IMPORTANCE OF A PERMEABLE DECOLLEMENT ZONE TO CONSOLIDATION OF SUBDUCTING SEDIMENTS: RESULTS OF NUMERICAL MODELING

Compaction of subducting sediments controls their mechanical properties and potentially affects the updip limit of seismogenic behavior.  For sediments to compact, pore fluids must be able to escape through either intergranular pore spaces or along fractures within fault zones. The plate boundary fault, or decollement, separates highly deformed overlying wedge sediments from slightly deformed underthrust sediments, and has been speculated to play an important role as a fluid conduit. This study examines how decollement permeability affects porosity loss within subducting sediments.  A transient modeling method was used that incorporates loading due to the weight of the overlying prism, fluid sources due to smectite dehydration, and fluid flow. Porosities were allowed to decrease with increasing effective stress, and a log-linear relationship between intergranular permeability and porosity was assumed.  Modeling used a generic geometry, moderate convergence rate (4 cm/yr), and properties representative of clay-rich sediments. Results indicate that without a permeable decollement zone, simulated porosities greater than 0.2 extended to depths greater than 10 km and distances greater than 70 km arcward of the deformation front. Addition of a moderately permeable (10^-15 m²) decollement zone allows much more rapid compaction and also affects the porosity profile.  A permeable decollement causes porosities to increase with depth (at a given distance from the deformation front), as opposed to a more normal profile of porosity decreasing with depth. Simulated porosity distributions are sensitive to whether the subduction zone is accretionary or non-accretionary, but this sensitivity decreases greatly as the decollement permeability increases.