Gravity and magnetic models at Rangitoto Volcano, Auckland Volcanic Field, New Zealand: Implications for basement control on magma ascent

In small-volume volcanism, pre-existing crustal structures can influence magma ascent processes. Rangitoto volcano in the Auckland Volcanic Field, New Zealand, provides a possible example of magma-structure interaction as this volcano was emplaced adjacent to an outcrop of a regional-scale basement fault, the Islington Bay Fault. In this study, we evaluated the magmatic plumbing system of Rangitoto using gravity and magnetic data acquired over the volcano and the adjacent, non-volcanic Motutapu Island. We modelled the Rangitoto internal architecture and magma plumbing using 2.5D forward and 3D inverse modelling methods. Both models are constrained by petrophysical data, while drill hole logs are only used to constrain the 2.5D model. Our model endmembers suggest that parallel magma pathways are present below the Rangitoto summit cones. In 3D magnetic models, this is evidenced by a fault-aligned pair of high-susceptibility bodies. Interpreting our models in conjunction with previously published geological and geophysical models allows us to hypothesise that the fault-parallel alignment of Rangitoto magma pathways reflects the primary influence of the Islington Bay Fault over the Rangitoto magma ascent. Magma diversion at shallow levels by other finer structures intersecting and adjacent to the fault could explain why Rangitoto erupted 3.5 km west of the Islington Bay Fault surface trace.