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 observati on and the prevailing stress state during measurement. We present results o f laboratory geotechnical tests on sediments cored from the Costa Rica conv ergent margin during Ocean Drilling Program Leg 170, By measuring the perme ability of samples of differing lithology before, during, and after shearin g we show that hydrological behavior is linked to the consolidation state o f 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 h igh permeability-effective stress dependence after being deformed at a high overconsolidation ratio. Under these conditions, shear zones in fine-grain ed sediments can dilate and thereby act as efficient fluid-flow conduits. S uch stress-dependent permeability typifies the cyclic pressure buildup and release mechanisms (valving) invoked for many tectonic settings. We infer t hat 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.