On the basis of field studies in the forearc domain of the Hikurangi subduction margin, in the eastern North Island of New Zealand, we analyze the evolution of the subduction since its onset 25 Myr ago. Analyses of brittle deformation within this forearc domain have revealed evidence of extensional deformation, contemporaneous with the subduction of the Pacific plate beneath the North Island. The type and origin of extensional deformation during the development of the active margin are discussed. The problem is crucial because this forearc domain was previously considered as having undergone almost continuous compression during the late Cenozoic. Two distinct events of late Cenozoic extensional deformation are identified. The youngest one is Quaternary in age: it affects limited areas where important uplift prevails. The orientation of extensional paleostress axes is perpendicular to that of uplift axes, hence consistent with the classical hypothesis of gravitational collapse affecting the upper part of the margin. Of particular interest is the older extensional event, middle-late Miocene in age. It affected most of the forearc domain during a long time span that was also characterized by widespread margin subsidence. Some structures attributed to this event are syndepositional, including rollover structures, and there are mainly high-angle normal faults that affect not only the Miocene sediments but also the pre-Miocene basement. The orientations of paleostress axes obtained by inversion of fault data sets are multiple; after corrections taking into account the rotational history of the margin during the late Cenozoic, they are found to be generally consistent with two major trends. Because these extensional structures developed throughout a long period of the margin subsidence, it is proposed that they reflect the process of tectonic erosion that affected the Hikurangi active margin during the middle and late Miocene, between about 15 and 5 Myr ago. ¿ 1999 American Geophysical Union