ALBERT

Description: We present a revised model for the formation of southwest Pacific backarc basins from 120 Ma to the present day. Our aim is to improve our understanding of the tectonic regime operating in the region and its consequences for global plate motions. Such an understanding helps explain present-day structures observed on the continental and oceanic lithosphere and the underlying mantle. Regional plate reconstructions were created using gravity and magnetic data from backarc basins, plate-circuit closure, global tomography and existing geological data. Our model predicts convergence between the Australian and Pacific Plates along the Norfolk Ridge from 120 to 100 Ma, followed by the fragmentation of East Gondwana. East-dipping subduction east of Australia was initiated at ca 90 Ma along the Loyalty-Three Kings Ridge and may have trapped Cretaceous quiet-zone crust In the Norfolk Basin. The inception of this subduction system may have provided a driving mechanism for the opening of the Tasman Sea by means of slab pull. A jump in subduction to the east was subsequently initiated along a west-dipping subduction system at ca 45 Ma driven by the collision of the Loyalty Arc with New Caledonia. Consequently, spreading in the North Loyalty Basin occurred by anticlockwise rotation of the subduction hinge between chrons 20 and 16 (43.8–35.3 Ma). This was concurrent to Norfolk Basin opening and formation of the Cook Fracture Zone. Backarc-basin formation then transferred to the South Fiji Basin where magnetic anomalles from chron 12 to 7N (30.9–25.2 Ma) have been identified as two contemporaneous triple junctions. The complex spreading regime witnessed in the South Fiji Basin appears analogous to the North Fiji Basin and may represent the surface expression of a hot, shallow mantle consistent in character to a superswell. The South Fiji Basin ceased forming at ca 25 Ma in response to a major plate reorganisation coinciding with the inception of the Alpine Fault, docking of the Ontong Java Plateau with the Melaneslan Arc and transpressional obduction of the Northland ophiollte. A lull in basin formation throughout most of the Miocene was followed by the reinitiation of backarc basin formation in the Lau Basin (during the past ∼7 million years) and North Fiji Basin (during the past ∼10 million years). All these apparent episodes of backarc-basin formation during the past 45 million years are possibly related to mantle-slab interaction at the 670 km discontinuity.