TECTONIC PROCESSES OF THE PAPUA-WOODLARK-SOLOMON ISLANDS REGION

Abstract

Globally-significant plate tectonic processes can be observed in detail in the Papua New Guinea – Woodlark Basin – Solomon Islands region. These include the rifting and break-up of a continent and the transition to oceanic seafloor spreading, the initiation and evolution of transform faults, propagating and overlapping spreading centers, the synchronous reorientation and jumping of spreading centers, island arc-plateau collision and the reversal of subduction, and the subduction of topographic rises, seamounts and very young lithosphere. The Woodlark Basin is an ideal location for studying these tectonic processes because it is completely mapped with high-resolution bathymetry, acoustic imagery and magnetic data, in addition to marine and satellite gravity data. A large part of the mapping was completed on R/V Kilo Moana Cruise 0418. Thin sediment covering the basement fabric of the young basin and a clear magnetic reversal history allow tight spatial and temporal constraints to be placed on its tectonic evolution. Most of the tectonic processes are currently active and occurring at high rates (rifting and spreading up to 20 mm/yr and subduction at >100 mm/yr), which allows them to be illuminated by studies of the associated earthquake seismicity over multiple fault rupture cycles. The three-fold increase in the volume and quality of the seismicity data reported post-2010 compared to pre-1995, together with improved procedures for preparing and processing the data, have significantly improved the relocation of catalogued hypocenters, especially their depth. Seafloor spreading in the Woodlark Basin separates the Australia and Woodlark plates and the formerly contiguous eastward extensions of the Papuan Peninsula. To the north, the basin is bounded by the Woodlark Rise rifted margin and the splaying Nubara Transform Fault. The conjugate margin on the south is the Pocklington Rise which, in the east, is bordered to the south by the Itina Trough, a young rift graben. The Woodlark Basin is subdivided at Moresby Transform Fault and Simbo Fracture Zone into the western, eastern and easternmost sections. The westward stair-stepping and propagating of the Woodlark Basin spreading center into the rifting Papuan continent occurred simultaneously with its subduction to the ENE beneath the Solomon Islands. Both occurred post ~8 Ma following the arrival of the 40-km-thick Ontong Java Plateau on the Pacific Plate, whose collision with the Solomon Islands led to a reversal of the subduction polarity, as previously recognized. I present a four-plate kinematic model of present day Woodlark, Australia, Trobriand and Solomon Sea plate motions that reconciles decadal GPS measurements with Brunhes-age seafloor spreading rates and includes subduction of the Solomon Sea Basin at the Trobriand Trough. Euler poles derived from the model are applied to a detailed 100,000-year-interval animation of the formation and evolution of the Woodlark Basin. I propose that the active spreading center being subducted beneath the New Georgia Group of the Solomon Islands comprises the arc-composition volcanoes of Simbo Island and another to its south. The Ranongga Transform Fault that connects this to the Australia-Woodlark spreading centers further west is concave to the east. The formation and rotation of microplates has helped minimize their collision with the Ontong Java Plateau that is still being subducted on the Pacific Plate from the north, as does the slab window shown by tomography and earthquake data to have formed between the conjugate rifted margins that have been subducted. The young Woodlark Basin lithosphere is too weak to sustain plate flexure and form a bathymetric trench yet is strongly coupled to the forearc by arc volcanoes on the subducting crust that nucleate tsunamigenic earthquakes.