A geophysical study of the Taupo Volcanic Zone New Zealand

Abstract

The Taupo Volcanic Zone is a region of intense volcanic activity, from which over 16,000 km3 of lavas and pyroclastics have been erupted since the Late Pliocene. Analysis of gravity and magnetic anomalies over the zone has revealed the presence of five major centers, each associated with an approximately circular negative gravity anomaly and a positive pseudogravity anomaly. These centers are interpreted as collapse features resulting from large‐scale withdrawal of magma from the upper crust. Four of the depressions occur beneath recognized volcanic centers or calderas; the fifth (Kapenga) is inferred to underlie an extinct hidden volcanic center. Three‐dimensional models of the thicknesses of low density and magnetized rocks in the Taupo Volcanic Zone have been determined by inversion of gravity and magnetic anomalies. Beneath three of the centers, the thicknesses of low density and magnetized rocks are approximately equal (2–3 km); that is, the collapse structures are infilled by magnetized volcanics. However, there is a significant discrepancy between the thicknesses of the gravity (5 km) and magnetic models (1.5 km) in the region of the Okataina Volcanic Centre. It is therefore suggested that a significant volume of low density, nonmagnetic rock, possibly acidic magma, underlies Okataina.

Seismic reflection imaging of the Haraharo Caldera boundary beneath Lake Tarawera, Okataina Volcanic Centre, New Zealand

“…4 suggest that subsidence is not uniform within the inner caldera, as reflectors on the east side of the lake also dip eastwards. Such observations are consistent with the gravity modelling of Rogan (1982). The total of 400 km 3 erupted from the central 190 km 2 Haroharo Caldera ( Fig.…”

“…Spread uniformly over the 190 km 2 central caldera area, the 215 km 3 suggests subsidence of as much as 1100 m in the last 65 000 yr. The total collapse over the lifetime of the caldera may be up to three times this amount (Rogan 1982). Nairn (1989Nairn ( , 2002 mapped in detail the volcanic sequences of the Okataina Volcanic Centre, both those that originated from within the Haroharo Caldera as well as those from external sources.…”

“…1) (Nairn 1989) simplistically corresponds to c. 2 km of localised caldera subsidence as a cylinder. Rogan (1982) modelled the gravity anomaly associated with the Haroharo Caldera by a c. 15 × 25 km collapse structure that sloped down from the margins to reach a maximum 2 km depth in the south-central part of the lake.…”

Electrical resistivity image of the upper crust within the Taupo Volcanic Zone, New Zealand

“…Geophysically, the eastern margin is marked by a steep, linear gradient in the residual gravity anomaly [Stern, 1979;Rogan, 1980] [1987] suggests, for example, that block faulting along the margin is a continuing process with a sequence of new faults forming to the east, carving successive slices from the Kaingaroa Plateau. In this process the margin would be migrating eastward toward the ranges.…”

Petrology and geochemistry of Maungatautari, a medium-K andesite-dacite volcano

“…Pureora and Titiraupenga are composed of basic or 10w-Si andesite with characteristically low K 2 0 contents (Cole & Teoh 1975;Cole 1979;Froude & Cole 1985) and have major and trace element compositions generally similar to the average Tongariro low-Si andesite (Cole 1978b;Cole et al 1983). The Mangakino Basin was most likely an active rhyolitic volcanic centre during the early Pleistocene and a source of large ignimbrite eruptions (Rogan 1982;Wilson et al 1984), but the region north of Mangakino is obscured by a succession ofthickmiddle Pleistocene ignimbrite sheets. This implies that the Quaternary trend of the volcanic front has migrated both to the east and to the south which is consistent with the trend for the youngest volcanic activity derived from vents towards the eastern boundary of the Taupo Voleanie Zone.…”