Carson, L.B.; Miller, C.A.; Massiot, C.; Villamor, P.; Alcaraz, S.; Leonard, G.S. 2025 3D Visualisation model of basement geology and caldera structure at Okataina Volcanic Centre. Lower Hutt, NZ: GNS Science. GNS Science report 2025/28. 22 p.; doi: 10.21420/4VWC-AJ33
Abstract
The Okataina Volcanic Centre (OVC) is an active caldera complex in the Bay of Plenty, New Zealand. It is situated near the north-eastern tip of Te Ahi Tupua – central Taupo Volcanic Zone (TVZ), an actively rifting, NE–SW oriented volcanic arc located within the Taupo Rift associated with the subduction of the Pacific Plate beneath the Australian Plate. The caldera complex is approximately 15 x 27 km, elongated along its N–S axis, and is infilled with Quaternary volcanic rock and sediments. Its eruptive history includes at least two periods of caldera collapse – the Matahina Eruption at 322 ka and the Rotoiti eruption at 50–60 ka, which respectively form the southern and northern segments of the caldera complex. Several geothermal systems are located near the caldera boundaries. The subsurface structure of the OVC is poorly constrained. It is located at a change in orientation of the Taupo; Rift; however, mapped rift-associated extensional faults terminate at or near the caldera boundaries and the relationship between rift structure and caldera architecture is unclear. Inherited structural fabrics in the underlying greywacke basement rocks are also inferred to have influenced caldera development, and potentially the development of the TVZ as a whole. A simplified 3D model of the top surface of the Mesozoic greywacke basement has been created using Leapfrog Energy software. The depth to basement is constrained within the caldera by recent 3D gravity inversion modelling that provides an estimate of basement depth, and outside the caldera by drilling intercepts of basement at Kawerau Geothermal Field, located ~10 km east of the caldera. The model is consistent with the New Zealand Active Faults Database and the New Zealand Community Fault Model. The 3D model and supporting data are available as Appendices to this report. The modelled structural network is based on a simplified interpretation of mapped NE–SW rift faults and N–S caldera collapse structures inferred from gradients in 3D gravity inversion modelling. The resulting model has an irregular rhombic network of faults and collapse structures bounding stepped fault blocks that produce a bowl-like caldera morphology. We discuss possible alternative fault models compatible with the sparse direct geological evidence. This model is a preliminary visualisation and conceptual model of the basement geology and structural framework of the OVC, within the broader context of the Taupo Rift setting. It provides insight into the relationships between tectonism, caldera volcanism and geothermal activity and will be a valuable resource to enable future research related to volcano-tectonic hazards and renewable energy potential of the area ( auths)
