A Geophysical Study of the Northern Taranaki Basin, New Zealand

Abstract

The northern Taranaki Basin is an informal name given to the sedimentary basin that lies off the west coast of the North Island, New Zealand between 37°S and the Taranaki Peninsula. This basin is situated within the Australian Plate close to the Pacific Plate boundary, so tectonic processes related to the evolution of the plate boundary have been the driving mechanisms for the basin's growth. The sedimentary sequence within the northern Taranaki Basin contains a useful record of the plate boundary processes, particularly that part of the sequence formed during the initiation and development of subduction in northwestern New Zealand. Modelling and understanding the tectonic processes that formed the northern Taranaki Basin is the subject of this thesis. Seismic reflection data are analysed and used to compile a set of 18 maps depicting isopachs and structure contours that make up the Cretaceous and Cenozoic sedimentary sequence. By combining these maps with others, previously published for the adjacent Northland and Taranaki basins, a complete and integrated map-set is obtained. This map-set is used to interpret the geological history and paleogeography of northwestern New Zealand. These interpretations indicate that Late Cretaceous to Eocene rifting and subsidence were followed by a period of compression, thrusting and foreland basin development in the Oligocene and Early Miocene and finally, by a period of rejuvenated extension from the Middle Miocene. Since the Oligocene the tectonic history of the northern Taranaki Basin has been governed by the relative position of the basin with respect to the subduction zone. As the subduction zone moved eastward, the basin was successively affected by reverse faulting (Early Miocene), arc volcanism (Middle and Late Miocene) and back-arc rifting (late Miocene and Pliocene). Identification of volcanic sequences from seismic reflection data enables an estimate of 8,500 ± 3,000 km3 for the volume of andesite rock erupted from volcanic centres in the northern Taranaki Basin during the Middle and Late Miocene. Further analysis suggests 1,000 to 2,500 km3 of magma were intruded into the basement beneath the volcanic centres during the period of volcanic activity. Thermal modelling predicts that heat from large igneous intrusions beneath the sedimentary sequence altered the maturity of a significant proportion of the petroleum source rocks in the northern Taranaki Basin. The area of source rocks in the northern Taranaki Basin can therefore be divided into regions that have been affected by Miocene volcanism and regions that have not. Modelling predicts Cretaceous non-marine source rocks that lie close to igneous intrusions are not currently generating hydrocarbons. In these areas, Tertiary marine sediments have now become more important as source rocks than the underlying Cretaceous non-marine rocks. The flexural strength of the lithosphere in northwestern New Zealand is examined by investigating the way it bends in response to surface and subsurface loads. Modelling of lithospheric flexure due to loading by the large offshore Manukau volcano suggests that the effective elastic thickness of the lithosphere (Te) in the vicinity of the volcano is 25 ± 12 km. A map of Te for northwestern New Zealand, derived from an analysis of isostatic admittance and the coherence between Bouguer gravity and topography, shows that Te in the Taranaki region (Te = 25 to 60 km) is about twice that of the Northland region (Te = 8 to 25 km). The lowest values occur in areas of present day high heat flow. Interpretation of seismic reflection data indicates that about 1.5 km of uplift has occurred in eastern parts of the northern Taranaki Basin during the Pliocene and Pleistocene. These data form a consistent pattern with published erosion estimates for the western North Island and together define part of a broad dome-shaped region of uplift/erosion centred on the Central Volcanic Region of the North Island. Geodynamic modelling of northwestern New Zealand shows that an upward loading pressure of 80 to 100 MPa within the Central Volcanic Region would give a pattern of uplift similar to that seen in Taranaki Basin. The Central Volcanic Region is interpreted as having a zone of anomalously hot mantle, 100 km thick, which is 80 to 100 kg/m3 (~3%) less dense than the surrounding material.