The last 25 Ma of rotational deformation in part of the New Zealand plate-boundary zone

Abstract

The Marlborough fault system is a zone of dextral transpression in continental crust, and marks the southern end of the Hikurangi subduction system between the obliquely convergent Australian and Pacific plates. Detailed mapping over an area of 100 km 2 has revealed a complex history of deformation during the last 25 Ma. The development of a thrust belt in the Miocene, referred to as D 1 deformation, was synchronous with the deposition of a thick submarine fan complex. The beginning of D 1 deformation may mark the initiation of the subduction zone in this part of the plate-boundary zone. Subsequently, in the Plio-Pleistocene, the region formed a zone of dextral shear, marked by major strike-slip faults and zones of distributed deformation, referred to as D 2 deformation. Crustal blocks, less than 5 km across, may have rotated ca 60° clockwise about vertical axes relative to the Pacific plate in the last 4 Ma. The pattern of deformation during the last 10 Ka to 100 years is consistent with the long-term D 2 deformation, and suggests that large crustal blocks resting on the subducted Pacific plate have rotated clockwise about vertical axes relative to the Pacific plate, accommodated by dextral shear further to the SW in the Marlborough fault system. The overall effect of the deformation has been the rotation of the edge of the southern part of the Hikurangi margin from a W to NW trend in the early Miocene to a NE trend today.