TECTONICS OF RIFT PROPAGATION INTO A CONTINENTAL-MARGIN - WESTERN WOODLARK BASIN, PAPUA-NEW-GUINEA

Oblique convergence along the irregular boundary between the Pacific and Indo-Australian plates in the SW Pacific has resulted in a transpressional regime. The drag of the overriding Pacific plate has produced a breakup of the Solomon microplate from the Indo-Australian plate and has induced its rotation within the mega-shear zone between the two major plates. The microplate is being separated from the Indo-Australian plate by passive rifting and seafloor spreading in the Woodlark Basin which is propagating westward along the continental margin of Papua New Guinea. The present-day pole of the microplate rotation with respect to Indo-Australian plate is estimated to be near the Owen Stanley Fault Zone which represents the suture after Paleocene-Eocene arc-continent collision. The average rate of the seafloor spreading propagation is estimated to be 150 mm yr-1 over the last 3.4 Ma. The transition between seafloor spreading and continental rifting is characterized by a dramatic reduction in the production of new oceanic crust and by a change from localized deformation within oceanic lithosphere to distributed continental extension. Variations between the two structural styles are being balanced by an accommodation zone and a newly developing transform fault. The style of continental deformation progressively changes due to a gradual decrease in lithospheric thinning along the rift axis toward the pole of opening. Three accommodation zones are proposed to balance differential extension and slippage between individual series of tilted blocks.