Magnarini, Giulia; (2021) Morphometric Study of Longitudinal Ridges in Long Runout Landslides on Mars, Earth, and the Moon. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Long runout landslides are hypermobile landslides ubiquitous in our solar system. The exact mechanism(s) that have to be invoked in order to explain their high velocity and exceptional travel distances over nearly horizontal surfaces has yet to be successfully determined. In this thesis I focused on the distinctive longitudinal ridges that mark the surface of long runout landslide deposits in the attempt to link these morphological features and their related internal structures to the mechanisms involved during the emplacement of such catastrophic events. I conducted a morphometric analysis of longitudinal ridges of three case studies: the Coprates Labes landslide in Valles Marineris on Mars; the El Magnifico landslide in Chile – on Earth; the Tsiolkovskiy crater landslide – on the Moon. For the first time in natural landslides, I found that the wavelength of the longitudinal ridges is consistently 2 to 3 times the thickness of the landslide deposit, in agreement with experimental work on rapid granular flows. The recurrence of the scaling relationship suggests a scale- and environment-independent mechanism. Therefore, I concluded that the existence of longitudinal ridges in long runout landslides cannot be used to infer the presence of specific lithologies forming the basal surface; nor environmental and climatic conditions at the time of landslide emplacement. Based on the agreement between the results obtained from my morphometric analysis and the laboratory experiments on rapid granular flows, I proposed that longitudinal ridges in long runout landslides are imparted by high-speed granular flow convection mechanisms. In order to ground truth such hypothesis, I conducted field work at the terrestrial El Magnifico landslide and studied the internal structures of the deposit and their relationship with the longitudinal ridges. I concluded that evidence cannot rule out a convection-style mechanism observed in laboratory experiments of granular flows but is also not equivocal in its support. I advanced an alternative hypothesis that longitudinal ridges may have formed by a mechanism that involves pattern-forming vibrations. Such proposed mechanism supports the existence of heterogeneous stress distribution and stress fluctuation within long runout landslide deposits, which are considered the hallmark of acoustic fluidization. I suggested the use of the scaling relationship between the wavelength of longitudinal ridges and the thickness of the deposit as a tool to infer the thickness of landslide deposits where its calculation is not otherwise possible using typical methods in geomorphology. I applied this novel idea to the Light Mantle landslide, Taurus-Littrow valley, at the Apollo 17 landing site: by calculating the representative wavelength of the longitudinal ridges I derived the thickness of its deposit, as it could not be derived through interpolation. I discussed the finding of my work within the context of the literature on frictional weakening in fault mechanics, on the similarity of weakening of shear zones in landslide and earthquake mechanics, and on the geomorphology of long runout landslide deposits. Following this, I defined a new framework under which the understanding of the formation mechanism of long runout landslides should be approached. Finally, I identified four possible directions for future work: extending the morphometric analysis of longitudinal ridges in martian double layer ejecta; investigating the importance of the roughness of the substrate in the formation of longitudinal ridges; performing friction experiments to study weakening mechanisms in lunar rock analogues; using martian long runout landslide deposits as stratigraphic markers in order to constrain the timing of geological processes.

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