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 are a, West Coast, South Island have been subsequently deformed into a ste ep monoclinal, WNW verging flexure by reverse movement on the structur ally inverted, east dipping Kongahu (Lower Buller) Fault. A sedimentar y description of the O'Keefe Formation is presented. Quaternary terrac es (Addison Formation) have been tilted up to 6 degrees since depositi on, 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 proximi ty to the Kongahu Fault. Post-Waipipian shortening across the Nikau ar ea 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 al so inferred to be taken up by a blind, east-dipping thrust fault to th e west of the Kongahu Fault in the central part of the study area. Sho rtening in the southern portion is accommodated on a low angle (18 deg rees) 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 degrees/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 p erpendicular to the major regional faults, and to the Alpine/Marlborou gh Fault systems.