The basin development and deformation associated with the Kongahu (Lower Buller) fault zone over the last 12 Ma, Mokihinui River, West Coast, South Island, New Zealand

Up to 1400 m of Waiauan to Waipipian shallow marine sediments (O'Keefe Formation, upper Blue Bottom Group), in the Mokihinui River mouth area, West Coast, South Island have been subsequently deformed into a steep monoclinal, WNW verging flexure by reverse movement on the structurally inverted, east dipping Kongahu (Lower Buller) Fault. A sedimentary description of the O'Keefe Formation is presented. Quaternary terraces (Addison Formation) have been tilted up to 6° since deposition, inferred at 340 ka BP. Uplift calculated from the terrace sequence indicate rates form 0.27 mm/yr to 0.42 mm/yr, increasing with proximity to the Kongahu Fault. Post-Waipipian shortening across the Nikau area increases from 1,000 m in the north, to 3,300 m in the south of the study area. Average shortening rates are 0.73 mm/yr. Shortening is also inferred to be taken up by a blind, east-dipping thrust fault to the west of the Kongahu Fault in the central part of the study area. Shortening in the southern portion is accommodated on a low angle (18°) plane of the Kongahu Fault. A clockwise rotation of the Kongahu Fault plane is accomodated by a sinistral transpressional fault in the north. A dextral rotation rate of 3-4°/Ma is inferred for the Kongahu Fault in this area, and is consistent with other data for the northern West Coast. The principal compressional axis in the area is perpendicular to the major regional faults, and to the Alpine/Marlborough Fault systems.