The contemporary force balance in a wide accretionary wedge: numerical models of the southcentral Hikurangi margin of New Zealand

Abstract

SUMMARYThe southcentral Hikurangi subduction margin (North Island, New Zealand) has a wide, low-taper accretionary wedge that is frontally accreting a >3-km-thick layer of sediments, with deformation currently focused near the toe of the wedge. We use a geological model based on a depth-converted seismic section, together with physically realistic parameters for fluid pressure, and sediment and décollement friction based on laboratory experiments, to investigate the present-day force balance in the wedge. Numerical models are used to establish the range of physical parameters compatible with the present-day wedge geometry and mechanics. Our analysis shows that the accretionary wedge stability and taper angle require either high to moderate fluid pressure on the plate interface, and/or weak frictional strength along the décollement. The décollement beneath the outer wedge requires a relatively weaker effective strength than beneath the inner (consolidated) wedge. Increasing density and cohesion with depth make it easier to attain a stable taper within the inner wedge, while anything that weakens the wedge—such as high fluid pressures and weak faults—make it harder. Our results allow a near-hydrostatic wedge fluid pressure, sublithostatic fluid overpressure at the subduction interface, and friction coefficients compatible with measurements from laboratory experiments on weak clay minerals.