Abstract
Summary The Bouguer map of New Zealand exhibits two major negative gravity anomalies. The Rangitikei-Waiapu Anomaly, which intersects tne axial ranges of the North Island, indicates a crustal downwarp which is not in isostatic equilibrium. This anomaly is closely parallel to the zone of intense seismicity, to the Taupo-White Island volcanic belt, to the Kaimanawa-Huiarau-Raukumara Ranges, to the thick Upper Pliocene sediments, and to the Hikurangi Trench; and it appears to be an expression of an active tectonic belt related to the Tonga-Kermadec-Hikurangi Trenches. The Rimutaka-Ruahine axial ranges are apparently not underlain by mountain roots. The Alpine Anomaly in the South Island is considered to be the gravitational expression of the crustal roots of the Southern Alps and the mountains south-west of Blenheim; the magnitude of the anomaly suggests that this region is' approximately in isostatic equilibrium.
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