The convex north trending Sangihe and Halmahera volcanic arcs face each other and are underlain by oppositely dipping Benioff zones whose surface intersection in the Molucca Sea forms a zone of crustal collision. Seismic reflection profiles across the Talaud-Mayu ridge between the arcs demonstrate that this topographic high consists exclusively of deformed rocks. Reflection profiles further indicate that this deformed mass, or collision complex, is being obducted onto the flanking arc aprons on both east and west sides along gently dipping thrust faults which crop out in troughs adjacent to the arcs. The near-surface layers of the arc apron are being incorporated into the collision complex by either plastic deformation or thrusting of coherent sediment wedges. Preliminary gravity and seismic refraction interpretation indicate that at least 8--10 km of low-density material underlies the Talaud-Mayu ridge. The total length of lithosphere subducted by the colliding arcs is at least 1000 km, the length of the Benioff zones.

The geometry of the collision zone suggests an obductive mechanism for emplacement of m¿lange and ophiolite belts against island arcs or continental margins. The central part of the mostly submarine Talaud-Mayu ridge stands 1--3 km higher than the flanking troughs, wherein the thrusts crop out. Gravitational flow of material outward and downslope away from the high translates at the edges to thrusting up over the adjacent arcs. The reconstructed history of the collision predicts at least two opposing vergence directions developed in the deformed rocks of the collision complex. The first, developed during subduction, verges away from the arcs, whereas the second, developed during the present phase of collision, verges towards the arecs. Thus vergence indicators alone are probably equivocal determiners of subduction polarity in ancient rocks.