Description: 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.

Description: Altered lavas have been dredged from three locations on the Resolution Ridge, west of New Zealand's South Island. On the basis of whole-rock geochemistry, Sr, Nd and Pb isotope data and Ar–Ar ages, they can be divided into two suites: 62–60 Ma enriched mid-ocean ridge basalt (E-MORB), and 57 Ma trachybasalt and trachyandesite of ocean island basalt (OIB) affinity. The E-MORBs from the Resolution Ridge are only the second place from which Tasman Sea abyssal oceanic crust has ever been sampled, they have Indian MORB-like isotope compositions, and their ages support a recent interpretation of a 100 km sinistral offset of the southern part of the Tasman Sea spreading ridge. The slightly younger OIB suite erupted shortly after oceanic crust formation and has FOZO to HIMU source characteristics similar to the well-known SW Pacific Diffuse Alkaline Magmatic Province (DAMP). The close occurrence and isotopic mixing relationships of both Paleocene volcanic suites on the Resolution Ridge may be explained by a heterogeneous upper mantle in which the more fertile OIB component was extracted during a later melting event away from the spreading ridge. The dredged lavas predate formation of Southeast Tasman oceanic crust that borders the Resolution Ridge to the south.

Description: Geophysical investigations of the northern Hikurangi subduction zone northeast of New Zealand, image fore‐arc and surrounding upper lithospheric structures. A seismic velocity (Vp) field is determined from seismic wide‐angle data, and our structural interpretation is supported by multichannel seismic reflection stratigraphy and gravity and magnetic modeling. We found that the subducting Hikurangi Plateau carries about 2 km of sediments above a 2 km mixed layer of volcaniclastics, limestone, and chert. The upper plateau crust is characterized by Vp = 4.9–6.7 km/s overlying the lower crust with Vp 〉 7.1 km/s. Gravity modeling yields a plateau thickness around 10 km. The reactivated Raukumara fore‐arc basin is 〉10 km deep, deposited on 5–10 km thick Australian crust. The fore‐arc mantle of Vp 〉 8 km/s appears unaffected by subduction hydration processes. The East Cape Ridge fore‐arc high is underlain by a 3.5 km deep strongly magnetic (3.3 A/m) high‐velocity zone, interpreted as part of the onshore Matakaoa volcanic allochthon and/or uplifted Raukumara Basin basement of probable oceanic crustal origin. Beneath the trench slope, we interpret low‐seismic‐velocity, high‐attenuation, low‐density fore‐arc material as accreted and recycled, suggesting that underplating and uplift destabilizes East Cape Ridge, triggering two‐sided mass wasting. Mass balance calculations indicate that the proposed accreted and recycled material represents 25–100% of all incoming sediment, and any remainder could be accounted for through erosion of older accreted material into surrounding basins. We suggest that continental mass flux into the mantle at subduction zones may be significantly overestimated because crustal underplating beneath fore‐arc highs have not properly been accounted for.