Mantle Tectonics of a Plate Boundary: the North Island of New Zealand

We constrain the extent of lithospheric and asthenospheric deformation by measuring and modelling anisotropy in the North Island of New Zealand through shear wave splitting of teleseismic waves, petrophysical analysis of mantle xenoliths and finite-element modelling of the mantle deformation in a back-arc spreading environment. We use seismic data collected on the GeoNet broadband stations deployed between 2001 and 2004 and a portable deployment in the Northland region. The results for the eastern part of the North Island confirm the earlier pattern. The fast directions are roughly NE/SW, which is parallel to the strike of the Hikurangi subduction zone. In the north of the North Island, two stations present back-azimuthal dependence of the splitting parameters. This dependence suggests a broader and more complex anisotropic effect, such as a dipping axis of symmetry or multiple layers, might be present beneath New Zealand, influencing the measurements on a regional scale. Results from the Northland deployment suggest that splitting from SKS phases decreases as the effects of present plate boundary deformation weaken and allow us to quantify the extent of the plate boundary deformation zone. The petrophysical analysis of mantle xenoliths from the Raglan (North Island) region allows us to constrain the lithospheric contribution to the observed seismic anisotropy. The spinel lherzolites analysed represent mantle material from depths shallower than 75 km and temperatures in excess of 1000°C up to 1150°C. Their extraction ages are less than 2 Myr. The maximum intrinsic anisotropy on the S-wave measured, 3.5%, seems too low to explain the delay times. This indicates that asthenospheric deformation plays the major role there. Through modelling of the lithospheric deformation, derived from surface deformation rates, in the North Island transtensive Central Volcanic Region, we will compare the anisotropy values obtained from the xenoliths to the modelled regional values derived from geodetic movement rates.