Fluid-flow pathways in actively deforming sediments: the role of pore fluid pressures and volume change

Abstract

The manner in which sediments respond to burial and shear depends largely on the ease with which fluids can escape. Rapid increase of overburden pressure or shear prevents efficient dewatering and can lead to the generation of excess pore fluid pressures. A principal control on the development of overpressure and hence mechanical strength is the sediments permeability. Observations from modern plate-margin accretionary prisms indicate that many faults are associated with both overpressuring and efficient fluid flow that operates in pulses, indicative of permeability that is constantly changing. We present the results of laboratory experiments exploring how the permeability of sediment retrieved from the Barbados accretionary prism and laboratory analogues varies with progressive strain. Volume changes are linked to permeability fluctuations and the drainage capacity of the material. We investigate the importance of the previous stress history imparted on the sediment and quantify the extent to which different deformation structures can acts as conduits for fluid flow under conditions of fluctuating effective stress. Material that has been initially consolidated and then subjected to reduction of effective stress, either by raised fluid pressure, or reduction of load (inducing overconsolidation) dilates during shear zone formation and displays an increase in permeability during strain. Shear fabrics can localise flow further where the principal mode of deformation is intensely localised and brittle, and effective stress approaches zero. Conversely, normally consolidated sediments deform by bulk volume loss without generating discrete fabrics, and hence do not enhance flow under reduced effective stress conditions. Permeability can increase during strain by up to one order of magnitude when deformation results in sudden porosity collapse and pore water expulsion. Microstructural analysis and permeability testing before, during, and after shear have helped constrain the principal factors that control the evolution of flow conduits in fine-grained low permeability sediments.