ESTIMATES FOR THE TIMING OF THE LAST COSEISMIC DISPLACEMENT ON THE ALPINE FAULT, NORTHERN FJORDLAND, NEW-ZEALAND

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.