The Cretaceous through Eocene tectonic history of this region is similar to that of other margins of Antarctica. East-west rifts between Australia and Antarctica, a result of northwest–southeast oblique extension, may have formed as early as the latest Jurassic (Willcox and Stagg, 1990). In the Early Cretaceous, east Gondwana was still intact and the Tasmanian region lay deep within present-day Antarctica, southeast Australia, and the continental block of Lord Howe Rise, Campbell Plateau, and New Zealand (LCNZ) (Fig. F6). Ocean currents are inferred to have flowed west and north of Australia and east of the LCNZ continental block. Early in the Late Cretaceous, rifting caused marine transgression into the AAG from the west, and seafloor spreading commenced between ~95 Ma (Veevers, 1986) and ~83 Ma (Sayers et al., 2001). A northwest–southeast, left lateral Tasmanian-Antarctic Shear Zone (TASZ) absorbed the relative motion of the two continents west of Tasmania, and AAG waters transgressed southward along the TASZ. Spreading propagated eastward, being fully under way west of Tasmania by the middle Eocene (Royer and Rollet, 1997), but the Tasmanian-Antarctic land bridge in the east allowed little to no water exchange between the AAG and the proto-southwest Pacific.
In the Late Cretaceous (~75 Ma), continental breakup and seafloor spreading began between Australia and the LCNZ (Cande and Stock, in press). Rifting propagated northward east of Australia, forming the Tasman Sea, and final breakup off northeastern Australia took place in the Paleocene (~60 Ma) (Gaina et al., 1999). Thereafter, major ocean currents could flow along the eastern coasts of Australia and Tasmania, the ETP and STR, and along the Antarctic margin to the south. However, the Tasmanian land bridge separating the AAG from the Pacific Ocean remained essentially intact until the latest Eocene. When a deepwater passageway developed between South America and Antarctica, to complete the Southern Ocean oceanographic circuit, remains disputed. Barker and Burrell (1977, 1982) argued that a deepwater pathway could not have developed in Drake Passage until close to the Oligocene/Miocene boundary. In contrast, Lawver and Gahagan (1998, 2003) suggested that the passageway opened somewhat later than the Tasmanian Gateway but no later than the early Oligocene, allowing the ACC to become established by then.
Leg 189 drill sites were located on four continental tectonic blocks: Site 1168 in the Sorell Basin on the west Tasmanian margin, Sites 1169 and 1170 in the Ninene Basin on the western STR block, Site 1171 in a small strike-slip basin on the central STR block, and Site 1172 on the ETP. The drill testing of seismic profiles has helped interpretation of the local tectonics (Hill and Exon, in press). According to Royer and Rollet (1997), the ETP rifted from Tasmania and the STR as part of Tasman Sea break-up in the Late Cretaceous (95 Ma), although Site 1172 subsided only slowly until the late Eocene.
Apatite fission track dating (O'Sullivan and Kohn, 1997) indicates a period of uplift and erosion near the Paleocene/Eocene boundary on the western and eastern margins of Tasmania. Between the eastern and central STR blocks near Site 1171, deformation ceased along the Balleny Fracture Zone at ~55 Ma, dating breakup between the southeastern STR and Antarctica. Northwest–southeast strike-slip movement along the west Tasmanian margin (Site 1168) ended in the middle Eocene (~43 Ma), when fast spreading between Australia and Antarctica transferred strike-slip movement to the north–south Tasman Fracture Zone on the west STR margin (Site 1170). Continent-continent movement ended only when Antarctica cleared the STR at the end of the Eocene (~34 Ma). However, the STR continued to move northward along the Tasman Fracture Zone relative to the western spreading center, finally clearing it in the early Miocene (~20 Ma). Heat from the passing spreading center caused uplift along the margin. At all Leg 189 sites the water started to deepen somewhat around the middle/late Eocene boundary (~37 Ma) (Hill and Exon, in press), but this may be attributable largely to a decrease in sedimentation rates rather than accelerated subsidence. A very rapid period of subsidence occurred near the Eocene/Oligocene boundary.