Estimating Plate Tectonic Forces at the New Zealand Plate Boundary Using Strain Rates Derived From a Kinematic Model of Fault Slip

We construct a model of forces at the New Zealand plate boundary from dynamical thin-sheet modeling. Stress magnitudes estimated are 10-50 MPa and effective viscosities are 0.5-5 x 10(21) Pa s within actively deforming regions. Models that include only far-field forces and forces from variations in topography and bathymetry can fit observations in most of the plate boundary, but basal tractions are required to fit extension in Havre Trough. For models that include nontopographic forces, we specify a rheology to acquire a unique solution for each of three rheologies: power-law rheology with n = 3, power law with n = 5, and a pseudo-plastic equal-stress rheology. The inclusion of nontopographic forces allows these models to fit observations very well. We predict forces in Havre Trough equivalent to basal tractions of 7-10 MPa at 20-km depth. Models with n = 3 and n = 5 require antiparallel forces on opposite sides of the plate boundary to drive deformation in South Island which would imply a plate boundary zone that is more localized at depth. The equal-stress pseudo-plastic model drives deformation with plate motion boundary conditions and localizes it with variations in effective viscosity. The effective rheology of South Island is most realistically modeled by an equal-stress pseudo-plastic rheology. The n = 3 or n = 5 power-law rheology models require a highly localized boundary zone in the lower lithosphere, but a broader zone in the upper lithosphere.