OCEANIA (INCLUDING FIJI, PNG AND SOLOMONS) 109 OCEANIA (INCLUDING FIJI, PNG AND SOLOMONS) H Davies, University of Papua New Guinea, Port Moresby, Papua New Guinea P Bani, Institut de la Recherche pour le De´veloppement, Noume´a, New Caledonia P Black and I Smith, Auckland University, Auckland, New Zealand E Garaebiti, Department of Geology and Mines, Port Vila, Vanuatu P Rodda, Mineral Resources Department, Suva, Fiji ß 2005, Published by Elsevier Ltd Introduction The islands of the south-west Pacific extend for 5000 km from New Guinea in the west to Fiji, Tonga, and Samoa in the east, and include the Solomon Islands, Vanuatu, and New Caledonia (Figure 1) The early European explorers called the area Melanesia because of the generally dark skin colour of the people Subsequently, the parallel island chains have sometimes been referred to as the inner and outer Melanesian arcs New Guinea was colonized from the west more than 40 000 years ago; New Britain, at least 35 000 years ago; New Ireland, at least 20 000 years ago; and the northern Solomon Islands, 28 000 years ago The other Melanesian islands, New Caledonia, Vanuatu, and Fiji, were settled only 3000 years ago The islands have many similarities in their geology, in that, for the most part, they have been constructed on oceanic crust by one or more cycles of volcanic activity Exceptions are the island of New Guinea, which is part of the Australian continent; the older rocks of New Caledonia, which first formed at the margin of the Australian continent; and the outermost Solomon Islands, which are elevated oceanic crust There are deep-sea trenches on either side of the island chains On the outer (north-eastern) side are the Manus, Kilinailau, Solomons, and Vitiaz trenches, which are only weakly active or are inactive, and the Tonga–Kermadec Trench east of Tonga (Figure 1), which is active On the inner (southwestern) side is an active system of trenches, the New Britain–Makira–Vanuatu–Hunter–Kadavu trenches Other seafloor features include the massive Ontong Java submarine plateau of thickened oceanic crust (Figure 1), smaller oceanic plateaus of rifted continental lithosphere in the Coral Sea, and a number of small ocean basins, some of which are actively spreading The islands occupy the boundary zone between the north-moving Australian plate and the west–northwest-moving Pacific Plate, and for this reason are tectonically and volcanically active (Figure 2) The plate boundary is not a simple one but, rather comprises a number of smaller plates that exhibit a variety of plate interactions, from seafloor spreading to subduction, transform faulting, and rotation The lines of earthquakes on the map (Figure 2) coincide with the plate boundaries, and give an idea of the high level of seismic activity in the region The plot of depth-coded hypocentres shows the trace of a subducted slab dipping westward from the Tonga–Kermadec Trench, in the east, and of a subducted slab dipping northward from the New Britain Trench, in the north-west The area has evolved since the Cretaceous by repeated episodes of crustal extension and crustal shortening Extension produced small ocean basins, and shortening resulted in subduction and thrust faulting The volcanic islands have grown through this time, initially (in the Eocene to Early Miocene) by volcanism related to subduction of the Pacific Plate on the outer trench system, and more recently (the past 10 million years) by volcanism related to subduction of the Australian Plate on the inner trench system Papua New Guinea Papua New Guinea (PNG) comprises the eastern half of the island of New Guinea, the adjacent islands of the Bismarck Archipelago, and the northernmost of the Solomon Islands (Bougainville and Buka) The geology of PNG can be described in terms of four geological provinces (Figure 3): a stable platform in the south-west, where the Australian craton extends beneath the island of New Guinea; a collisional zone in the centre and north of the island, where terranes have been sutured to the Australian craton; a province built by volcanic arc activity in the north-east; and the Ontong Java submarine plateau Platform and Foldbelt The stable platform comprises a continental basement of Palaeozoic and partly Precambrian metamorphic and igneous rocks overlain by a 4-km-thick paraconformable sequence of shelf facies Mesozoic siliciclastic, and Cenozoic carbonate and siliciclastic, sedimentary rocks (Figure 3) Rocks along the northern margin of the platform are buckled and faulted to form a basement-involved foreland foldand-thrust belt Northward of the fold-and-thrust belt, the Mesozoic sediments are transitional into a kilometres-thick sequence of black mudstones These