The origin of the Papua New Guinea tsunami that killed over 2100 people on 17 July 1998 has remained controversial, as dislocation sources based on the parent earthquake fail to model its extreme run-up amplitude. The generation of tsunamis by submarine mass failure had been considered a rare phenomenon which had aroused virtually no attention in terms of tsunami hazard mitigation. We report on recently acquired high-resolution seismic reflection data which yield new images of a large underwater slump, coincident with photographic and bathymetric evidence of the same feature, suspected of having generated the tsunami. T -phase records from an unblocked hydrophone at Wake Island provide new evidence for the timing of the slump. By merging geological data with hydrodynamic modelling, we reproduce the observed tsunami amplitude and timing in a manner consistent with eyewitness accounts. Submarine mass failure is predicted based on fundamental geological and geotechnical information.

The major ophiolite complexes of Papua New Guinea lie on the northeastern margin of the Australian craton and are flanked, externally, by Palaeogene volcanic arcs. The ophiolites are segments of oceanic lithosphere which occupied the forearc zone prior to arc-continent collision. In the case of the easternmost ophiolite, the Papuan ultramafic belt, there is evidence that emplacement was preceded by development of a secondary rupture in the down-going plate, at some distance from the subduction zone, and was followed by extension, buoyant emergence, and reversed movement of the ophiolite. The westernmost ophiolite (April ultramafics) has more complex structure and probably developed as a series of thrust sheets in a subduction system; the thrust sheets were subjected to renewed deformation and thrusting by continued convergence after arc-continent collision.

In many highly extended rifts on the Earth, tectonic removal of the upper crust exhumes mid-crustal rocks, producing metamorphic core complexes. These structures allow the upper continental crust to accommodate tens of kilometres of extension, but it is not clear how the lower crust and underlying mantle respond. Also, despite removal of the upper crust, such core complexes remain both topographically high and in isostatic equilibrium. Because many core complexes in the western United States are underlain by a flat Moho discontinuity, it has been widely assumed that their elevation is supported by flow in the lower crust or by magmatic underplating. These processes should decouple upper-crust extension from that in the mantle. In contrast, here we present seismic observations of metamorphic core complexes of the western Woodlark rift that show the overall crust to be thinned beneath regions of greatest surface extension. These core complexes are actively being exhumed at a rate of 5-10 km Myr(-1), and the thinning of the underlying crust appears to be compensated by mantle rocks of anomalously low density, as indicated by low seismic velocities. We conclude that, at least in this case, the development of metamorphic core complexes and the accommodation of high extension is not purely a crustal phenomenon, but must involve mantle extension.

New Guinea Trench has caused the deformation of Plio-Quaternary sediments in the Mamberamo Basin, deformation and Pliocene igneous activity in the central range, and the southwestward motion of the Bird's Head. The island has significant resources of economic minerals and hydrocarbons.

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