VOLCANIC AND STRUCTURAL EVOLUTION OF TAUPO VOLCANIC ZONE, NEW-ZEALAND - A REVIEW

The Taupo Volcanic Zone (TVZ) in the central North Island is the main focus of young volcanism in New Zealand. Andesitic activity started at c. 2 Ma, joined by voluminous rhyolitic (plus minor basaltic and dacitic) activity from c. 1.6 Ma. The TVZ is c. 300 km long (200 km on land) and up to 60 km wide, as defined by vent positions and caldera structural boundaries. The total volume of TVZ volcanic deposits is uncertain because a sub-volcanic basement has not been identified, but present data suggest bulk volumes of 15-20,000 km(3), and that faulted metasediments form most of the immediate subvolcanic basement. Rhyolite (greater than or equal to 15,000 km(3), bulk volume, typically 70-77% SiO2) is the dominant magma erupted in the TVZ (mostly as caldera forming ignimbrite eruptions), andesite is an order of magnitude less abundant,and basalt and dacite are minor in volume (< 100 km(3) each). The history of the TVZ is here divided into 'old TVZ' from 2.0 Ma to 0.34 Ma, and 'young TVZ' from 0.34 Ma onwards, separated by the Whakamaru eruptions, which obscured much of the evidence for older activity within the zone. The TVZ shows a pronounced segmentation into northeastern and southwestern andesite-dominated extremities with composite cones and no calderas, and a central 125-km-long rhyolite-dominated segment. Eight rhyolitic caldera centres have so far been identified in the central segment, of which two (Mangakino and Kapenga) are composite features, and more centres will probably be delineated as further data accumulate. These centres account for 34 inferred caldera-forming ignimbrite eruptions, in the c. 1.6-Ma lifetime of the central TVZ. The modern central TVZ is the most frequently active and productive silicic volcanic system on Earth, erupting rhyolite at c. 0.28 m(3) s(-1), and available information suggests this has been so for at least the past 0.34 Ma. The rhyolites show no major compositional changes with time, though the extent of magma chamber zonation may have changed with the incoming of rifting and crustal extension in the past c. 0.9 Ma. Within the central TVZ, non-rhyolitic compositions have been erupted apparently irregularly in time and space; in particular there is no evidence for a geographic separation of basalts from andesites. Between 0.9 and 0.34 Ma, a major episode of uplift affected areas around the TVZ, while at the same time the main focus of activity may have migrated eastwards within the TVZ accompanying rifting along the axis of the zone. The modern TVZ is rifting at rates between 7 and 18 mm a(-1) and restoration of the thin (15 km) 'crust' (V-p less than or equal to 6.1 km s(-1)) beneath the central TVZ to its pre-rifting thickness (25 km) implies that rifting at such rates may have begun only at c. 0.9 Ma. The TVZ is a rifted are, but its longitudinally segmented nature, high thermal flux and voluminous rhyolitic volcanism make it unique on Earth.