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. Plain language summaryA 3D model of shear wave speeds in the crust provides new information on the geological structure beneath the Auckland Volcanic Field (AVF). The field is positioned above the contact between ocean floor rocks of the Dun Mountain-Maitai Terrane (DMT) in the west with fast wave speeds and a prominent wedge-shaped low-speed zone in the sandstone-dominated Waipapa/Caples Terrane in the east. The model images a fold structure of the DMT similar to that seen throughout New Zealand and at depth delineates the contact as a major sub-vertical discontinuity. A second vertical discontinuity just east of the AVF is very abrupt, is also oriented NNW-SSE and is probably caused by faulting. The model has also imaged the NE-oriented Port Waikato-Mangatangi cross-fault system in the south of the region and the large massif of coarse-grained intrusive rocks in the DMT that causes the previously recognised Takapuna gravity anomaly.