A large rock avalanche in the Craigieburn Range, Canterbury

Abstract

A 0.5 km³ high-speed rock avalanche fell from the summit and western flanks of the Craigiebum Range above Lake Coleridge, Canterbury, New Zealand, about 300 years ago. Avalanche debris covers about 4 km² and is at least 300 m thick at some localities where it infilled a former glacial meltwater channel.

The source rocks were well-indurated, highly fractured sandstones and mudstones of the Torlesse Supergroup. The sides of the rock avalanche source were bounded by faults, and although it is unclear exactly what surfaces provided the failure planes, it appears that failure occurred at least in part down bedding planes, though not directly down dip.

Rock avalanches are most frequently triggered by large earthquakes. The presence of this and 6 other rock avalances of widely different age in the Craigiebum Range area suggests that the region is prone to rock avalanches and that perhaps at least 7 moderate to large magnitude earthquakes have occurred in this area in the last 6000 years.

Keywords:

• rock avalanche
• Craigieburn Range
• earthquakes