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Gravity and magnetic modelling of southern Northland Basin, New Zealand

Summary A combined interpretation of gravity, magnetic and seismic data has been used to investigate some key structural elements of the southern part of the Northland Basin which lies off the west coast of North Island, New Zealand. 2-D seismic reflection data reveal a thick sedimentary sequence, a number of significant structural highs and large Miocene volcanic complexes. The basement reflector is generally poorly imaged, especially below locations where volcanics are evident at shallower levels in the seismic sections. Gravity and magnetic maps clearly show the locations of the major volcanic centres which have strong anomalies (up to 50 mGal and 800 nT). In addition, a belt of positive magnetic anomalies (up to ~200 nT) trends N-S in the south and NW-SE in the north. This follows the strike of the main structural trends in the region and suggests the occurrence of a highly magnetic basement terrane. Preliminary 2.75D gravity and magnetic modelling shows that the Miocene volcanic bodies are broader than previously interpreted and extend to basement depths. Modelling also shows that the top of the basement is deeper (up to 9 km) than is evident in the seismic sections. The gravity and magnetic data therefore are invaluable in resolving some of the ambiguities in the seismic interpretation.