A 3D crustal shear speed model of the Auckland volcanic field, New Zealand, from multi-component ambient noise tomography

Abstract

We present a 3D shear-wave speed model of the crust in the intra-plate Auckland Volcanic Field (AVF), that provides new depth information on the basement geology and is a step towards determining lithospheric controls on volcanism of the AVF, within the Zealandia continent. Beneath the AVF, our model finds a mid-crustal contact of the upturning ophiolite-rich Dun Mountain-Maitai Terrane (DMT), and the western end of the accretionary Waipapa (Caples) Terranes which are dominated by terrigenous sedimentary rocks. Westward and at depth, the DMT presents as a high-speed, thick, and horizontal part of the Gondwana-ward verging syncline seen in the DMT elsewhere in New Zealand. Along the western side of the Junction Magnetic Anomaly (which indicates the presence of the DMT), a major vertical discontinuity probably marks the contact between the horizontal DMT and the Waipapa (Caples) Terranes. The Waipapa (Caples) Terrane contain a large wedge-shaped low-speed zone tapering eastward and extending NNW-SSE alongside the DMT. In addition, the model identifies a sharp vertical discontinuity at the eastern edge of the AVF that extends from the near-surface through the low-speed wedge. We interpret this discontinuity as a relatively young subvertical fault with a significant downthrow to the west. It appears to be a reactivation of a fault zone inherited from the break-up of Gondwana. The model also imaged the largest massif of crystalline ultramafic rocks in the DMT that causes the Takapuna Gravity Anomaly and the NE striking Port Waikato-Mangatangi cross-fault system. The strike directions of the faults mirror a rhombic-rectangular fault grid inherited from the breakaway of Zealandia from Gondwana. The shear speed model was produced using frequency-time analysis of multi-component Rayleigh wave Green's functions, reversible-jump Markov chain Monte Carlo inversion of group arrival times, and the inversion of dispersion relations for shear speed as a function of depth.