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ODP Leg 189 was designed to test the hypothesis that opening of the Tasmanian Seaway and initiation of circumpolar circulation contributed to the thermal isolation of Antarctica, leading to the development of initial ice-sheet and oceanic thermohaline circulation. The clay assemblages of the Tasmanian region contain the traces of two tectonic stages associated with ocean opening south of the south Tasman Rise near the Palaeocene–Eocene boundary and strike-slip activity between the western Tasmanian land-bridge and Antarctica during the Late Eocene. Earliest Oligocene clays indicate that cooling of Antarctic margins and activity of western boundary circulation progressed with the regional subsidence.

The Lord Howe Rise (LHR) is a large, complex, and poorly studied fragment of thinned continental crust submerged 750–3000 m beneath the oligotrophic waters of the central Tasman Sea (Figure 1). Deep seismic profiles taken during recent cruises in the region have revealed an intriguing phenomenon: the eastern slope of the LHR has a prominent and extensive bottom simulating reflector (BSR) that cross‐cuts lithology at 0.65–0.75 s two‐way travel time (twt) [Exon et al., 1998; Lafoy et al., 1998]. Given best estimates for sonic velocity (1600 m/s) and thermal gradients (0.9+0.04°C/m) in sediment on the LHR, predicted temperatures and pressures at the depth of the BSR lie on the CH4‐CH4 hydrate‐seawater equilibrium curve. Thus, the BSR on the LHR most likely represents an interface between gas hydrate and free gas [e.g., Dickens et al., 1997]. Although BSRs and gas hydrates are increasingly found in the marine realm, the LHR discovery has raised an interesting conceptual problem c oncerning the origin of gas hydrates.

A geological and geophysical study of active submarine volcanoes offshore of Epi Island and between Epi and Efate Islands, New Hebrides Arc, has delineated five large, mainly submerged, calderas over a 90-km length of the arc. These vary in diameter from 15 to 30 km, and the two northern calderas have active post-caldera basaltic cones. Karua volcano lies on the submerged northern rim of the caldera northwest of Tongoa, and has periodically erupted medium-K20 basaltic andesite in this century. Submarine cones Epia, Epib, and Epic are aligned east-west and spaced 3.5 km apart on the rim of the northernmost caldera. Rocks dredged from these cones include basalt, dacite, and cognate gabbroic inclusions with magmatic affinities similar to those of the Karua lavas. A pronounced bimodal Si02 content is noted for the large caldera volcanoes in the New Hebrides and other oceanic arcs. This bimodality has important implications for physical processes of magma differentiation within arc-volcano magma chambers. The bulk of the uppermost level of the surficial (and seafloor) dacite pumiceous pyroclastics in the area of northeast Epi and Tongoa was derived from cataclysmic eruptions of more than one of the now largely submerged calderas.