GEOLOGIC AND KINEMATIC CONSTRAINTS ON LATE CRETACEOUS TO MID EOCENE PLATE BOUNDARIES IN THE SOUTHWEST PACIFIC

Within global plate tectonic reconstructions, our ability to link the kinematic history of the Pacific plate is strongly dependent on unravelling the geodynamics of the southwest Pacific region. The kinematics of plate boundaries within and around Antarctica, Australia, and Zealandia profoundly affect how we link Pacific plate motions to the Indian and Atlantic domains, with implications for understanding hotspot motions, plate motion change, and alignments between seafloor spreading and absolute plate motion. However, the geodynamic evolution of the southwest Pacific has been the subject of starkly contrasting plate reconstruction models, reflecting sparse and ambiguous data in a region where much of the geological record lies submerged. Within the Late Cretaceous to Eocene time period, disparate models have been proposed: (1) west-dipping subduction and back-arc basin opening to the east of the Lord Howe Rise (northern Zealandia); (2) east-dipping subduction and back-arc basin closure to the east of the Lord Howe Rise, and (3) tectonic quiescence with no subduction or spreading between Zealandia and the Pacific. To help resolve this long-standing problem we review the geological evidence for deformation and plate boundary processes from around Zealandia; we also test a range of plate kinematic models, to explore the consequences of alternative plate circuit calculations for implied plate kinematics in candidate plate boundary regions. Based on our synthesis, our preferred kinematic model incorporates east to northeast directed rollback of a west-dipping subduction zone between 85 and 55 Ma, accommodating opening of the South Loyalty back-arc basin to the east of New Caledonia. Around 55 Ma, west-dipping subduction and back-arc basin spreading end, and there is initiation of northeast dipping subduction within the back-arc basin. Significant ambiguity remains in these reconstructions, confounding attempts to more tightly constrain rapid changes in Pacific plate motion around the time of the Hawaii-Emperor bend within a global reconstruction framework. Nonetheless, some anomalous characteristics of the Pacific plate history, such as the high degree of skewness between spreading and absolute motion directions in the Late Cretaceous, persist independent of the specific plate circuit used.