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 subduction of the Pacific plate at the Tonga trench 1 . It is not known why this slab generates so many more earthquakes than other subducting slabs worldwide. Above the subduction zone the active Tofua (Tonga) volcanic arc is separated by the V-shaped Lau basin from a remnant arc, the Lau ridge, located at the eastern edge of the Australian plate 2 . The irregular and discontinuous magnetic lineations within the basin have 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 increases northwards to a maximum of ∼160 mm yr −1 No straining is observed within the northern Tonga ridge, suggesting that it comprises part of a rigid microplate. Convergence rates across the Tonga trench increase northwards to a maximum of ∼240 mm yr −1 . The extraordinary seismic activity of the subducting slab is probably related to this unusually rapid subduction.