Green Lake landslide : an ancient large-scale wedge failure in glaciated terrain, Fiordland, New Zealand

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Hancox, G.T.; Perrin, N.D. 1994 Green Lake landslide : an ancient large-scale wedge failure in glaciated terrain, Fiordland, New Zealand. Lower Hutt: Institute of Geological & Nuclear Sciences Institute of Geological & Nuclear Sciences science report 93/18 50 p.

Abstract: Green Lake Landslide is a very large ancient rock slide of schistose gneiss and granodiorite located in the deeply glaciated Hunter Mountains of Fiordland, New Zealand. Geology and geomorphic evidence suggests the slide occurred just after the end of the last (Otira) glaciation about 12,000 to 13, 000 years ago. With an estimated volume of about 27 km3 and area of 45 km square it is considered to be the largest documented sub-aerial landslide of its type on earth. Geomorphic evidence indicates that the slide occurred as a rapid failure, possibly in two phases. Collapse of the 1500 m-high alpine ridge on the east side of the former Monowai (now Grebe) valley resulted in the destruction of a 9 km segment of the southern Hunter Mountains. Slide debris moved up to 2.5 km laterally and 700 m vertically, forming a landslide dam about 800 m high, that is inferred to have blocked and truncated a proto-Lake Monowai, and impounded a lake (Lake Grebe) about 11 km long. This lake was gradually infilled with glacial sediments, and later peat and swamp deposits. Radiocarbon (14C) dating of silts shows the final infilling Lake Grebe occurred about 11,000 to 11,500 years ago. Dating of peat deposits indicates the lake was drained about 8600-9000 years ago, after ice recession had allowed lake water to flow north into Lake Manapouri. Today the swampy headwaters of the Grebe River are all that remains of the former lake area. Based on the 14C dates the currently estimated age of Green Lake Landslide is about 12,000-13,000 years. This is in close ageement with estimates based on geology and geomorphology. Main features of the landslide include a large area of hummocky, bush-covered slide debris up to 1000 m thick comprising a number of large, semi-intact blocks up to 2.5 km long, a prominent V-shaped head scarp that extends for about 14 km, and four large pull-apart basins. Three of these basins contain large landslide ponds, notably Green Lake and Island Lake. Tne landslide occurred following glacial retreat, which caused the withdrawal of lateral support from an oversteepened ice-carved slope underlain by a low-angle (25-30deg.), southwest-dipping potential failure surface within the Mount Cuthbert Fault Zone. Slope stability analysis using realistic inferred parameters suggests that stability of the slope that collapsed would have been reduced by flooding of the potential failure surface by a proto-Lake Monowai when the glaciers retreated. However, failure was most probably triggered by strong (MM VIII-X) earthquake shaking, possibly associated with a large (>= Ms 7.5) earthquake on the Alpine Fault off the Fiordland coast. This earthquake may also have triggered some of the other old large landslides identified in the Fiordland region. Green Lake Landslide is considered to be a significant feature because of its enormous size and catastrophic failure mechanism, which resulted in the collapse of a substantial part of a high mountain range . Today, the landslide dam remains essentially intact, and there appears to be little potential for reactivation. (auths)