The structure, morphology, and surface texture of debris avalanche deposits : field and remote sensing mapping and analogue modelling

Flank collapse generates avalanches and large landslides that significantly change the shape of a volcano and alter the surrounding landscape. Most types of volcanoes experience flank collapse at some point during their development. In the Philippines, for example, the numerous volcanoes with breached edifices belong to the cone, subcone, and massif morphometric classes. Debris avalanches occur frequently on both volcanic and non-volcanic terrains making it an important geologic event to consider for hazard assessment. Debris avalanche deposits (DAD) preserve surface and internal structures, morphology, and texture that can be used to determine transport type, deformation history, causal mechanism, and emplacement kinematics. However, natural DAD are often too vast and chaotic-seeming in the field so that structural and morphological mapping by remote sensing is a good complement to studying them. This study describes and analyses recurrent structural and morphological features of analogue models and natural DAD at Mt Iriga and Guinsaugon (Philippines), and uses several other examples at Mt Meager (Canada), and Storegga Slide (Norway). The study explores the use of analogue models as landslide kinematics, dynamics, and emplacement and causal mechanism indicators. Hummocks are identified as a key structural element of DAD. Hummocks, a major DAD topographic feature, are formed as the mass in motion slides and evolves by progressive spreading and break up. Internally, high angle normal faults dissect hummocks and merge into low angle shear zones at the base of the slide zone. Hummock size distribution is related to lithology, initial position, and avalanche kinematics. Hummocks provide information on the transport conditions and initial composition of the landslide. Their geometry (size and shape), internal structures, and spatial distribution are kinematic indicators for landslides from development until emplacement and provide a framework for interpreting emplacement dynamics. Experiments with curved analogue ramps show the development of an area of accumulation and thickening, where accelerating materials reach a gently sloped depositional surface. Experiments with straight ramps show a longer slides with continued extension by horst and graben structures and transtensional grabens. A thickened mass is found to subsequently remobilise and advance by secondary collapse. This set of experiments show that failure and transport surface morphology can influence the emplacement mechanism, morphology, and avalanche runout. Structural and morphological mapping by remote sensing, and description of recurrent features at the remote and previously unmapped Suphan Dagi (Turkey), Cerro Pular-Pajonales (Argentina), and Tacna (Peru) DAD suggest scenarios, causes, triggering and emplacement mechanisms of these DAD. These are used to explain their avalanche kinematics and dynamics. Mapping DAD is a necessary step for identifying past events and existing hazards in specific areas. Identifying and describing the DAD structures and morphology will help understand the kinematics and dynamics of the emplaced avalanches.