Summary

The southern Pacific Ocean has a complex geological structure incorporating two major temporal and spatial elements. Prior to the Late Jurassic (150 Ma B.P.), the present continental margin elements of the South Pacific formed part of the convergent, or actively subducting, margin of Gondwana. The oceanic crust of the proto-Pacific Ocean was being subducted under Gondwana, but no trace remains of this older crust at present. About the late Jurassic, the Gondwana continent broke up with rifts forming between New Zealand/Australia and Antarctica and between South America and Antarctica. This breakup of Gondwana and the subsequent drift of the other continents from Antarctica led to the development of the southern Pacific Ocean as a feature of Cenozoic age, the continuation of the West Antarctic margin as a convergent plate margin and the development of continental rift (passive) margins between Australasia and Antarctica.

The structure and geological history of the old Gondwana margin (South America, West Antarctica and New Zealand) therefore results from the continued existence of a convergent margin from at least the early Mesozoic (200 Ma B.P.), probably as early as the Precambrian (600 Ma B.P.), to the present. A major unconformity (erosional event) resulted from the breakup tectonics of Jurassic (150 Ma B.P.) - Early Cretaceous (100 Ma B.P.) age. Other perturbations reflected in the geological history of the margin include the development of the active plate boundary through New Zealand during the mid-late Cenozoic and the sequential cessation of convergence at the West Antarctic subducting margin as the Aluk-Phoenix spreading centre impinged on the West Antarctica trench.

In contrast, the conjugate margins of Antarctica, Australia and New Zealand reflect younger (Cretaceous and later) rift and passive continental margin development. Sedimentation, in general, has been low and the rift margins are relatively sediment starved. The associated continental plateaus, such as Campbell Plateau, Ross Sea and South Tasman Rise, are therefore dominated by late Mesozoic-Cenozoic extensional tectonics with rift structures and crustal downwarp.

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