Abstract:
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.
