The Fiji Plateau is a high, hot area of young oceanic crust. It is bounded on the north by a Cretaceous Pacific archipelago, and to the east and west by the Tonga and New Hebrides island arcs which go back to the Eocene. The Fiji Islands are an Eocene and younger continental mass formed within the ocean basin. Plate tectonics provides the key for understanding the area. Marine geological and geophysical data from Scripps Institution of Oceanography expeditions, especially Nova, and published seismic, gravity, and island geologic information provide the basis for the interpretation.
Fiji is now flanked by three active sea-floor spreading centers which are part of a very complicated transform linking the Tonga and New Hebrides crustal consumption zones. Extension in the Lau Basin is also taking place. Magnetic anomalies and seismicity permit six small blocks and the large Pacific and Australian plates to be distinguished, and some idea of their relative motions to be gained.
From published magnetic anomaly and fracture zone data, a detailed history of the Tertiary motions of the Pacific and Australian plates with respect to Antarctica has been deduced. By a suitable choice of plate boundaries, horizontal movements of the larger tectonic units of the Fiji Plateau region can be worked out for the entire Tertiary. This reconstruction successfully accounts for many hitherto unexplained bathymetric and geologic features of the area. The history proposed for the Fiji area is probably unique among the world's oceans.