A major submarine mountain chain called the d'Entrecasteaux zone (DEZ) collides with the central part of the New Hebrides arc. Three migrated multichannel seismic sections obtained across this collision zone show considerable structural variability within slope rocks. The subducted top of the north ridge of the DEZ can be traced beneath the accretionary wedge. Strata forming this wedge could be uplifted remnants of a wedge that predated collision, or they could have been accreted from rocks that form the ridge crest. Large slope anticlines and major vertical-offset faults are absent from the vicinity of this ridge; therefore, such structures do not necessarily form in collision zones. Between the two ridges of the DEZ, rocks forming the accretionary wedge dip toward the trench and terminate obliquely downward at the decollement between arc and subducted rocks. This attitude is unique to the collision zone and probably records the progradation of a flood of sediment eroded from nearby islands that underwent rapid uplift during the late Quaternary. Near the south ridge, deep-ocean sedimentary rock curve upward and form the west and south faces of the Bougainville spur, a peak that towers over the rest of the lower arc slope. An accreted guyot may underlie this spur. Fragmentary geologic and geophysical evidence suggests that the north ridge, from which mid-ocean-ridge basalt has been dredged, may be denser and stiffen than the south ridge, which appears to be made up mainly of volcaniclastic rocks. These differing physical properties of the two ridges probably determined the type of structures that developed within lower-slope rocks.