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| Titel |
Rock-slope failure activity and geological crises in western Norway |
| VerfasserIn |
Paula Hilger, Reginald L. Hermanns, Kristin S. Myhra, John C. Gosse, Susan Ivy-Ochs, Bernd Etzelmüller |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250141759
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| Publikation (Nr.) |
 EGU/EGU2017-5300.pdf |
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| Zusammenfassung |
| In Norway a compilation of terrestrial cosmogenic nuclide (TCN) ages of rock-avalanche
deposits suggests a close link of rock-slope failures related to deglaciation. Although ages
spread over several thousand years at the end of the Late Pleistocene, 50% of all documented
events occurred within 1000 years after deglaciation. It is therefore likely that debuttressing
triggered most of the events. The same data set suggests that 25% of the events occurred
during a period stretching until the Holocene thermal maximum (HTM). These events might
be interpreted as possible reactions to additional factors such as the thawing of high-altitude
permafrost.
An example of a geological crisis following deglaciation and before the HTM are seven
lobate rock-avalanche deposits mapped under the slope of the Vora mountain (1450 m asl.) in
the Nordfjord area of western Norway. Three events of this rock-slope failure cluster date
within a short time period of 2000 years, where modelling studies indicate that high-altitude
permafrost was present.
After the HTM rock-slope failures are distributed temporally and spatially rather
evenly throughout the Holocene and western Norway. But there are two independent
local clusters with frequent rock slides during a short time span. (1) At the active
Mannen rock-slope instability several rock-avalanche and rockslide deposits were
mapped on the valley bottom. Stratigraphic relations combined with TCN dating
suggest that at least one event occurred when the valley bottom was below the marine
limit. TCN ages of further four lobes cluster around 5.2 ka BP, which does not
coincide with any other rock-avalanche occurrence in the region. The top of the north
facing 1295 m high unstable slope concurs with the currently estimated permafrost
boundary. Preliminary TCN ages of the sliding surface indicate that larger parts of the
mountain did not become active until the climate maximum. It is likely that due
to structural complexity not allowing for any easy kinematic failure process, it
required several thousand years of rock-slope deformation prior to the multiple
failures.
(2) The youngest independent rock-avalanche cluster is historic with 5 rock avalanches
sourcing from Ramnefjellet in 1905, 1936 (three events), and 1950 entering into Loen lake in
western Norway. Subsequent displacement waves killed 61 people in 1905 and
73 people due to the first failure in 1936. The back scarp does not exceed 850 m
elevation and lies hence below the present day and Little Ice Age permafrost limit. It is
therefore unlikely that permafrost dynamics contribute to this sequence of rock-slope
failures.
Local clusters or a geological crisis by rock-slope failures seems to be related to different
main factors, such as glacial debutressing, influence of ground thermal regime changes
(Mannen) and probably more disconnected to major climate variability (Loen). For an
integrated risk management it is therefore important to understand that large rock-slope
failures do not necessarily have to occur in single events but can occur over several decades
or centuries and thus complicate severely land use management after catastrophic
events. |
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