Abstract

The shoshonitic Tavua Volcano in Viti Levu, Fiji, developed from a submarine to a subaerial environment, and comprises lava flows, dominantly of absarokite composition, with debris flows and conglomerates on its flanks. The Tavua Caldera formed in the centre of the volcano. Within the caldera, avalanche deposits are overlain by the Turtle Pool Formation, which consists of lacustrine sediments, shoshonite lavas and hydrovolcanic/pyroclastic lapilli tuffs. Subsequently the Inner Caldera formed in the centre of the Tavua Caldera. This was infilled by the Morrisons Pool Formation, which comprises lavas and pyroclastic rocks of banakite composition, with minor intercalated sediment. Thus the magmas became progressively more evolved.

In addition to normal movement along regional and concentric faults, subsidence (which exceeds 3 km) was accomplished via substantial downsagging. Evidence for this is the consistent centripetal dip of caldera fill material, which locally exceeds 50° in parts of the Turtle Pool Formation. Centripetal dips are also recognized in pre-caldera rocks immediately outside the Tavua Caldera.

Subsidence was episodic, caused by periodic, non-explosive withdrawal of underlying magma to erupt on the flanks and in part within the caldera. Downsagging and fault movement were responsible for approximately equal amounts of subsidence, and their interaction produced a caldera with the shape of a stepped funnel.

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