Extensional deformation across an active margin, relations with subsidence, uplift, and rotations: The Hikurangi subduction, New Zealand

On the basis of field studies in the forearc domain of the Hikurangi subduc tion margin. in the eastern North Island of New Zealand, we analyze the evo lution of the subduction since its onset 25 Myr ago. Analyses of brittle de formation within this forearc domain have revealed evidence of extensional deformation, contemporaneous with the subduction of the pacific plate benea th the North Island. The type and origin of extensional deformation during the development of the active margin art: discussed. The problem is crucial because this forearc domain was previously considered as having undergone almost continuous compression during the late Cenozaic. Two distinct events of late Cenozoic extensional deformation are identified. The youngest one is Quaternary in age; it affects limited areas where important uplift preva ils. The orientation of extensional paleostress axes is perpendicular to th at of uplift axes, hence consistent with the classical hypothesis of gravit ational collapse affecting the upper part of the margin. Of particular inte rest is the older extensional event, middle-late Miocene in age, It affecte d most of the forearc domain during a long time span that was also characte rized by widespread margin subsidence. Some structures attributed to this e vent are syndepositional, including rollover structures, and there are main ly high-angle normal faults that affect not only the Miocene sediments but also the pre-Miocene basement. The orientations of paleostress axes obtaine d by inversion of fault data sets are multiple: after corrections taking in to account the rotational history of the margin during the late Cenozoic, t hey are found to be generally consistent with two major trends. Because the se extensional structures developed throughout a long period of the margin subsidence, it is proposed that they reflect the process of tectonic erosio n that affected the Hikurangi active margin during the middle and late Mioc ene, between about 15 and 5 Myr ago.