The Finsch Deep is an asymmetric rhomboidal basin, with a maximum depth of 5400 m, situated to the north of the Solomon Sea Triple Junction, Papua New Guinea. Anomalous slip vector azimuths, regional plate velocities, earthquake hypocenters, and bathymetric depth relative to surrounding terranes all indicate that the Finsch Deep is not a passive feature developed behind the subduction front. Our interpretation of all available data, including seismological studies, detailed bathymetry, side-scan character, and seismic sections, suggests that the Finsch Deep has developed due to N-S extension in the transition zone from continental collision west of the Solomon Sea Triple Junction to oceanic subduction to the east. The key mechanisms thought to drive convergent margin extension within the New Guinea Collision are slab pull and oblique subduction. Researchers studying other convergent margins have recognized underplating and tectonic erosion as driving mechanisms for extension; though we have no evidence to either support or discount their presence within the New Guinea Collision.¿ 1997 American Geophysical Union