Nonlinear stress dependence of permeability; a mechanism for episodic fluid flow in accretionary wedges

Recent studies of the hydrogeology of accretionary wedges demonstrate that permeability is a dynamic property that depends upon the scale of observation and the prevailing stress state during measurement. We present results of laboratory geotechnical tests on sediments cored from the Costa Rica convergent margin during Ocean Drilling Program Leg 170. By measuring the permeability of samples of differing lithology before, during, and after shearing we show that hydrological behavior is linked to the consolidation state of the sediment at the onset of shear, and to the formation or reactivation of deformation fabrics. One sample obtained from a fault zone displayed a high permeability-effective stress dependence after being deformed at a high overconsolidation ratio. Under these conditions, shear zones in fine-grained sediments can dilate and thereby act as efficient fluid-flow conduits. Such stress-dependent permeability typifies the cyclic pressure build-up and release mechanisms (valving) invoked for many tectonic settings. We infer that a fracture permeability, opened up at high fluid pressures, is several times to several orders of magnitude greater than the matrix permeability. Our results help quantify the degree to which hydromechanical coupling can enhance flow in the actively deforming parts of accretionary wedges.