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.