Geophysical anomalies and igneous bodies off the West Coast, North Island

Abstract

Gravity and magnetic anomalies off the west coast of the North Island are caused by igneous bodies much larger than any observed on land in New Zealand. North of Auckland the bodies resemble seamounts in shape and size; they appear to be volcanic piles about 5-6 km thick with steep sides and flat tops which form the sea floor, and are surrounded by sediments. South of Auckland the anomalies can only be interpreted as due to composite bodies. The greater parts of the gravity anomalies are probably due to igneous material intruding Tertiary sediments; the greater parts of the magnetic anomalies are probably due to igneous rocks intruding late Paleozoic basement. Unlike the northern anomalies which occur as isolated features, the southern anomalies lie on an extensive zone of magnetic activity which may run into the Norfolk Ridge. The relationship of the offshore bodies to the much smaller onshore volcanic features is by no means clear, but it would appear that the coastline separates regions of quite different tectonic styles.