Analysis and Implications of Large Martian and Terrestrial Landslides.

Abstract

I address two long-standing scientific problems in this thesis: the mechanism(s) of long-runout in large landslides; and the activity of water on the surface of Mars late in its history. Long-runout landslides form significant geologic risks. My research has aimed at understanding the factors that control the initiation and runout of large landslides wherever they occur. A second objective of this research has been to use martian landslides to gauge the activity of liquid water on Mars' surface over the past quarter of its history. To achieve these objectives, I made field observations of six moist and dry landslides in the western United States, I studied all the high-resolution Viking Orbiter images for large landslides on Mars and I collated all the available literature data on large landslides, to develop the clearest view possible of the nature of the large landslide process. I then used this information to evaluate all the previously published models of long runout, and to develop my own theory when I found that none of the existing theories provided an adequate explanation of the observations. I conclude that large landslides primarily slide and spread into place over fluidized natural basal lubricants. This concept follows upon the air-layer lubrication theory of Shreve, but does not call upon a gas lubricant, a possibility ruled out by the presence of Blackhawk-like long-runout landslides on Mars. Rather, the lubricants appear to be fine-grained materials from the substrate or from the landslide debris itself that deform plastically at the high shear stresses and strain rates present beneath large rapid landslides. Large dry landslides grade into moist debris flows as water becomes incorporated into their fine-grained component. Moist and dry landslides differ markedly in sedimentology and morphology, but not in a variety of quantitative relations. Seven landslide events on Mars appear to have involved water during runout, based on comparisons with water-bearing landslides of similar morphology and volume on Earth. These deposits occur primarily in Valles Marineris and indicate that liquid water has been sporadically available on Mars at various times over the last ~1 billion years of its history.