands, result primarily as accommodation structures in response to progressive burial. Superimposed on these structures are bedding-parallel deformation bands, interpreted as shear bands that formed by rotation of the principal stress and subsequent shear compaction. They display intensely aligned clay particles arising from porosity collapse, fabrics that result in a bulk decrease in permeability that do not focus fluids extensively.
The décollement zone is characterized by heterogeneous grain-size distribution, deformation style, and clay mineral fabric. Diffusive flow occurs in more silty zones, with more focused flow along brittle fracture surfaces. Experimental data confirm that permeability can vary along such fractures in response to fluid pressure. The bulk flow pattern along the décollement is interpreted here to represent prolonged periods of fluid pressure buildup interspersed with short-lived, transient episodes of enhanced fluid flow as fractures interconnect and allow dissipation of pressurized fluids. Both the hydrological and structural features point to shear failure under conditions of either reduced effective stress due to raised fluid pressure after consolidation or within regions where incipient cementation has provided enough internal strength to allow strain softening and localized slip to occur.
The underthrust unit displays markedly different deformational style from both the prism and décollement, indicative of efficient decoupling between the two plates, as observed for other margins (e.g., Barbados; Housen et al., 1996). Comparison of physical properties profiles between the underthrust sections and the reference site show that much of the compactional thinning of individual units occurs through porosity loss alone. However, the presence of bedding-parallel deformation bands and normal faults suggests that strain has become localized in certain regions but are unlikely to affect dewatering processes.
The results have implications for other convergent margins that, while containing lithologically distinct sediments, exhibit similar patterns of focused flow and geochemical anomalies, for example, Barbados (Moore, Klaus, et al., 1998); Nankai (Taira, Hill, Firth, et al., 1991); and Cascadia (Westbrook, Carson, Musgrave, et al., 1994). Fault zones need not have intrinsically higher matrix permeabilities than surrounding sediment. Indeed, fracture-induced flow is more likely to occur in sediments that are extremely overconsolidated and hence more susceptible to brittle failure. The accompanying response may be a drop in fluid pressure, implying that overpressure is not a prerequisite for dilation and development of a fracture permeability.