114 OCEANIA (INCLUDING FIJI, PNG AND SOLOMONS) Figure Geological map of the Solomon Islands, including Bougainville and Buka Islands (Papua New Guinea), showing the Makira (San Cristobal) active trench, the Kilinailau inactive trench, and the North Solomons slow acting trench The dashed line marks the international border Map drawn by L Cotterall, Geology Department, Auckland University sediments and intrusive rocks These extend from Bougainville (PNG) and the Shortland Islands in the north-west, to the near end of Guadalcanal in the south-east There is an outlier of Pliocene volcanics and intrusive rocks in central Guadalcanal, at Gold Ridge Bougainville Island, politically a part of Papua New Guinea, is the north-westernmost of the Solomon Island chain and combines characteristics of both the central and south-western geological provinces A basement of Eocene and Oligocene arc-related volcanic rocks is partly overlain by Miocene limestone and Plio-Quaternary volcanoes and is intruded by Pliocene dioritic stocks The Solomon outer eastern islands are far removed from the central and western islands and are shown on the Vanuatu geological map (Figure 5) The islands of the Santa Cruz Group comprise Miocene to Holocene volcanic rocks and sediments The Duff Islands, and the islands of Anuta, Fatutaka, and Tikopia (not shown on Figure 5) comprise Quaternary volcanic rocks and sediments Tinakula in the Santa Cruz Group is the only active volcano Geological Evolution The islands of the north-east province developed as a result of continued convergence across the line of the Kilinailau Trench (Figure 4), and resulting delamination and eastward overthrusting of an upper part of the Ontong Java volcanic plateau The rocks of the lower part of the Ontong Java plateau continue to be subducted steeply beneath the Solomon Islands The central province association of ultramafic rocks, Cretaceous basalt, and Eocene metamorphic rocks is thought to have developed as a result of subduction of Pacific plate at the Kilinailau Trench in the Eocene, but the apparently rapid exhumation of these rocks (exhumed before the Oligocene) is not readily explained The same cycle of subduction yielded the Oligocene to Early Miocene arc-type volcanic rocks that overlie basement on the islands of the central province The Late Miocene to Holocene volcanic rocks of the south-west province were generated by subduction of the Australian plate at the Makira (San Cristobal) Trench Economic Geology Gold associated with Pliocene intrusives has been mined at Gold Ridge on Guadalcanal Island, and soils on ultramafic rocks on Santa Isabel Island have been tested for lateritic concentrations of nickel and cobalt