Crust–mantle structure of the central North Island, New Zealand, based on seismological observations

Two geophysical features characterise the anomalous crust–mantle structure beneath central North Island of New Zealand: the apparent thinning of the felsic crust by a factor of at least 50%, and upper mantle P-wave velocities (Pn phase) about 10% less than normal (∼7.3–7.4km/s). Pn velocities increase slowly to a maximum of ∼7.8km/s at a depth of about 80–100km. Above a depth of 15km seismic P-wave velocities (Vp) are 6km/s or less, representing what is likely to be a mix of the original greywacke crust that has been stretched, along with an unknown volume of intruded igneous rocks that are in various stages of cooling. Defining the Moho is problematic in this dynamic region. Below a depth of 15km there is a continuous increase in Vp such that velocities ∼7km/s are reached at a depth as shallow as 20km on a east west line and ∼7.4km/s on a north–south line. We interpret these wave-speeds as a transition zone into low Pn velocities, rather than high lower-crustal values of Vp. Pn wave-speeds as low as 7.4km/s are detected by tomographic methods to depths of at least 65km and laterally over an area of at least 10,000km2. A 1–4% partial melt in the mantle wedge beneath the central North Island is estimated, based on Qp, seismic anisotropy, low values of Pn and amplitudes of deep reflections. These upper mantle features are more indicative of asthenosphere than lithosphere and, coupled with the geological evidence for rapid uplift here at 5Ma, are consistent with convective removal of the mantle lithosphere from western and central North Island in late Miocene times.