Seismic and Gravity Investigations of the Central Volcanic Region, North Island, New Zealand
Gravity and seismic refraction studies were undertaken in order to investigate the geological structure of the Central Volcanic Region. A detailed analysis of density determinations from bore-hole rock samples, three seismic refraction surveys and a spectral analysis of the magnetic anomaly field are described. Interpretation of the observed gravity anomaly fie ld for the Central Volcanic Region is initially undertaken by analytically separating the observed anomaly field into its regional and residual components; the almost entirely negative residual anomaly field is then interpreted in terms of varying thicknesses of near-surface, low-density volcanic rocks. At Mangakino and just west of Taupo, however, it is found that the calculated gravity anomaly effect of the seismically determined thickness of low-velocity, and hence low-density, volcanic rocks is less negative than the observed residuals; at both locations "secondary residuals" of about -200 μN/kg remain unexplained. Models are presented that account for these secondary residuals as being due to discrete volumes of low-density molten rhyolite emplaced within the seismic basement. The second method of gravity interpretation used in this study involves modelling all components of the observed gravity anomaly field . This necessitated giving consideration to both the gravity effect of the subducted Pacific plate and to seismic data bearing upon the variation of crustal thickness and mantle density throughout the central North Island. A gravity model for the central North Island is developed for which the important features are: i) The crust of the Central Volcanic Region is deduced to be only about half the normal continental thickness, and underlying the crust is an "anomalous", low-density upper mantle. This finding from the gravity model is supported by the results of a previous study of upper mantle seismic velocities and from the interpretation of a longrange seismic refraction survey carried out within the Region. These seismic data indicate the depth to, and the velocity of the upper mantle beneath the Region to be 15 km and 7.4 km/s respectively. ii) The positive gravity anomalies that predominate over the western and northwestern North Island can largely be explained by gravity edge-effects associated with variations in the crustal thickness and mantle density within the back-arc areas of the North Island. The gravity model is interpreted as lending support for a previously made proposal that the Region is the site of asymmetric back-arc spreading, and that the crustal rocks now being created are transitional in character between typical oceanic and typical continental.