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G T Hancox and N D Perrin, March 2000 Green Lake Landslide is a very large ancient rockslide
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 13,000 years ago. The landslide has an
estimated volume of ~27 km3 with a surface area of 45 km2, and is
considered to be the largest documented landslide of its type on
earth (although larger submarine slides are known). The landslide
is pictured in Figure 1 and its location shown in Figure 2. The
geology and geomorphology of the landslide are shown in Figure 3,
while Figure 4 is a satellite image of the landslide showing its
vast size, which can only be fully appreciated when viewed from
space. |
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 Figure
1. Aerial photo of Green Lake Landslide inFiordland, just 2-3 km north of Lake Monowai. This view looks southeast across hummocky, semi-intact blocks of landslide debris towards the 14 km long, v-shaped head scarp. Green Lake is a large landslide pond formed at the foot of the head scarp. Island Lake is another pond within the landslide debris. The flat swampy ground in the foreground is the head of the Grebe Valley. This area is actually the infilled part of the original Lake Monowai, which was cut in half when the landslide occurred 12,000-13,000 years ago. Outflow from the former lake once flowed south into Lake Monowai (via an over flow channel right), but the lake was gradually infilled with glacial sediments and swamp deposits. This swampy area now forms the head of the Grebe River, which flows north into Lake Manapouri. |
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| Geomorphic evidence indicates that the
landslide probably occurred as a rapid failure, possibly in two phases.
The collapse of the 1500 m-high mountain ridge on the east side of
the former Monowai (now Grebe) valley resulted in destruction of a
9 km long section of the southern Hunter Mountains. Slide debris was
transported up to 2.5 km laterally, and fell about 700 m vertically
into the deeply glaciated former Monowai valley, which at the time
of the landslide was probably filled with a glacial lake that extended
south towards Lake Monowai. Landslide debris formed a landslide dam
about 800 m high in the valley, which cut the original Lake Monowai
in two, impounding a lake ~11 km long (Lake Grebe), which was gradually
infilled with glacial sediments, and later peat and swamp deposits.
Radiocarbon (14C) dating of lake silts in the Grebe valley shows
the final infilling Lake Grebe occurred about 11,000-11,500 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 estimated age of Green Lake Landslide is about
12,000-13,000 years. This is in close |
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 Figure
2. Map of southern Fiordland showing the geology and main physiographic
features of the region, and the location of Green Lake Landslide and
other very large landslides.
The main features of Green Lake Landslide include a large area
of |
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| Green Lake landslide was analysed using
appropriate inferred parameters suggests. This showed that flooding
of the potential failure surface by lake water when the glaciers retreated
would have reduced slope stability. However, this was probably not
sufficient to cause such a large mountain mass to collapse. Further
analysis simulating earthquake shaking showed that the landslide was
most probably triggered by strong earthquake shaking (MM8-MM10), possibly associated with a large earthquake (Ms7.5 or greater) 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. Lake Landslide is considered to be a significant feature because
of its enormous size and catastrophic effects, which resulted in
the collapse of a substantial part of a high mountain range (see
Figure 4). It illustrates the sort of environmental effects that
occur after deglaciation in a mountain area, especially, in an area
of high seismicity that is periodically shaken by very large earthquakes.
Today, the landslide dam remains essentially intact, and apart from
local failures around the steep head scarps |
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| Figure 3. Geological and geomorphic map of Green Lake Landslide. |
Figure 4. |
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References Hancox, G. T. and Perrin, N. D., 1994: Green Lake landslide: a very large ancient rock slide in Fiordland, New Zealand. IGNS Science Report 93/18, June 1994. Hancox, G.T. and Perrin, N.D., 1994: Green Lake landslide: a very
large ancient rock slide in Fiordland, New Zealand. Proceedings
of the VIIth I.A.E.G. International Congress, Lisbon, Portugal,
5-9 Sept 1994. For more information contact Graham
Hancox
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