Potential field and bathymetric investigation of the Monowai volcanic centre, Kermadec arc: implications for caldera formation and volcanic evolution

Abstract

The dynamics of caldera formation are affected by internal factors, such as magma chamber structure and properties and regional tectonic stresses. Understanding their interrelationship is important for assessing volcanic hazard and reconstructing the evolution of a volcano. The Monowai volcanic centre, located at the Tonga–Kermadec Arc, is a basaltic–andesitic submarine stratovolcano and caldera. Recent surveys have shown that it is active, but little is known about its structure and evolution. Here we present a combined analysis of swath bathymetry and potential field data from Monowai, acquired during cruise SO215 on the R/V Sonne, in 2011 April–June. The Monowai caldera is associated with a 20–25 mGal Bouguer gravity anomaly high and a broad positive magnetic anomaly. Short-wavelength magnetic anomalies of up to +1400 and −800 nT are observed along the caldera rim and on the summit of Monowai cone. Inversion of the Bouguer gravity anomaly data shows that the caldera high is caused by a buried dense body with density contrast of up to +450 kg m−3 with respect to the surrounding arc crust. The body has a volume of ∼250 km3 and is divided into a main unit extending from 3 to 6 km depth and a shallower ring structure that underlies the rim of Monowai caldera. We estimate the density of the body to be 2650–2850 kg m−3, suggesting a mafic composition. The ring structure is interpreted as set of ring dykes and the main unit as a solidified or partly solidified magma chamber system (the Monowai pluton). The observed magnetic anomalies are consistent with pluton intrusion within the Bruhnes magnetic polarity. Analysis of the swath bathymetry data shows that Monowai is located in a 20-km-wide graben, part of an en-echelon, left-stepping horst and graben system that spans the length of the arc and backarc. The Monowai caldera is elongated perpendicular to the direction of rifting, suggesting that it is affected by the regional stress regime. Geomorphological analysis suggests that the caldera was created by repeated collapse of a single caldera structure and that its complex shape and multiple ring faults can be attributed to prolonged activity and multiple cycles of inflation and collapse. We propose a multiphase evolution of the Monowai volcanic centre comprising initial magma emplacement and crustal inflation; magma chamber evacuation and caldera collapse; caldera resurgence and mass waisting; southward migration of the eruptive vent; and formation of Monowai cone. It remains unclear whether the current eruption at the Monowai cone is fed directly from a small underlying magma chamber, or laterally from the Monowai pluton. Observation of radial fissure ridges on its flanks suggests that a shallow magma body may have been recently emplaced beneath the summit causing inflation and the formation of extensional cracks. The summit appears unstable and may be prone to lateral or vertical collapse. Therefore, the region remains a potential source of regional volcanic and tsunami hazard.