The d'Entrecasteaux zone (DEZ) collides with the central New Hebrides island arc and consists of two subparallel ridges that strike east-west, stand 1–2 km above the surrounding oceanic plate, and subduct obliquely (15°) northward beneath the arc. Rocks dredged from the north ridge as well as reflections evident in multichannel seismic reflection data indicate that this ridge has a volcanic origin. Crystalline volcanic rocks are common along the lower flank of the ridge, but sedimentary, probably volcaniclastic, rock caps the ridge. Seismic reflection data collected over the lower arc slope reveal that mass wasting deposits locally make up most of the accretionary wedge. These deposits appear to form discrete bodies, suggesting that mass wasting occurred episodically. Large anticlines and thrust faults having large vertical separation are not readily evident where the colliding ridge intersects the arc slope; apparently, slope rocks have low strength so that mass wasting deposits formed instead of large-relief structures. Mass wasting is thought to occur as the accretionary wedge is uplifted in response to the northward oblique subduction of the north ridge. The toe of the north ridge flank marks an abrupt transition in the lithologies that make up the footwall of the interplate decollement. Footwall lithologies change from ocean basin to volcaniclastic ridge material, and this transition probably marks a discontinuity in friction along the decollement or in rock mechanical properties because north of the transition, thrust faults deform the accretionary wedge whereas south of the transition, steep reverse faults crosscut the wedge and pierce the north flank of the ridge. This piercement means that the decollement at least locally lies within the ridge and that ridge material exotic to the New Hebrides arc may be incorporated into the accretionary wedge.