Estimates for the timing of the last coseismic displacement on the Alpine Fault, northern Fiordland, New Zealand

Abstract

Stratigraphic sections through sag ponds developed along the Alpine Fault trace between Milford Sound and John O'Groats River record drastic changes in sedimentation. Swamp or forest-floor material has been inundated by up to 0.5 m of cataclasite-derived sand and gravel. Two events, inferred to represent degradation of newly created scarps on the Alpine Fault, occurred at, or later than, 230 ± 50 yr B.P., and 1980 ±60 yr B.P., respectively. At John O'Groats River, large silver beech trees growing on the Alpine Fault scarp have been topped at heights of 8–15 m above ground level by movement on the fault. Unbroken beeches have mean ages calculated at 266 +90, -60 years. On a regional scale, a colluvial fan-forming event at Kaipo River and rotational slumping at Gorge Plateau are broadly synchronous with inferred fault movements near John O'Groats River. It is suggested that a large seismic event, related to the last major movement on the Alpine Fault, occurred in the middle 17th to early 18th century.