Gravity anomalies in Fiordland, south-west New Zealand

Abstract

Abstract A large positive isostatic gravity anomaly (+ 160 mgal) in Fiordland (south-west New Zealand), 40 km wide and 90 km long, is paired by a — 140 mgal negative anomaly off the coast to the west. The negative anomaly, centred about 25 km from the coast, is about 50 km wide and at least 180 km long parallel to the coast, and extends south as far as Dusky Sound. The positive anomaly is interpreted as being due to the upthrusting of mantle material to within 10 km of the surface in the area between Doubtful and Breaksea Sounds, and the negative anomaly to a thick sequence (> 30 km) of crustal rocks off the coast, which are probably in part sediments derived from the uplifted Fiordland block. The crust-mantle boundary between these two contrasting regions has a steep dip (> 70°) and could be identified with the southward extension of the Alpine Fault. Interpretation of the gravity and geological data also suggests that a more recent low angle thrust fault has brought the exposed acidic metamorphic rocks ...