The origin of the modern thermohaline ocean circulation system must postdate the tectonic
creation of a continuous Southern Ocean. Particularly important for the origin of the ACC-DWBC
was the opening of the Australian-Antarctic (South Tasman) and South American-Antarctic
(Drake Passage) deep-water flow gateways (Molnar et al., 1975; Lawver et al., 1992). The South
Tasman gateway, including the Balleny Fracture Zone (Lonsdale, 1988), opened to deep water in
the early Oligocene (~32 Ma), thereby allowing connection between the Indian and Pacific Oceans
for the first time (Kennett et al., 1972; Kennett, 1977). Later, at ~20 Ma (earliest Miocene), the
opening of Drake Passage (Boltovskoy, 1980) allowed the establishment of the full
circum-Antarctic ocean circulation. During the critical late Eocene to Miocene period, the New
Zealand Plateau was located downcurrent from the evolving South Tasman gateway (Watkins and
Kennett, 1971), directly in the path of the evolving ACC-DWBC system (Fig. 3).
To 181 Scientific Report: Geological Setting