Magnetic anomalies over southern New Zealand

Abstract

Total force magnetic anomalies at a height of 3.05 km over southern New Zealand are presented. Over the Haast Schist Group magnetic relief is low with the exception of a narrow band of positive anomalies of 100 to 300 nT running from the Alpine Fault southwards to Lake Wakatipu. The northern part of this band coincides with the extent of a lamprophyre dike swarm and with seismic reflections from about 3 km depth. The major anomaly in the northern part of the Southland Syncline is over the Darran Diorite. Restoring the continuity of the Southland and Nelson Synclines demonstrates that this anomaly can be correlated with similar anomalies in the Nelson region, invalidating a previous suggestion that the Nelson anomalies are due to ultramafic rocks. The Dunedin Volcanic Complex produces an anomaly of only 100 nT. This is consistent with magnetisation of the extrusive rocks only and with the neutralisation of the remanent component by intercalation of members of normal, reversed, and scattered polarity. Over the southern boundary of the Anglem Complex in Stewart Island is an unusually large negative anomaly of -500 nT amplitude. The anomaly is consistent with the polar low of a normally magnetised complex of Longwoods-Bluff and Anglem rocks overthrusting the granites to the south-west. Total force anomalies at sea are used to examine the Macquarie-Fiordland junction. The Macquarie Ridge anomaly swings northwards as it approaches Fiordland and parallels the coast as far as Dusky Sound. A positive anomaly of 200 nT lies parallel to and west of Resolution Ridge and other north-east-trending anomalies occur west of the Fiordland coast.