THE ROLE OF SEDIMENT PERMEABILITIES IN RESTRICTING FLOW AND MAINTAINING HIGH PORE PRESSURES IN SUBDUCTION ZONES

Seafloor sediments carry fluids into subduction zones in their pore space and in hydrated minerals such as clay and opal. Once the sediments enter a subduction zone at the trench they are rapidly buried and subjected to increasing stress and temperature. The resulting consolidation and mineral dehydration provide an ongoing source of fluids at depth. Drainage to the seafloor is limited by the low permeability of the sediments, so that pore pressures rapidly increase with distance from the trench. Data from two long-term borehole observatories located in the Northern Barbados subduction zone show that, pressures reach 60% of the lithostatic load after 4 km of subduction. Results from steady-state hydrogeologic modeling of Barbados show excellent agreement with measured pressures. This agreement supports the general validity of using steady-state hydrogeologic models and laboratory permeability data to predict pressures in subduction zone environments.

Critical taper theory provides a quantitative estimate of pore pressures required to maintain the observed range of subduction angles and seafloor slopes in subduction complexes worldwide. Given equal convergence rates and seafloor sediment thicknesses entering at the trench, the subduction complex with the higher permeability sediments will have lower pressures. According to critical taper theory, the lower pore pressure complex will have a stronger prism base and steeper taper angle. The Cascadia Accretionary complex with an 8º taper angle and sand-rich sediments is an example of this type of system. In contrast, low permeability, clay-rich sediments are expected to result in high fluid pressures and low taper angles. The Barbados prism with a 3º taper angle is a good example. The Peru margin is puzzling case of an accretionary setting characterized by a high taper angle and convergence rate, but relatively low sediment permeabilities. Model results for Peru indicate that flow downward through the sediments to the basement can drain the complex and maintain the low pore pressures consistent with its steep taper angle.