Four Brothers and a Waka: Investigating Lithospheric Accommodation of Shear and Convergence Underlying the South Island of New Zealand

New seismic anisotropy and P- and S-wave mantle tomography models produced from passive source seismic data provide insight into how mantle lithosphere deforms due to lateral shear and oblique convergence across the South Island of New Zealand. The shear wave splitting measurements show a zone of mantle seismic anisotropy 100 - 200 km wide with orientations of the fast quasi-shear wave nearly parallel to relative plate motion between the Pacific and Australian plates. Anisotropy of increased obliquity to relative plate motion bookend this 100 - 200 km wide zone. A model representing distributed lithospheric deformation matches the observed orientation and amounts of shear wave splitting. Likewise, given certain conditions of grain size in the asthenosphere, localized lithospheric deformation with diffuse shear in the asthenosphere also matches the shear wave splitting measurements. P- and S-wave tomograms show high-speed structure under the northwestern South Island reaching 400 - 450 km deep. This is evidence for oblique westward subduction of Pacific lithosphere since 45 Ma, consistent with estimates from finite rotations of 850 - 1000 km of subduction thought to have occurred since this time. The high-speed zone reaches 200 - 300 km below the depths of the deepest intermediate depth earthquakes, suggesting that ~ 200 - 300 km of slab below them is required to produce sufficient weight to induce the intermediate depth seismicity. Low seismic speeds seen to 200 km along the east coast of the South Island coincide with regions of volcanism and thinned lithosphere. In the central South Island directly under regions of mountain building, high speeds are found to about 200 km deep. This suggests the lack of an unstable drip due to convergence across the South Island, though is consistent with accommodation of convergence via either lithospheric thickening or intracontinental subduction