Backstops of accretionary wedges must play a critical role in the evolution of wedge development, but no agreement exists as to their significance or fundamental structure, whether backstops dip toward or away from the toe of the wedge. No region is sufficiently well imaged to answer this fundamental question satisfactorily. We propose a large source, multichannel study of the forearc and backarc areas of the eastern Sunda arc, Indonesia, where we have found evidence of backthrusting in the forearc and backarc thrusting in the backarc. Imaging the backthrust (the Sawu thrust near Sumba Island) and backstop geometry will have significant implications for: a) defining the tectonic incorporation of forearc basin strata into the rear of the wedge during backthrusting; b) identifying the earliest accretionary products of the wedge, and thus initiation of subduction accretions; and c) determining relative timing relations between backthrusting and back arc thrusting in a zone of arc-continent collision. The latter will require a program of drilling, and the multichannel study will provide a framework for such a program. These questions will also be addressed in a small accretionary wedge developed along the zone of backarc thrusting, where we can completely image the structure of the backstop and lower plate beneath the wedge. In addition, we propose to carry out 8 days of digital single-channel seismic profiling within the Banda Sea, using a 1,000 cubic inch tuned source array. The objective will be regional seismic stratigraphy in preparation for drilling in l988.