Potential field and high-resolution bathymetry investigation of the Monowai volcanic centre, Kermadec Arc: implications for caldera formation and volcanic evolution

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 similar to 250 km(3) 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 volume of the main unit is estimated to be 200-300 km(3). 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 consists of a single collapse structure and that its complex shape and multiple ring faults can be attributed to prolonged activity and multiple collapse episodes. It remains unclear whether the current eruption at the Monowai cone is fed directly from a small underlying magma chamber, or laterally from a magma body adjacent to the Monowai pluton. However, 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.