THE NADI BASIN - NEOGENE STRIKE-SLIP FAULTING AND SEDIMENTATION IN A FRAGMENTED ARC, WESTERN VITI-LEVU, FIJI

The Nadi Basin in western Viti Levu is one of a number of sedimentary basins controlled by E-W to NE-SW faults that developed on the Fiji Platform in the Late Miocene. The initial basin fill, belonging to the Upper Miocene Nadi Sedimentary Group, consists of mainly basement-derived breccias and sandstones, deposited at bathyal depths by sediment gravity flows on a basin floor dissected by braided E-W faults. The main source area lay to the south, across the actively down-faulting basin margin. In the latest Miocene, a change in provenance took place as debris spread southward from a newly developed E-W chain of shoshonitic volcanoes (Koroimavua Volcanic Group) some 12 km north of the southern basin margin. Continued normal displacement along the southern margin allowed the deposition of a thick sequence of shoshonitic sandstones at bathyal depths on the downthrown side. In the earliest Pliocene, the onset of basin inversion was marked by shallowing to neritic depths and the onlap of sandstones and shoshonitic conglomerates southward onto pre-Nadi Sedimentary Group basement. As inversion proceeded the basin fill was folded and southward-directed reverse displacement took place along the southern basin margin. Evidence from the Nadi and other Late Miocene basins indicates that they developed as a result of broadly E-W-directed-sinistral strike-slip, probably related to a shearing collision of the Melanesian Border Plateau with the northern margin of the Fiji Platform during the breakup of the formerly continuous Outer Melanesian arc. It is suggested that prior to the establishment of the Fiji fracture zone at 7-6 Ma, much of the left-lateral displacement currently taking place along that plate boundary was accommodated by strike-slip deformation within the Fiji Platform.