Seismic reflection profiling east of the North Island, New Zealand, has defined the northeastern margin of an area of elevated basement rocks referred to as the Hikurangi-Chatham Plateau. The northeastern boundary of the plateau extends from the southern Kermadec Trench to the eastern end of the Chatham Rise. The plateau is bounded to the south by the Chatham Rise and to the west by the Hikurangi Trough.

Gravity modeling and inversion of earthquake travel-time data indicate that the crust of the plateau is 10-15 km thick. Numerous volcanic peaks are observed throughout the plateau. The age and type (oceanic versus continental) of the plateau crust is unknown. The plateau is being subducted beneath the North Island in the Hikurangi Trough. I suggest that the buoyancy of the plateau, compared with the oceanic crust to the north, is responsible for the contrasts in subducting plate dip, trench morphology, and accretionary prism elevation between the Hikurangi subduction system and the Kermadec subduction system to the north.

Postulated Triassic-Jurassic subduction southward beneath the northern Chatham Rise probably ceased following arrival of the buoyant Hikurangi-Chatham Plateau at the margin.