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 (P_n phase) about 10% less than normal (∼ 7.3-7.4 km/s). P_n velocities increase slowly to a maximum of ∼ 7.8 km/s at a depth of about 80-100 km. Above a depth of 15 km seismic P-wave velocities (V_p) are 6 km/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 15 km there is a continuous increase in V_p such that velocities ∼ 7 km/s are reached at a depth as shallow as 20 km on a east west line and ∼7.4 km/s on a north-south line. We interpret these wave-speeds as a transition zone into low P_n velocities, rather than high lower-crustal values of V_p. P_n wave-speeds as low as 7.4 km/s are detected by tomographic methods to depths of at least 65 km and laterally over an area of at least 10,000 km². A 1-4% partial melt in the mantle wedge beneath the central North Island is estimated, based on Q_p, seismic anisotropy, low values of P_n 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 5 Ma, are consistent with convective removal of the mantle lithosphere from western and central North Island in late Miocene times.