A 3D Gravity Inversion of the Puysegur Trench, New Zealand, with Insights into Subduction Initiation

We have constructed a linear 3D gravity inversion model of the Puysegur Trench, south of New Zealand, where oceanic lithosphere of the Australian plate has underthrust Puysegur Ridge and Solander Basin on the Pacific plate since the Miocene. These models provide insights into the density contrasts, structures, and state of isostatic balance in the region, which are key to understanding the process of subduction initiation. Our procedure inverts the Sandwell et al. (2014) 1 min gravity grid to determine the best fitting differential densities of spatially discretized subsurface prisms in a least squares sense. We employ a Bayesian approach to incorporate both data error and prior constraints based on seismic data from the South Island Subduction Initiation Experiment cruise. Subsurface densities and local structure along our survey lines were estimated from seismic refraction velocity models and horizons picked from seismic reflection data, respectively. Based on these densities, Gaussian priors were applied to the appropriate model parameters as absolute equality constraints. To stabilize the inversion and provide relative equality constraints on the parameters, we utilize a combination of zeroth, first, and second order Tikhonov Regularization, which enforces a smooth solution in the x and y directions between the seismically constrained regions, while allowing for sharp contacts in the vertical direction. Our best fitting models reveal that the large negative gravity anomaly over the Snares Zone requires a low-density root beneath Puysegur Ridge, which is composed of continental crust. The Solander Basin is composed of rifted continental margin crust. Structural profiles from our final models, in combination with the associated gravity, can be used to determine the state of isostatic compensation both along and across the subduction system. The continental composition of Puysegur Ridge and Solander Basin implies large density contrasts are key for subduction initiation, and subsidence of buoyant crust implies the system is transitioning to a slab pull state, with a greater component of slab pull in the north than in the south.