THE HIKURANGI PLATEAU

Recently collected geophysical data reveal the basement and sedimentar y structure of the Hikurangi Plateau presently being subducted beneath the North Island of New Zealand. We have subdivided the plateau into 6 areas on the basis of basement structure. Two basins separated by a basement high trend subparallel to the Chatham Rise, probably filled w ith Mesozoic sediments. Another is parallel with the present plate mar gin along the east coast of the North Island and is filled with Late C enozoic sediments. The northern part of the plateau is characterised b y shallow volcanic basement. The southeast part of the region is compl ex but has several kilometers of sediments deformed by volcanic intrus ions. At least two episodes of volcanism have occurred. The older, pro bably Mesozoic episode, produced numerous large, isolated edifices, so me exposed as seamounts and others buried by sediments. Activity resum ed in the Neogene, resulting in the emplacement of smaller knolls and widespread intrusions. Variation in crustal thickness north of the Cha tham Rise supports the hypothesis of a convergent margin along the nor th flank of the Chatham Rise in the Mesozoic. The structure of the Hik urangi Plateau may explain the origin of the postulated physiographic variations and segmentation of the downgoing slab along the active Ker madec-Hikurangi subduction margin.