The Awhitu volcanic complex, an offshore Pliocene volcano in the northern Taranaki Basin, New Zealand

Interpretation of seismic reflection data indicates that a volcanic complex of Pliocene age (between 5 and 2.2 Ma) lies offshore in the northernmost Taranaki Basin at 173.8 degrees E, 37.4 degrees S. The volcanic complex, here named the Awhitu volcanic complex, appears to be completely buried beneath late Pliocene and younger sediments. It has an irregular, unsymmetrical shape with at least three distinct peaks, and interpretation of all available seismic reflection data indicate it to be <700 m thick. Simple gravity and magnetic models, constrained by the seismic reflection data, predict that the Awhitu volcanic complex is comprised of rocks with a bulk density of 2350-2600 kg/m(3) and a bulk magnetisation of 3.5-12 A/m. A review of the density and magnetisation of rocks from the region indicates that Miocene and Pliocene andesitic volcanoes in the Taranaki Basin have a similar bulk density, but a lower bulk magnetisation than the Awhitu volcanic complex. The magnetisation is, however, within the observed range of basaltic rocks from volcanoes on the west coast of the North Island, and although it is possible for the Awhitu volcanic complex to be of andesitic composition, results from magnetic modelling suggest that it is composed of basaltic rocks. The morphology of the volcanic complex is also more characteristic of basaltic volcanic centres. The Awhitu volcanic complex is likely to have a similar origin to the onshore Pliocene-Pleistocene volcanoes of the South Auckland and Waikato districts, and if it is subduction-related, its location relative to onshore volcanoes may reflect the eastward migration of the subduction zone in the Pliocene-Pleistocene.