Geological structure and volcanism of the Taupo-Tarawera district

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Abstract
The structure and volcanism of the Quaternary volcanic belt in the North Island of New Zealand are discussed in the light ot geophysical evidence. The two major structural features outlined by the gravity survey are the Kaingaroa Plateau, in which the Mesozoic basement rocks are above sea-level, and the Taupo-White Island Depression, in which the basement reaches depths of at least 12,000 ft. Between the two are basement blocks at various stages of subsidence. These features can be closely correlated with surface topography, which indicates recent structural evolution; there is also evidence for a genetic relation between the hydrothermal fields and basement faults. The distribution of volcanic rocks is shown by magnetic surveys; apart from the ignimbrites, these rocks are concentrated in the structural depressions; the dacites and andesites occur over partly subsided basement blocks, whereas rhyolites appear in the deepest depressions. This suggests an andesite-rhyolite sequence of volcanism, repeated as subsidence has progressed from one part of the area to another. A hypothesis is put forward to account for the appearance of the basalt in very small quantities. Although this does not explain the underlying causes, it correlates most of the observed features of the Taupo volcanism and suggests some factors responsible for the different types of volcanism here and on the West Coast of the North Island.

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