Subduction in eastern Indonesia: how many slabs?

Seismicity associated with arc-continent collision in eastern Indonesia tes tifies to past north-directed subduction of Indian Ocean lithosphere beneat h the Banda Sea. The complex patterns of deep seismicity have been cited as evidence for simultaneous south-directed subduction at the northern margin of the sea but this interpretation has not been universally accepted. Rece ntly available recomputations of hypocentre locations have provided increas ed resolution of eastern Indonesian Wadati-Benioff Zones (WBZs). Shallow to intermediate depth seismic activity around the Banda Arc appears to suppor t models involving subduction of two separate and distinct lithospheric sla bs, but between 150 and 500 km the WBZ has a continuous 'shoehorn' shape. T his shape confirms the presence of subducted lithosphere beneath Seram, in the north, as well as beneath Timor, in the south, is incompatible with ind ependent subduction of two unconnected plates and implies rapid eastwards r etreat of the subduction trace across a now vanished northern spur of the I ndian Ocean. This 'roll-back' is unlikely to have been driven by local grav itational forces alone and may have been sustained by injection behind the Banda slab of asthenospheric material escaping from the Molucca Sea arc-arc collision. (C) 2001 Elsevier Science BN. All rights reserved.