Interpretation of seismic refraction data from the southwestern coast of the Papuan Peninsula and the northwest Coral Sea gives consistent results using several inversion techniques. Sediments over the Papuan Plateau are 5 km thick; to the west and northwest they thicken to 10 km along the axes of the Moresby and southern Aure Troughs. Farther out into the Coral Sea, over the Eastern Plateau, they are 1 to 2 km thick. Beneath the sediments, a layer with a P-wave velocity of 6.07 km s^-1 was inferred over the region. It is underlain along the southern coastline of the peninsula by a lower crustal layer with a velocity of 6.9 km s^-1, which is probably also present under the Eastern Plateau. We cannot say whether the lower crustal layer also occurs offshore under the Moresby Trough, but it is not present under the Aure Trough in the north. Intervals where the velocity increases with depth are likely in the lower crust (> 17 km) under the peninsula, but we have insufficient data to say if they are present offshore. The Moho is 27 to 29 km deep along the southwestern coast of the peninsula. It shallows to 19 km under the Moresby Trough and deepens to 25 km under the Eastern Plateau. The crust is therefore continental under the Papuan Peninsula and Eastern Plateau and, excluding the sediments, oceanic under the Moresby Trough. We have examined several tectonic models for the region that imply different stress patterns at the time of formation of the Moresby Trough. We favour one in which northern Australia, the Eastern and Papuan Plateaus and the Papuan Peninsula once formed a continuous, continental crust. With the opening of the Coral Sea Basin, crustal thinning extended northwards along the axis of the Moresby Trough, probably into the Aure Trough. This model implies a regional tensional stress pattern at the time of formation.
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