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

The 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 decollement 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 decollement beneath the outer wedge requires a relatively weaker effective strength than beneath the inner (consolidated) wedge. Increasing density and cohesionwith depthmake it easier to attain a stable taper within the innerwedge,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.