The Banda Arc lies at the intersection of three rapidly converging tectonic plates. This convergence has created a complex pattern of subduction, as plates are dragged beneath each other into the Earth’s mantle. It contains what may be the largest fold in the Earth’s crust, reaching depths of up to 650 kilometres.

Conflicting theories for the arc’s origin have divided scientists: some believe it results from the subduction of two slabs from the north and south, while others suggest it was caused by the deformation of a single slab.

Now, using detailed paleogeographic reconstructions, scientists from Royal Holloway, University of London and Utrecht University in the Netherlands have proposed a new explanation: the ‘rollback’ of a single slab of oceanic crust. They suggest that as the slab sank into the mantle, the subduction zone rolled back into the Australian plate as it migrated northwards. As it folded, the sinking slab peeled away the underlying mantle from the continental crust.

The slab is now obstructing the Australian plate’s movement, and the resulting deformation of the Earth’s crust is causing rapid elevation of the Banda Arc islands – some of which have risen from several kilometres below sea level to three kilometres above it in just two million years.

‘These are remarkable vertical movements on geological time scales,’ said Robert Hall, one of the study’s authors. ‘We’re seeing a mountain belt forming before our eyes.’     

KARA MOSES
October 2010