Stratigraphy and structural outline of the Taupo Volcanic Zone

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Abstract
The Taupo Volcanic Zone is a complex basin of volcano-tectonic subsidence in the central North Island. Here eruptive activity has persisted from approximately midPliocene to the present time. It is a basalt-andesite-rhyolite volcanic province well known for its ignimbrites, for the rocks are dominantly stratiform rhyolitic pyroclastics, and of these, welded ignimbrites are the most conspicuous. Massive rhyolite domes and andesite volcanoes are common, but associated dacites and basalts sparse. The ignimbrites are widespread, easily recognized stratigraphic units, which form the basis of stratigraphic subdivision. Except for surficial pyroclastic deposits, the ignimbrite sheets are the only rock units in the volcanic zone that can be distinguished from one another on the basis of lithology and mineral composition, and hence can be correlated over large distances. The ignimbrites average 100 ft in thickness and many have extended over areas exceeding 1000 sq. miles. The following litho-stratigraphic units, in order of increasing age, are described here: These are all newly named, except for those marked + which are redefined or modified from earlier literature. On the basis of surface exposure, the ignimbrite formations appear segregated into three areas: a western “fringe” or apron, a down-faulted central area, and an eastern marginal area. The western ignimbrites are mainly old, thin, but extensive sheets veneering an apron of Tertiary and Mesozoic sediments extending west from the Rangitoto and Hauhungaroa Ranges. The ignimbrites in the central area vary widely in age and are interbedded with thick sequences of tuffs and tuffaceous sediments, such as those of the Huka Group. The eastern ignimbrites cover the extensive Kaingaroa Plateau and fill valleys along the high Te Whaiti greywacke ranges to the east. The oldest of these are down-faulted and now deeply buried in the central area. Numerous unconformities and irregular gravity profile5 testify to a complex volcanic history in which periods of highland erosion and lowland sedimentation were frequently interrupted by large-scale ignimbrite eruptions. Lateral gradations in several ignimbrites suggest that they were erupted from different parts of the Taupo Zone, each presumably from vents tapping a separate magma chamber at depth. Following each voluminous eruption, piecemeal collapse occurred, partly along normal faults, contributing to the formation of subsidence areas such as the Mangakino Basin, Lake Taupo, and others within the Taupo-Rotorua Graben. Displacements of the order of 1500 ft or more are shown by the Paeroa and Rangitaiki Faults. Proven ignimbrite feeder dikes or vents have not yet been recognised, and it appears that most of those in the Taupo Volcanic Zone are in areas that have undergone maximum subsidence and burial.

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