Abstract
The tectono-sedimentary evolution of the southern Tonga Ridge during the latest Miocene to Pleistocene is described from seismic reflection profiles and drilling results from Site 840, located on the western margin of the Tonga Ridge. The evolution took place in three episodes that have been related to the rifting of the Tonga/Lau Ridge and spreading in the Lau Basin. Uplift of the Tonga Ridge in the latest Miocene is attributed to deformation caused by the rifting of the Tonga/Lau Ridge. The uplift confined sedimentation to the western side of the Tonga Ridge, although the (mainly volcanic) sediment sources remained unchanged from the Lau Ridge. A phase of minor fiexuring and faulting in the early Pliocene precedes a period of effusive volcanism, during which depositional limits expanded. The volcanic source was again from the Lau Ridge, and the volcanism is interpreted as taking place during a final rifting phase before the sundering of the Tonga/Lau Ridge. A further phase of minor fiexuring in the early Pliocene is attributed to the sundering of the Tonga/Lau Ridge. Subsequent deposition on the Tonga Ridge takes place in isolation from the Lau Ridge. Late Pliocene volcanic sediments are sourced from the Tofua Arc, newly formed on the western margin of the Tonga Ridge.
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