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.

Identity of igneous bodies beneath the continental shelf of west Northland, New Zealand

“…The interpretations by Davey (1974) and Hatherton et al (1979) ofthese large positive gravity and magnetic anomalies as large, basic igneous bodies, seems well founded. Kamp, however, does not appear to have used the higher quality seismic data (Geco N.Z.…”

“…Kamp, however, does not appear to have used the higher quality seismic data (Geco N.Z. 1982) now available and appears to have relied on Hatherton et al's (1979) interpretation of inferior quality seismic data (Australian Gulf Oil Company 1973) that the igneous bodies are flat-topped volcanic seamounts.…”

“…Kamp's inferred Late Eocene or Oligocene age for the west Northland igneous bodies depends largely on his hypothesis of mid-Cenozoic rifting and the acceptance of the age inferred by Hatherton et al (1979). The age adopted by Hatherton et al is based solely on a K-Ar date of 31 m.y.…”

Heavy mineral rich beach sands of the Waitakere coast, Auckland, New Zealand

“…Hatherton et al (1979) considered that they indicate a very large volcanic body, of basicintermediate composition, and of probable Tertiary age. Again, such a body would represent a possible source of sand of the mineralogy observed on Waitakere beaches, but there is no way of testing the supposition.…”

“…1). It was designed to: (1) establish any longshore trends in mineralogy and grain size; (2) investigate the source of the titanomagnetite and ilmenite, and other heavy minerals; (3) consider the possible role of the offshore ironsand concentration (Carter 1980) and the large offshore magnetic igneous body (Hatherton et al 1979) in supplying heavy minerals to the beach system; and (4) draw conclusions about the results of coastal processes.…”