Late low-angle thrusting and the Alpine Fault, central Westland, New Zealand

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Abstract
Low angle, east and southeast dipping thrusts cut Fraser Formation mylonitic gneisses and Tuhua Group granite in the Mikonui River area of central Westland, New Zealand. The Bald Hill Range Thrust has displaced Fraser Formation 1–2 km westwards over Greenland Group/Tuhua Group basement, Tertiary sediments, and Pleistocene-Recent glacial and outwash deposits. A sequence of periglacial, marine, laminated silts and conglomerates immediately west of the Bald Hill Range Thrust records an abrupt vertical transition from Alpine schist-derived, to Fraser Formation-derived, clasts, indicating rapid uplift, with erosion, of Fraser Formation along the Bald Hill Range Thrust during the Pleistocene. The western boundary of Fraser Formation is a combination of two entirely separate fault systems—the Bald Hill Range Thrust, and a older mylonitic fault. The Fraser Fault (of Young) is redefined as a major fault zone along which mylonitic gneisses of the Fraser Formation have been juxtaposed against basement rocks to the northwest, which include Tuhua Group granite and Greenland Group metasediments. The fault zone is characterised by a zone of cataclasite, found in Geologist Creek and west of Mt Upright, and specifically excludes subsequent thrust faulting. The Bald Hill Range Thrust and other thrusts, distinct in structural form, timing, and location from the Fraser Fault, are linked with recent thrusting of the Alpine Fault. Two remnant schist nappes in Surveyor Creek and The Doughboy are described, and they are considered, along with the Bald Hill Range Thrust, to be inactive surface traces of the Alpine Fault, which have migrated with time at least 4 km northwest of the most recent trace. Thrusting extending back into the Pleistocene is in accord with the rapid increase in convergence rate across the Alpine Fault, predicted by plate motion studies to have occurred within the last few million years.

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