Sediment thickness and crustal structure of offshore western New Zealand from 3D gravity modelling

3D modelling of satellite gravity data covering the offshore area west of New Zealand has been used to predict the depth to the seafloor, basement, and Moho. The models assume constant densities for each layer, with no lateral changes of rock properties. The modelling was done on a 2.5 km grid. Depths to the seafloor, basement, and Moho were interpreted from seismic data or taken from published sources, and were used to constrain the solutions. Where control points are available, the models generally match the interpreted depths to within the expected level of accuracy. Basement depth is underestimated in the deeper parts of the basin. This is due to the assumption of constant density layers. Use of a more appropriate density/depth function would have resulted in a better match with the interpreted depths. The modelling highlights Cretaceous rift structures along the margins of the Challenger Plateau, Lord Howe Rise, and West Norfolk Ridge. Horsts and graben oriented parallel to the Challenger Plateau and Lord Howe Rise margin formed prior to seafloor spreading in the Tasman Sea. They are typically 10-20 km across and 1-2 km deep. The northwest-southeast trend is interrupted by north-south-trending faults associated with a later phase of subsidence in the Bellona Trough. The basement structure of the West Norfolk Ridge extends to the southwest as a buried series of horsts and graben which separate sedimentary basins in Northland and the New Caledonia Basin. The sediment thickness predicted by the models in the New Caledonia Basin is thinner than that interpreted on seismic sections, emphasising the importance of using density gradients in models of deep sedimentary basins. The modelled depth to Moho is c. 13 km in the Tasman Sea and 25 km beneath the Challenger Plateau. Crustal thinning of c. 30% is predicted for the Bellona Trough, New Caledonia Basin, and Lord Howe Basin.