Abstract. The Taupo Volcanic Zone (TVZ) forms a region of back-arc extension within the continental crust of New Zealand and is notable for its caldera volcanism, high rate of magmatic production, and high heat flow. On the eastern side of the TVZ, volcanism terminates abruptly along a linear margin. We present the interpretation of three multiple-source bipoledipole resistivity surveys across this margin. The major features of the measured data can be reproduced by using two-dimensional models of the resistivity in the upper 10 km of the crust. Along the margin the high-resistivity graywacke basement (300 f•m increasing to 1300 f•m) is downfaulted to the west across a zone about 6 km wide, with a total vertical offset of approximately 2 km. The easternmost fault is overlain by an unbroken 0.24 Ma ignimbrite sheet, suggesting the fault has been inactive for this period. Further to the west, at depths greater than 2 kin, a conductive zone about 15 km wide and of total conductance 180-280 S has been identified. This conductive zone can be modeled by using a range of resistivities (40-3 f•m) and thicknesses (8-2 kin). Models incorporating resistivities less than 15 f•m require the low-resistivity zone to be wedge shaped, so that conductance increases to the west. The low resistivity is believed to be caused by the presence of clay alteration within volcanoclastic rocks. The large volume of volcanic rocks at depth can be linked to a series of caldera-forming eruptions that have taken place along the eastern side of the TVZ.

In order to study the regimen of the McMurdo Ice Shelf three holes, 31, 32, and 57 m deep, were drilled near the seaward edge of the shelf; ice thicknesses at the drill sites were calculated to be 33, 48, and 94 m respectively. The rate of melting at the bottom of the shelf was found to be 1 m per year at two drill sites. The vertical density gradient in each of the holes is larger than that observed at Little America Station, and the density increases abruptly by about O· 1 g cm-" at the top of a brine-soaked layer which was estimated to be less than 6 m thick in each hole. Temperature profiles can be explained on the assumption that the brine moves horizontally through the shelf from the seaward edge to the interior and supplies heat to the shelf by convection and by liberation of latent heat during freezing. The observed brine level in the holes is about 20 percent lower than the hydrostatic level.

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The solutions for the potential due to a current source near a hemispheroidal body of finite resistivity, which have been given by Cook and Van Nostrand, are adapted for use with dipole‐dipole resistivity arrays. In the limiting case where the current dipole is at a large distance from the hemispheroid a simple expression is obtained for the apparent resistivity measured at the center of the hemispheroid. For an arbitrary placement of current electrodes, apparent resistivities are obtained by summing a double series involving the associated Legendre functions. Theoretical results using differently shaped hemispheroids are compared with field data obtained from several dipole‐dipole resistivity surveys of the hot water reservoir of the Broadlands Geothermal Field, New Zealand. The comparison enables an estimate to be made of the depth of the geothermal reservoir.

A study of temperature measurements taken over an interval of 2·4 years in a 7·6 m deep hole on Hut Point Peninsula (Ross Island) has shown that the vertical heat flux is 164 ± 60 mW 1m2 (4 X 10-6 cal/cm 2 s) which is about 2· 5 times greater than the world average heat flux. The result has been obtained by fitting simultaneously all data to a series of harmonic functions using a least-squares method of curve fitting. The thermal effect of Quaternary basaltic extrusions which cover the peninsula is too small to explain the observed flux.

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