The integrated history of repeated caldera formation and infill at the Okataina Volcanic Centre: Insights from 3D gravity and magnetic models

Multistage collapse caldera create complex geological structures that are often buried by kilometers of pyroclastic infill and late stage lava domes, making study of their origins difficult from outcrop alone. Here we present new gravity and aeromagnetic data compilations derived from terrestrial, lake and airborne surveys to investigate the buried internal structure of the Okataina Volcanic Centre (OVC) and interpret its stages of development. Magnetic highs (1300 nT) are caused by a combination of thick lava flows and domes that infill the collapse structures, and a deeper feeder structure/dyke complex that extends below the basement to at least 6 km depth. Magnetic lows caused by hydrothermal alteration are associated with topographically low areas near the topographic and structural collapse margins suggesting fluid circulation within the caldera exploits both deep and shallow structures. Several large rhyolite lava flows contain low magnetisation rims reflecting the glassy outer carapace and crystalline core. The gravity data show a - 62 mGal residual gravity low associated with the OVC, with the steepest gradients occurring inside the topographic margins. The gradient of the gravity low is stepped towards its lowest point near the outlet of Lake Tarawera. Each step is interpreted as relating to the buried structural collapse margins of the Utu, Matahina, Rotoiti and possibly Kawerau, catastrophic caldera forming eruptions, creating an overlapping and nested caldera structure, however there is no evidence for an OVC encircling ring structure. Buried caldera margins associated with oldest Utu eruption may play a role in the location of the youngest Tarawera eruptions. We propose smaller amplitude gravity lows that extend outside the topographic margins of the caldera are related to lateral magma migration to the south-west, exploiting regional tectonic stress regimes, towards eruption vents within the caldera. 3D gravity inversion, including models numerically constrained by a 3D magnetotelluric model, suggest a caldera depth of 5000 +/- 500 m that accumulated over multiple collapse episodes along with rifting induced subsidence in the past 550 ka. Our multiphysics approach to investigating caldera structure has revealed buried structural margins and their relationships to eruptive vents and hydrothermal features at the OVC. Detailed geophysical models of caldera structure are rare globally, and our findings are consistent with analogue and numerical models of caldera formation that accompanies the most violent volcanic eruptions on Earth.