Geology of the Okataina Volcanic Centre : sheets part U15, part U16, part V15 & part V16, scale 1:50,000

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Nairn, I.A. 2002 Geology of the Okataina Volcanic Centre : sheets part U15, part U16, part V15 & part V16, scale 1:50,000. Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences geological map 25 156 p. + 1 fold. map

Abstract: Okataina Volcanic Centre (OVC)is the most recently active of the eight majoy rhyolite eruptive centres in the Taupo Volcanic Zone of New Zealand. Within the OVC lies Te Haroharo Caldera, a complex of coalescing collapse structures formed by a succession of voluminous pyroclastic eruptions. At least two major, and probably more than two smaller caldera-forming eruptions have occurred in the OVC during the last 400 000 years. Early pyroclastic eruptions from Okataina may include the ''quartz-biotite tuffs'' which appear to form part of the voluminous sequence of ''Whakamaru-group ignimbrites'' erupted from multiple source claderas in the Central TVZ at around 0.34 Ma. The Matahina Ignimbrite (0.28 Ma) was erupted from the OVC, and is interbedded with the Bonisch-Pokopoko-Omuku Pyroclastics (undated, and probably erupted in part from teh OVC). The Kaingaroa Ignimbrites were erupted from the adjacent Reporoa Caldera at 0.23 Ma. The Rotoiti Pyroclastics were erupted from Okataina at c.65 ka. These caldera-forming rhyolitic pyroclastic eruptions were followed by the Mangaone Pyrockastics sequence of eruptions between 45 ka and 28 ka, and the post-22 ka intracladera sequence during which the Haroharo and Tarawera volcanic complexes were built up in 11 distinct eruptive episodes. About 80 km3 of rhyolitic magma and 1 km3 of basalt were erupted from more than 40 vents within the OVC during the last 22 ka. During each episode, eruptions occurred from multiple separated vents along underlying fissures. The eruptive vents define two, subparallel, northeast-trending zones, apparently related to fundamental basement fractures that have controlled the location of the Haroharo and Tarawera volcanic complexes. The recent volcanic history of the OVC indicates that future eruptions will occur from multiple vents on both Tarawera and Haroharo vent zones. The next eruption may be of either rhyolitic or basaltic nature, with phreatomagnetic explosions occurring if vents are located in or adjacent to lakes. Pyroclastic falls, flows and surges and the extrusion of lava domes and flows, followed by lahars and floods, can be expected. Basaltic fissure eruptions may also occur in apparently random locations around Haroharo Caldera. Until potential vent locations are indicated by precursory activity, hazard zoning for futre eruptions can only be indicated and in very general terms. The volcanic history of the OVC suggests that a large magmatic heat source still underlies Haroharo Caldera. Little surface hydrothermal activity occurs within the caldera, however peripheral geothermal fields at Tikitere, Waimangu, Waiotapu, Waikite (and possibly Kawerau) may be related to Haroharo Caldera heat sources. Measured and estimated chloride fluxes into lakes and rivers at the OVC indicate that the total convective heat flow from Haroharo Caldera approaches 1500 MW, about five times greater than the volcanic heat flow represented by the eruption of 500 km3 of rhyolitic magma over the last 400 000 years. Hydrothermal convection has thus played a dominant role in the transfer of heat from deep in the crust to the surface. The evolution of Haroharo Caldera and its subsidiary structures between 65 ka and the present has greatly affected regional drainage patterns. Drainage fromt he 500 km<sup3 Lake Rotorua catchment, previously dammed by eruption of the Rotoiti Pyroclastics, was captured by headward erosion from Haroharo Caldera at c. 24 ka. The Rotorua basin then drained through Haroharo Caldera into the Whakatane Graben, via the ancestral Tarawera River, until this channel was blocked by extrusion of the northern lavas of the Haroharo complex, diverting Rotorua drainage northward into the Kaituna River at some time between 21 ka and 7.5 ka; most probably at 9 ka (auth)

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