Initiation of the Fiordland-Puysegur subduction zone, New Zealand

The Australia-Pacific plate boundary south of New Zealand was an active ridge 45 Ma to 30 Ma, generating oceanic crust between the Resolution Rifted Margin and the Campbell Rifted Margin. Referred to as the Macquarie Ridge Complex (MRC), this boundary progressively evolved into a strike-slip boundary from 30 Ma to 20 Ma; the northern segment, the Fiordland-Puysegur subduction zone (FPSZ), has had a substantial transpressional component. Over the last 20 Myr, 600 km of highly oblique plate motion occurred at the MRC, and resulted in a maximum total convergence of 150-200 km at the FPSZ, which some simple models suggested might be near the threshold for a self-sustaining subduction. The morphology of the Puysegur Ridge shows a diagnostic change from uplift to subsidence expected for the transition of a subduction zone from being forced externally to being internally driven by the negative buoyancy of the slab. The large negative gravity anomalies over the Snares Zone, in the middle of the FPSZ, imply strong vertical forces pulling downward the lithosphere. To better understand these observations, we use a viscous flow forward model with a free surface to simulate the geodynamics of the FPSZ since 20 Ma. The forward model describes the dynamics of an incompressible, Stokes fluid. Brittle-ductile behavior of the material within the crust-asthenosphere is modeled by using a fluid viscosity defined via a composite flow law comprised from an Arrhenius and a Drucker-Prager rheology. The well-constrained relative plate motion between the Australian and Pacific plates is used to define a Dirichlet boundary condition for velocity within the lithosphere. In the mantle, we apply the hydrostatic pressure as a normal stress boundary condition. A simplified surface process model consisting of linear diffusion is applied at the free surface to simulate short-range erosion and sedimentation. Our models show that the topographic variations within the Puysegur Ridges may correspond to different stages of subduction initiation. The Snares Zone may be self-sustaining, while the northern and southern parts of the FPSZ are approaching such a self-sustaining threshold.