Abstract
Rock avalanches are low frequency natural hazards that can alter landscape morphology, and constraining the timing, volume and emplacement dynamics of pre-historic rock avalanches is crucial for understanding the hazards posed by these events. Here we perform cosmogenic nuclide dating, topographic reconstruction and runout modelling and of the Molveno rock avalanche, located north of Lake Garda in the Province of Trento, Italy. The unique morphology of the deposits, which features numerous large scarps and prominent lineaments, have previously led researchers to interpret the Molveno rock avalanche as being the result of multiple events. Our results show that the Molveno rock avalanche had a volume of approximately 600 Mm3, and failed from a prominent niche located on Monte Soran. 36Cl cosmogenic nuclide dating results shows that the deposits were emplaced as a single event approximately 4.8±0.5 ka, and suggests that the unique deposit morphology is due to the emplacement processes acting during and soon after failure. Numerical runout modelling shows that this morphology could have resulted from a combination of runup and extensional spreading of the debris along the complex valley floor topography. The ages we determined for this event are coincident with the nearby Marocca Principale rock avalanche (5.3±0.9 ka), which may suggest a common trigger. Our results have important implications for interpreting the morphology of rock avalanche deposits, and contribute to the evolving understanding of rock avalanche processes in the Alps.
Highlights
Large rock avalanches are rare mass movements that can dramatically alter landscape morphology
Our results show that the Molveno rock avalanche had a volume of approximately 600 Mm3, and failed from a prominent niche located on Monte Soran. 36Cl cosmogenic nuclide dating results shows that the deposits were emplaced as a single event approximately 4.8 ± 0.5 ka, and suggests that the unique deposit morphology is due to the emplacement processes acting during and soon after failure
Our analysis has revealed several important preconditioning, preparatory, triggering, and emplacement factors that contributed to the Molveno rock avalanche
Summary
Large rock avalanches are rare mass movements that can dramatically alter landscape morphology. These are especially prominent between the Central North and Buse dei Pravebili regions, as well as in Moline (Figures 2A,B) These features have been interpreted as evidence that the deposit is composed of multiple events (Chinaglia, 1992), and it was previously unknown if these features formed during or after the emplacement of the rock avalanche(s). At 60 s the landslide collides with the opposite valley, and spreads to the North and the South This symmetrical spreading is not expected from field mapping, it is explained though by the large depression which was created at the location of present-day Lago di Molveno (see section “Reconstruction of pre-failure topography and volume analysis”). Rheology Unit weight (kN/m3) Friction angle (◦) Pore pressure coeff. (Ru; –) Friction coefficient μ (–) Turbulence coefficient ξ (m/s2) Internal friction angle (◦)
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