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The Austradec Ill, seismic survey carried out in 1975 by the IFP-CEPM (*)-ORSTOM
group explored the structure of the archipelagoes of the Solomon Islands and New Ireland. The following results were obtained.
I ) During most of their history the two archipelagoes were part of a single coherent system similar to the New Hebrides island arc.
2) In the Solomon archipelago, the active margin is superposed on a former island arc not directly related to it. The genesis of the two structures thus must be considered separately.
This former island arc is defined from NE to SW by the trench (West Melanesian-North
Solomon), the inner wall, the frontal arc (N .E. Ridge), the sedirrentary basin (New Ireland
and Solomon Islands Basins) with a volcanic arc located in the sedimentary basin, and the marginal basins (Manus Basin and part of the Solomon Sea and Woodlark Basin).
The geological evolution of this region is visualized as follows : the observed polarity is evidence of the subduction of the Pacific plate under the Austral-Indian plate during the middle Tertiary. In the Miocene, the arrival of the Ontong Java Plateau in the subduction zone caused this subduction to stop. The relative movements of the plates reated a new active margin along New Britain and the southern flank of the Solomon archipelago.