Gravity field derived from the altimetric geoid and its implications for the origin,driving force and evolution of microplate-type marginal basins in the southwestern Pacific

The thickness anomaly of lithosphere in the North Fiji Basin, Lau Basin and the surrounding area was obtained from gravity data derived by satellite altimetry, using crustal thickness determined by available seismic refraction studies. It was revealed that the North Fiji Basin is an isolated asthenospheric dome and that the lithospheric thickness is about 50km less than that of the surrounding area. Such a lithospheric structure and surface topographic feature suggest a plume-like mantle upwelling beneath these basins. The result of numerical simulation shows the possibility of opening of the back-are area and change of subduction polarity which occurred at the first stage of the evolution of the North Fiji Basin. As the lifetime of the southwestern Pacific marginal basins is limited to some 10 million years in most cases, the heat source (plume) which initiates these marginal basins might be sporadic, and their expansion will end when the heat source is wholly consumed.