The Northland Basin, New Zealand: Analysis of Gravity and Magnetic Data

Abstract

The offshore Northland Basin, New Zealand, is the northern extension of the petroleum-producing Taranaki Basin. The basin has a complex history involving Mid- to Late-Cretaceous rifting, associated with the separation of the New Zealand micro-continent from the Australian/Antarctic plate, and early-Miocene thrusting and calc-alkaline volcanism, associated with the onset of plate convergence. 2-D seismic reflection data show that a sequence of Cretaceous to Recent sedimentary rocks up to 3-4 km thick occurs within the main depocentres. Gravity and magnetic data from the basin are complex and reflect the distribution of basement and volcanic rocks in the basin. Significant positive gravity anomalies, typically sub-circular, are associated with Miocene volcanics, whose upper surfaces are often evident in the seismic reflection data but whose overall geometries are poorly imaged. More linear, but less pronounced, gravity highs are attributable to up-thrown basement ridges whilst strong gravity lows mark the main depocentres. Strong magnetic anomalies are a characteristic feature of the basin and similarly mark the locations of the Miocene-Pliocene volcanics. In the northern part of the basin especially, more linear magnetic anomalies appear to be associated with structural units in the basement, however the nature of basement rocks in the region is poorly known. Gravity and magnetic data can therefore play a critical role in resolving some important features of basin development.