Gravity Modeling of Crustal Thickness and Lithospheric Structure of the Puysegur Trench, South Island, New Zealand: Insights into Subduction Initiation

This research is part of the South Island Subduction Initiation Experiment (SISIE), which in February and March of 2018, aboard the R/V Marcus G. Langseth, mapped and imaged the Puysegur Trench, south of the South Island of New Zealand. The survey utilized multichannel seismic (MCS) reflection and ocean bottom seismometer (OBS) refraction data, along with gravimetric, bathymetric, and magnetic data. The Puysegur subduction zone is a unique location for studying the process of subduction initiation because it is making the transition from a forced to a self-sustaining state, a process by which slab pull begins to dominate the force balance, leading to the development of a mature subduction system. Horizons for the seafloor, top of basement, decollement, and boundary between younger sediments and older accreted sediments in the inner trench wall were picked from the seismic reflection profiles that crossed the Puysegur Trench and Ridge. These horizons were used to constrain sediment and crustal thickness volumes for gravity modeling. Depths to the horizons, from two way travel time, were determined with velocity modeling from the OBS data. Densities in accord with the seismic velocities and standard porosity-depth relationships, constrained by industry borehole data from offshore New Zealand, were assigned to blocks of the structure model. We forward modeled the gravity signal from the density model and compared it to satellite gravity from Sandwell et al. (2014) to determine appropriate densities and structural heterogeneities. Specifically, we investigate the origin of the gravity anomaly over the Snares Zone and test whether Puysegur Ridge is composed of oceanic or stretched continental crust, the nature of which may exert control on the interaction between the strike-slip Puysegur Fault and active subduction. Additionally, we use an inverse approach, inverting the regional satellite free air gravity data for crustal thickness and density, which are also constrained by the OBS data. These results provide a more comprehensive understanding of the lithology and structure of the Puysegur margin and the rheological and mechanical controls on subduction in the region, and can be applied to broader geodynamic models of subduction initiation as a global process.