Caldera Volcanoes of the Taupo Volcanic Zone, New Zealand

Abstract

The Taupo volcanic zone (TVZ) has been active since 2 Ma and has erupted > 10 '• km 3 of dominantly rhyolitic magma during the last 1 m.y. Most of the volcanism is concentrated in a 125 x 60 km area forming the central TVZ and is expressed largely as six major caldera volcanoes, Rotorua, Okataina, Kapenga, Mangakino, Maroa, and Taupo, marked by localized collapse of the underlying basement and clustering of known or inferred vent sites. These centers have activity spans from 150 to 600 ka and have each erupted at least 300 to 1000 km 3 of magma. All centers except Rotorua are known or inferred to have had complex histories of multiple caldera collapse, which have occurred alongside general basement collapse within the TVZ accompanying regional extension. Deep-seated NE trending basement lineations and/or faults have influenced vent sites at Okataina, Maroa, and Taupo. Welded ignimbrites are prominent in the pre-140 ka record; their absence since then is attributed to the effects of surface water on eruption styles rather than to a change in eruptive behavior. Volcanism from the centers has been overwhelmingly rhyolitic (> 97%; SiO 2 69-77 wt %) with minor high-A1 basalt and dacite and traces of andesitc, mostly as lithic fragments in ignimbrites from Okataina and Mangakino. Although insignificant in volume, the basalt is important as a low-Si end-member in mixing relationships with the rhyolite (at one extreme generating the dacites) and occasionally as a trigger for the rhyolitic eruptions. The current average rhyolite magma eruption rate from the central TVZ is ~0.27 m 3 s -x, equally divided between Okataina and Taupo, a figure close to the long-term average for the last 1.1 Ma. However, geothermal heat flow data imply that a further 1.4-1.8 m 3 s-x of magma may be intruded within the crust. The ratio of inferred intruded material to erupted material is higher at centers where lava extrusions are volumetrically significant (Okataina, Maroa), and this is correlated with lower phenocryst equilibration temperatures in the eruptives. Evidence for resurgent doming and long-term (> 105 years) magma cycles documented at similarly sized rhyolitic calderas in the western United States is absent from the TVZ centers; this is attributed to the young faulted crust of the region, preventing the formation of sufficiently large high-level magma chambers. In overall terms, the central TVZ is comparable in size and longevity to the Yellowstone system, but its individual eruptions have very much shorter recurrence intervals and smaller volumes. INTRODUCTION The Taupo volcanic zone (TVZ) is a complex volcanotectonic depression which is the locus of Quaternary volcanism (and geothermal activity) in the central North Island of New Zealand (Figure 1; Healy [1962]). It is broadly interpreted as a volcanic arc [Ballance, 1976] and/or marginal basin [Cole and Lewis, 1981] parallel to the southern continuation of the Tonga-Kermadec subduction system. The TVZ is only the latest expression of southward younging Cenozoic calc-alkaline vol...

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