The thermal and crustal structure of a continental back-arc basin: offshore Bay of Plenty, New Zealand

Abstract

The back arc basin resulting from the subduction of the Pacific Plate beneath the Australian Plate at the Hikurangi Margin, North Island, New Zealand is assessed using newly acquired marine geophysical data. These geophysical data provide insight into the thermal and crustal consequences of back-arc processes in continental lithosphere. The data are used to create an image of crustal thinning and the resulting enhancement of the thermal structure. The remarkably high heat flow observed in the Central Volcanic Region of New Zealand is a symptom of back-arc processes. Frequency domain analysis of magnetic data in the region indicates that these processes have compressed the geotherm resulting in a shallow, 9.4 ± 3 km, Curie point isotherm depth. This shallow Curie point isotherm, when combined with the limited depth of crustal seismicity and the elevated topography and bathymetry observed in the region, provides an image of a lithosphere that is thermally enhanced throughout its thickness. The mechanism of heat transfer within the lithosphere is not known with certainty. In order to explain the high magnitude heat flow observed and in the presence of evidence against extremely high geothermal gradients it is necessary to invoke advection of igneous material to at least the base of brittle-ductile transition. At this point an effective system of convection of meteoric water is thought responsible for the transfer of heat to the surface. Thinning of the crust is evident in the upper crust in interpreted multi-channel seismic data. Acoustic basement forms large scale half-graben structures typical of an environment where pre-spreadirig rifting is taking place. Correlation of acoustic basement with outcropping greywacke basement within the back-arc basin suggest that greywacke may be a continuous feature throughout the region, however thermal arguments make this unlikely. Thinning in the lower crust is consistent with the region's gravity field which is characterised by a broad positive gravity anomaly and a strong gradient that is perpendicular to the plate boundary. The positive gravity anomaly (~50 mGal) is suggested to be the southern continuation of the arc and back-arc high observed at the site of oceanic-oceanic subduction to the north. The gravity gradient represents a northwest to southeast decrease of ~ 50 mGal over ~ 65 km. Two-dimensional modelling using existing constraints suggests that the gravity field results from the replacement of approximately 10 km of the lower crust by igneous material and the juxtaposition of relatively thin and thick crust. The scenario presented in this study is that of a crust that has been thinned by rifting and intruded by igneous material. Normal faulting and half-graben structures are evident in the upper crust, an enhanced thermal regime is present throughout the crust, and replacement of the lower crust by igneous material has occurred. These phenomenon are all attributable to back-arc processes in continental lithosphere.