LATE CRETACEOUS TO MID EOCENE TECTONIC EVOLUTION OF THE SOUTHWEST PACIFIC

The SW Pacific has a complex tectonic history dominated by episodes of subduction, back-arc spreading and continental deformation. Its Late Cretaceous to mid Eocene (~85-45 Ma) evolution has been the subject of very contrasting reconstruction models, reflecting sparse and ambiguous data. Uncertainty in the timing of and motion at plate boundaries has also produced controversy around how to implement a robust SW Pacific plate circuit. It is agreed that the SW Pacific comprised three spreading ridges during this time: in the Southeast Indian Ocean and Tasman and Amundsen seas. However, one and perhaps two other plate boundaries also accommodated plate motions: in the West Antarctic Rift System (WARS) and between the Lord Howe Rise (LHR) and Pacific. Geologic and kine­matic data are reviewed to better constrain its plate motion history during this period, and determine the time-dependent evolution of the regional plate circuit. A model of (1) west-dipping subduction and basin opening to the east of the LHR from 85-55 Ma, and (2) initiation of northeast-dipping subduction and basin closure east of New Caledonia at ~55 Ma is supported. West-dipping subduction and basin opening were not driven by convergence, as has previously been proposed. Our plate circuit analysis suggests that between at least 74 Ma and subduction initiation at ~55 Ma there was little net relative motion between the Pacific plate and LHR, <20 km of convergence with a component of strike-slip motion. Subduction must therefore have been primarily driven by the negative buoyancy of the slab, or perhaps forced trench retreat due to orogenic collapse. Although at least two plate boundaries may have separated the Pacific plate and LHR during this time, as there was little to no motion between these plates then a plate circuit which treats the Pacific plate and LHR as a single plate (“Australian” circuit) will produce similar kinematic results to a circuit that leaves their relative motion unconstrained (“Antarctic” circuit). After 55 Ma we advocate using an Antarctic plate circuit as the tested Australian plate circuit models predict net compression in the WARS which is unsupported by current geologic observations. Our preferred model makes testable predictions, such as burial of an arc beneath the Tonga and Vitiaz ridges, to be investigated by future work.