Is the Bounty Trough, off New Zealand, an aborted rift?

Abstract

Remarkably little is known about the Cretaceous rifting process between New Zealand and Antarctica as well as within the submarine parts of the microcontinent of New Zealand itself. The Bounty Trough offers good insights into these break-up processes. Here we present results from a combined gravity, multichannel seismic and wide-angle reflection/refraction seismic transect across the Bounty Trough and interpret this on the basis of velocity distribution and crustal composition derived from Poisson's ratio and P-wave velocity. The lower crust exhibits a high-velocity (vp ca. 7 - 7.7 km/s, vs ca. 3.9 - 4.5 km/s), high-density body (rho equals 3.02 kg/cm³) at the location of the most thinned crust of the Bounty Trough. In this part, crustal thickness is reduced from 22 - 24 km beneath Chatham Rise and Campbell Plateau to about 9 km. We interpret this high-velocity/density body as a magmatic intrusion into a thinned continental crust. Our results show that the Cretaceous rifting of the Bounty Trough is very likely not the result of back-arc extension caused by the Hikurangi Plateau subduction in the Gondwana margin, but of continental break-up processes related to the separation of New Zealand from Antarctica. Rifting ceased at the onset of seafloor spreading, so that only little oceanic crust was produced in the Middle Bounty Trough. Comparisons with the Oslo Rift and the Ethiopian/Kenya Rift indicate analogue systems and imply a stretching model that combines uniform stretching and simple shear extension.