Two-dimensional and three-dimensional finite element modelling of mantle processes beneath central South Island, New Zealand

Abstract

SUMMARY Numerical models that include pre-existing faults, elasto‐visco‐plastic rheology, thermal structure, realistic plate boundary conditions, and surface erosion are built to investigate alternative hypotheses of mantle flow geometry beneath central South Island, New Zealand. Bouguer gravity anomalies, teleseismic traveltime delays and seismic velocity structures are used to constrain models. Model predictions are compared with topography, uplift, magnetotelluric resistivity structure, isostatic gravity anomalies (IGAs), and seismicity. We find that the wedge model, in which the mantle lithosphere of the Australia plate is wedged between crust and mantle lithosphere of the Pacific plate, gives reasonable predictions of topography, uplift rate, IGAs, and stress and strain state in the lithosphere. Coupled erosional-tectonic models suggest that tectonic compression primarily determines the asymmetric surface relief, with erosion influencing its detail and having small effects on shallow stresses. Both 2-D and 3-D models suggest that the neotectonics of central South Island is dominated by weak faults. 3-D model results show that the Alpine fault and mantle shear zone accommodate most convergent and strike-slip plate motion, confirming non-partitioned plate motion in this region. Adding the regional strike-slip component has little effect on the convergent velocity field, and only changes the stress field by rotating the horizontal principal stresses out of the plane of the cross-section.