Crustal strain phenomena in the Solomon Islands, constraints from field evidence, and relationship to the India-Pacific plates' boundary |
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| Authors: | WRH Ramsay |
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| Abstract: | The Solomon Islands lie along the India-Pacific plates' margin and have recorded a history of deformation resulting from the interaction of these two plates. Various kinematic models have been proposed for the Solomons and these have involved a variety of plate tectonic processes. It is pointed out that almost without exception these models have been based on a provincial geological classification of the island group in which it is assumed that two of these provinces—Pacific and Central provinces—commenced their geological development in regions distant from one another. Invariably such models require that Santa Isabel represents part of a collision zone between these two provinces, though field evidence from Santa Isabel for such a collision has in the past been largely lacking.These various kinematic models are examined in the light of more recent field evidence, and a premise on which they have been based—initial separate development for two of the provinces—is questioned. Rather it is here suggested that the Central and Pacific provinces developed in roughly similar positions, one with the other as they occur today, and that they were at least in part separated from Oligocene time onward by a linear peridotite-gabbro ridge, Korighole-Florida high, which acted as a sediment barrier to much of the coarser clastic and volcanogenic sedimentation.The initial development of the Solomon Islands began in an oceanic environment with the extrusion of extensive submarine tholeiitic ‘flood basalts’ and intrusion of associated gabbroic and ultramafic rocks at depth, during the Late Mesozoic to Early Tertiary. This igneous phase occurred with the whole of the island group representing the western margin of the Ontong Java Plateau. Subsequent asymmetric development of the Solomons during the Eocene and Oligocene resulted in uplift, shearing, and the initiation of arc volcanism, plutonism, and arc-related sedimentation in the Central province to the west. In contrast, through much of the Tertiary the Pacific province to the east continued to receive dominantly pelagic sediments before undergoing uplift and renewed deformation in the Pliocene. The recognition that the ophiolite crust in the Solomon Islands represents an autochthonous entity, which has acted as basement to subsequent arc volcanism, has significant implications on geochemical studies of these islands now being undertaken. |
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