THE RELATIONSHIP BETWEEN VOLCANIC ISLAND GENESIS AND THE INDO-AUSTRALIAN PACIFIC PLATE MARGINS IN THE EASTERN OUTER ISLANDS, SOLOMON TSLANDS, SOUTH-WEST PACIFIC

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The Eastern Outer Islands form a group of small islands situated in the south-west Pacific Ocean, to the south-east of the main Solomon Islands chain, and to the north of the New Hebrides chain.
They are founded upon the northern part of the submarine Fiji Plateau and flanked by deep sea trenches on the western, northern and eastern sides, but by an east-west trending fracture zone on the southern side. Two south-easterly trending island chains can be recognized, the western chain includes the islands of Tinakula, Nendö, Vanikolo and Utupua, and the eastern chain includes the islands of the Duff Group, Anuta and Fatutaka. The island of Tikopia is situated midway between both chains. The Torres and Vitiaz Trenches form the western and eastern flanks respectively of the north Fiji Plateau, which is itself bordered to the west and east respectively by the Indo-Australian and Pacific lithospheric plates. A westerly extension of the Vitiaz Trench, known as the Cape Johnson Trough, forms the northern boundary. The south side of the region is delimited by the Hazel Holme Fracture Zone.
Volcanic rocks within the group range from picrite basalts, through basalts and andesites, to dacites. Mafic lavas predominate in the older islands of the western chain, two islands of the eastern chain and in samples dredged from the Vitiaz Trench. More sialic lavas are exposed in two islands of the eastern chain, the isolated island of Tikopia and also in the active volcanic island of Tinakula.
Petrologic, petrochemical, seismic and heat flow evidence suggests that the Eastern Outer Islands represent two, discrete south-easterly trending island arcs, each associated with the adjacent trench. The sequence of volcanic episodes can be best explained in terms of plate tectonics, in which the islands of the western chain were produced during two episodes of volcanic activity associated with an easterly inclined Torres subduction zone. Late Oligocene to early Miocene subduction produced the island of Nendö, but the islands of Utupua, Vanikolo and Tinakula were produced during the late Pliocene to Recent. The westerly-dipping Vitiaz subduction zone is postulated as a source from which the east facing Duffs-Anuta-Fatutaka island arc was produced during the Middle Miocene to late Pliocene.