Geophysical evidence for post-Miocene rotation of the island of Viti Levu, Fiji, and its relationship to the tectonic development of the North Fiji basin

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Geophysical studies of the island of Viti Levu, Fiji, show large amplitude magnetic anomalies of up to 1500 nT to be located above the Tavua volcanic center. Field sampling of the volcanic rocks from the intrusive and extrusive centers of the volcano show the rock samples to have initial intensities of natural remanent magnetization of up to 380 A/m. Radiometric dating of fifteen oriented samples show an age range of 2.9 to 7.0 m.y. B.P. for the suite of rocks collected from the Mba basaltic group. Paleomagnetic analysis of the dated rocks shows declination vectors ranging from 325+/-9° for an age of 2.9+/-0.2 m.y. B.P. to 290+/-4° for an ageof 7.0+/-0.4 m.y. B.P.. As a group, the paleomagnetic declination values of the samples systematically decrease with age. This decrease is attributed to progressive anticlockwise rotation of Viti Levu during the process of formation of the North Fiji Basin. A linear least squares analysis of the paleomagnetic data indicates that the island has rotated in an anticlockwise direction through 90° during the past 7 m.y. at a rate of 13.7° per million years. Declination values as low as 245° measured for some sites (not dated) suggest that anticlockwise rotation of Viti Levu and development of the North Fiji Basin could have commenced as early as 10 m.y. B.P. following the breakup of the ancient Solomons-New Hebrides-Fiji-Lau island arc. Large rotations of microplates may have occurred along other island arcs and marginal basin system elsewhere in the Western Pacific and such rotational histories may reflect in detail the evolutionary development of these arcs and basins.

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