Volcanic and structural evolution of the Okataina Volcanic Centre; dominantly silicic volcanism associated with the Taupo Rift, New Zealand

The Okataina Volcanic Centre (OVC) contains the northeastern most caldera complex in onshore Taupo Volcanic Zone (TVZ), New Zealand, sited largely within the Taupo Rift. Pyroclastic fall deposits and ignimbrites deposited in the Bay of Plenty coast area between 420 and 625 ka were probably erupted from OVC, and these provide a maximum age for the centre. The earliest ignimbrite for which there is good evidence for eruption from OVC is the c.550 ka Quartz-biotite Ignimbrite, exposed to the west of Lake Okataina and to SE of OVC. This ignimbrite probably correlates with one of the early ignimbrites found in the Kawerau geothermal wells, and is large enough to have been accompanied by caldera collapse. It was followed by extensive rhyolitic explosive eruptions of the Murupara Subgroup. culminating with eruption of the Matahina Ignimbrite at c.325 ka. This c.160 km(3) (magma Volume) eruption was accompanied by caldera collapse to form the southern part of the present day Okataina caldera complex. A long duration sequence of rhyolite lavas and pyroclastics was then erupted on the southern and western sides of OVC, before eruption of the > 100 km(3) Rotoiti Pyroclastics at c.61 ka was accompanied by caldera collapse on the northern side of the centre. The Rotoiti episode was followed by an intensive period of intra-caldera volcanic activity which is still going on today. The Mangaone Subgroup pyroclastics were erupted between 40 and 31 ka, and include the c.33 ka Kawerau Ignimbrite (similar to 20 km(3)), large enough to have caused further minor caldera collapse. In the last 26 ka, nine rhyolite eruption episodes have built the Haroharo and Tarawera lava and pyroclastic massifs (>85 km(3) magma volume) within the caldera complex. The structural boundaries of the OVC calderas are buried by the products of later eruptions, but are probably controlled by regional tectonic features. Both the Matahina and Rotoiti calderas appear to have embayments which represent downsags where magma has migrated along regional structures associated with the Taupo Rift. OVC is sited at a major offset within the young Taupo Rift and represents a structurally complex transfer zone. Some early rhyolite domes are aligned north-northwest suggesting control by structures in the subvolcanic basement, while more recent domes are aligned northeastwards, reflecting the orientation of the Taupo Rift. Southwestward propagation of the axial rift of the Whakatane segment and northeastward propagation of the Kapenga segment have created two linear vent zones through OVC (Harcharo and Tarawera). At Tarawera, fissures and near surface dikes formed during the 10 June 1886 basalt eruption are oblique to the vent lineation suggesting some near surface strike-slip component consistent with OVC being in a zone of transtension. (C) 2009 Elsevier B.V. All rights reserved.