Displacement on the Alpine Fault at Haast River, South Westland, New Zealand

Abstract

On the north bank of Haast River, the Alpine Fault trace has an 047–057° trend. The trace comprises left and right stepping en echelon, linear segments, each up to 200–300 m long, with a prominent terrace upthrown by between 5 and 10 m on the southeast side. A creek channel, deeply incised into the terrace 1.1 km north of the river, is offset dextrally by 94 m at the fault scarp. Exposed in the creek walls is a sequence of subterrace sediments, up to 9 m thick, that show an upward transition from clean, well‐sorted sands, through fine‐grained sands and carbonaceous clays containing a persistent peat horizon, to an erosive, coarsegrained, wood‐bearing gravel unit. The sequence is interpreted as a regressive transition from beach, through swamp or overbank fluvial, to high‐energy fluvial environments. As in other South Westland localities, the postglacial regression is in marked contrast to the pronounced global transgression predicted from rising sea levels. Radiocarbon dating of the peat horizon has yielded ages of c. 7300 yr B.P., and wood from the gravel unit is dated at c. 4000 yr B.R Although the age of the terrace surface is poorly constrained, it is unlikely to be older than c. 4000 years. Horizontal and vertical offsets of the terrace surface and the stream course have occurred at minimum rates of 23.5 mm/ yr and 2.25 mm/yr, respectively, for the last 4000 years.

The area immediately north of Haast River marks a major change in character of the Alpine Fault. The fault trace is strongly segmented 10 km to the northeast, but at Haast River and farther south, the fault trace has a northeast trend, with a single fault surface accommodating strike‐slip and a decreasing dip‐slip displacement.

Keywords:

• Alpine Fault
• displacement rate
• postglacial paleoenvironments
• neotectonics