The accretionary margin of the Hikurangi forearc on the southeast coast of the North Island of New Zealand is part of the leading edge of the Australian plate, which is overriding the obliquely coverging Pacific plate. We investigate the last 10 m.y. of deformation history of the innermost (western) quarter of the total width of the forearc through analysis of the sedimentologic and structural evolution of the Eketahuna area on the east coast of the North Island. The Eketahuna area is ideal for such a study because emergence of the margin in the Quaternary has exposed a complete late Neogene rock record. This record has allowed us to chronicle the strain history. From 10 Ma to about 2.5 Ma this forearc region was the locus of subsidence and marine deposition.

In the lastest Pliocene this part of the margin began to shorten through of folding and reverse faulting, bringing an end to basin filing. The period of shortening was brief, and by the late Pleistocene, reverse faulting has ceased and was immediately succeeded by dextral strike-slip faulting, in some cases along the same faults. Presently, the dominant strain regime in the inner quarter of the forearc is strike-slip faulting.

This structural history illustrates that, over time, the pattern of strain partitioning has changed in the Hikurangi forearc. The switch from crustal shortening to dextral shear along the major faults in this area in the last 1 m.y. may be a response to more than 10¿ of clockwise rotation in the southern Hikurangi forearc in Pliocene and Pleistoecene time. This rotation is a consequence of the fact that the accretionary margin is undergoing continuous deformation between the obliquely converging Australian and Pacific plates in this area at the southernmost end of the Kermadec-Hikurangi subduction system. The inboard portion of this young accretionary margin is exceptionally well exposed today, probably in part because of the late Neogene subduction of relatively thick, buoyant crust of the Hikurangi-Chatham plateau. ¿ American Geophysical Union 1995