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Abstract

In December 1991, 11.8 ± 2.4 × 106 m3 of rock and ice fell from Mount Cook (3754 m), the highest peak in New Zealand's Southern Alps. It fell 2720 m in 2 min and traveled 7.5 km (averaging 60 m/s). It generated a magnitude (ML) 3.9 earthquake, becoming finely comminuted and doubling in mass through erosion.

In December 1991, rockfalls began at Mount Fletcher (2467 m) 30 km northwest of Mount Cook. They continued until the ridge north of Mount Fletcher fell in rock avalanches in May and September 1992. They dropped 1440 m along similar 3.8 km paths in 50 s, generating magnitude 2.8 and 2.7 earthquakes. The first displaced 7.8 × 106 m3 of water from a lake, the second, ~5 × 106 m3.

In February 1996, rockfalls of ~105 m3 fell from Mount Thomson (2642 m), 15 km southwest of Mount Cook. It was about an annual event in the park, and a 10-year event at Mueller Glacier. The sequence was similar to earlier phases at Mount Fletcher.

The collapses all were of steep, east-facing slopes of intensely fractured rock on the hanging wall of the Main Divide fault. The larger three were of cohesionless, anisotropic materials having weaknesses along steeply dipping joints almost parallel to, but steeper than the slope. Initial failures were on lower, but steeper slopes. Retrogressive failures quickly took the entire slopes.

The peaks are at limiting slope equilibrium. Collapse is their major erosion process, maintaining relief in balance with uplift and valley erosion. The hanging wall of the Main Divide fault has the fastest uplift rate, and the highest frequency of failure. The frequency of >106 m3 collapse is 1 per 20–30 yr.

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