Asymmetric, multiple-block collapse at Rotorua Caldera, Taupo Volcanic Zone, New Zealand

Calderas worldwide have been classified according to their dominant collapse styles, although there is a good deal of speculation about the processes involved. Recent laboratory experiments have tried to constrain these processes by modelling magma withdrawal and observing the effects on overlying materials. However, many other factors also contribute to final caldera morphology. Rotorua Caldera formed during the eruption of the Mamaku Ignimbrite. Collapse structure and evolution of Rotorua Caldera is interpreted based its geophysical response, geology and geomorphology, and the stratigraphy of the Mamaku Ignimbrite. Rotorua Caldera is situated at the edge of the extensional Taupo Volcanic Zone, in which major faults strike NE–SW. A second, less dominant fault set strikes NW–SE. These two fault sets have a strong influence on the morphology of Rotorua Caldera. No one style of collapse can be applied to Rotorua Caldera; it was formed during a single eruption, but subsided as many blocks and shows features of trapdoor, piecemeal and downsag types of collapse. Here Rotorua Caldera is described, according to its composition, activity and geometry, as a rhyolitic, single event, asymmetric, multiple-block, single locus collapse structure. The Mamaku Ignimbrite is the only ignimbrite to have erupted from Rotorua Caldera. Extracaldera thickness of the Mamaku Ignimbrite is up to 145 m, whereas inside the caldera it may be greater than 1 km thick. The Mamaku Ignimbrite can be separated into a basal tephra sequence and main ignimbrite sequence. The main ignimbrite sequence contains no observable flow unit boundaries but can be split into lower, middle and upper parts (LMI, mMI, uMI respectively) based on crystal content, welding, jointing, devitrification and vapour phase alteration. Juvenile clasts within the ignimbrite comprise three consanguineous silicic pumice types and andesitic fragments. Only the most evolved pumice type occurs in the basal tephra sequence. All three pumice types occur together throughout the main ignimbrite sequence, whereas the andesitic fragments are only present in uMI. Lithic lag breccias in uMI indicate a late stage of caldera collapse. Concentration of lithic fragments increases towards the middle of the ignimbrite, and may also reflect increased subsidence rate during an earlier stage. Collapse of Rotorua Caldera is thought to have occurred throughout the eruption of the main ignimbrite sequence of the Mamaku Ignimbrite, allowing simultaneous eruption of all the different pumice types and causing the abrupt transition from deposition of the basal tephra sequence to the main ignimbrite sequence.