Investigating crust and upper mantle structure of northwestern New Zealand: from receiver functions and surface wave phase velocities

Abstract

We employ a joint inversion of teleseismic receiver functions and surface wave phase velocities to determine the shear wave velocity structure in the crust and upper mantle beneath north-western New Zealand. Receiver functions primarily contain information on velocity contrasts, while surface waves are sensitive to the average shear velocity with depth. By performing a joint inversion we reduce the limitations of each method, resulting in a more robust shear wave model. Inversion results reveal regions of low shear wave velocity of ~2.8 km/s in the mid crust (10-19 km depth) and ~4.0 km/s in the upper mantle (70-90 km depth) beneath Quaternary intraplate basalt fields. We infer that the mid-crustal low velocity zones (LVZ) are bodies of partial melt, most likely rhyolite intrusions. We suggest the upper mantle LVZ is a zone of 2-3% partial melt and a source region for the basalts of the Auckland Volcanic Field. This is in agreement with hypothetical models for a shallow upper mantle source rather than a deep seated mantle plume for the Auckland volcanism. Average shear velocities for the upper crust are 3.4-3.6 km/s, increasing to 3.6-4.0 km/s in the lower crust. The Moho is interpreted to be 29±1 km deep in the southern end of the array, shallowing to 26±1 km towards the edge of the continental shelf of northern New Zealand. Low upper mantle shear velocities of 4.2±0.1 km/s are observed, and are thought to represent a small percentage of partial melt in the upper mantle beneath this back-arc region. These low velocities extend to greater depth (~90 km) beneath the southern stations, indicating that, mantle deformation associated with the mantle wedge to the south may extend as far north as the southern end of our array. Near surface low velocity layers extend to 6 km depth beneath a station at the northern tip of New Zealand, likely representing the expression of a thick sequence of sediments from the Northland Allochthon and the more recent Northland Basin.