GEODETIC OBSERVATIONS OF VERY RAPID CONVERGENCE AND BACK-ARC EXTENSION AT THE TONGA ARC

THE Earth's most active zone of mantle seismicity arises from the subd uction of the Pacific plate at the Tonga trench(1). It is not known wh y this slab generates so many more earthquakes than other subducting s labs worldwide. Above the subduction zone the active Tofua (Tonga) vol canic are is separated by the V-shaped Lau basin from a remnant are, t he Lau ridge, located at the eastern edge of the Australian plate(2). The irregular and discontinuous magnetic lineations within the basin h ave proven difficult to interpret(3,4), and so the regional kinematic framework has been obscure. We report geodetic measurements of crustal motion within the Tonga-Lau system, which reveal the fastest crustal motions yet observed. The Lau basin is opening at a rate which increas es northwards to a maximum of similar to 160 mm yr(-1). No straining i s observed within the northern Tonga ridge, suggesting that it compris es part of a rigid microplate. Convergence rates across the Tonga tren ch increase northwards to a maximum of similar to 240 mm yr(-1). The e xtraordinary seismic activity of the subducting slab is probably relat ed to this unusually rapid subduction.