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 × 1021 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. Plain Language Summary: New Zealand is prone to earthquakes because it lies on the boundary between two tectonic plates. Plates are the strong outer shell of the Earth that is broken into moving pieces. We construct a set of models of forces at the New Zealand plate boundary, to better understand why plates move. Tectonic forces originate in three ways: (a) transmitted through the plate from far away; (b) from topography that gravity is trying to make collapse; and (c) from local tractions between the shallow plate and differential movements deeper within the Earth. We find that local forces are needed to fit all observations, but not all our models seem realistic. The model that is most plausible has tectonic stresses that are approximately even across the plate boundary region, rather than being much higher within the deforming zone. Our estimates of tectonic stress are similar or slightly larger than previous estimates and we can fit all observations of how the faults in New Zealand are moving. Key Points: Stress magnitudes are 10–50 MPa and effective viscosities are 0.5–5 × 1021 Pa s in actively deforming regions of New ZealandExtension in Havre Trough is driven by local, nontopographic forces that are equivalent in magnitude to basal tractions of 7–10 MPaRheology of lithosphere in South Island plausibly fits a pseudo‐plastic model, while power‐law models require unlikely force patterns [ABSTRACT FROM AUTHOR]