Remarkably little is known about the Cretaceous rifting process between New Zealand and Antarctica that affected the submarine regions within the New Zealand microcontinent. Bounty Trough provides insights into these breakup processes. Here we present results from a combined gravity, multichannel seismic, and wide-angle reflection/refraction seismic transect across the Middle Bounty Trough and interpret these results 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 $cong$ 7 -- 7.7 km/s, vs $cong$ 3.9 -- 4.5 km/s), high-density (ρ = 3.02 kg/cm3) body at the location of the thinnest crust on the profile. Here the crustal thickness is reduced to about 9 km from 22--24 km beneath Chatham Rise and Campbell Plateau. We interpret the high-velocity/density body as a magmatic intrusion into thinned continental crust. Our results show that the Cretaceous opening of Bounty Trough was very likely not the result of back-arc extension caused by collision of the Hikurangi Plateau with the Gondwana margin, but of continental breakup processes related to the separation of New Zealand from Antarctica. Rifting ceased in the Middle Bounty Trough at the onset of seafloor spreading. Comparisons with the Oslo Rift and the Ethiopian/Kenya Rift indicate that all three rift systems show analogous extensional features. From this we derive a stretching model for the Bounty Trough that combines elements of pure shear and simple shear extension.