Longitudinal Ridges in Long Runout Landslides: Are High-Speed Granular Flow Mechanisms Applicable?

The mechanisms of emplacement of long runout landslides and the formation mechanisms of longitudinal ridges associated with their deposits remain subjects of debate. The presence of an icy surface can be used to explain both the reduction of friction associated with the deposition of long runout landslides and the development of longitudinal ridges. However, laboratory experiments on rapid granular flows show that ice is not a necessary requirement for the development of longitudinal ridges, which instead may form from convective cells within high-speed flows. Results demonstrate a relationship between the wavelength (S) of the ridges as 2-3 times the thickness (T) of the flow. At the field scale, the same scaling relationship has been found for the first time in a martian long runout landslide. H ere, we present the case study of the El Magnifico landslide, in the north part of the Atacama Desert, Chile, which exhibits longitudinal ridges. We find that the wavelength of the ridges and the thickness of the deposit scale in good agreement with the scaling relationship found for rapid granular flows and a martian long runout landslide. We also study the internal structures of the landslide deposit and their relationship with the longitudinal ridges in order to determine the emplacement mechanism. Their interpretation is not conclusive of a mechanism of convective-style motion. In fact, the internal structures can also be associated to internal folding of the sliding mass derived from the existence of pattern-forming vibrations. Our observations, which include interaction with no chaotic mixing between different lithologies and the presence of megablocks that exhibit preserved bedding planes associated with fluctuations in stress, are qualitatively similar to numerically modelled rapid granular slides, which were suggested, to some degree, to be associated with acoustic fluidization.