Structural analysis of debris avalanche deposits

VANWYK Résumé: Debris avalanche deposits are commonly found on volcanoes, and present a significant hazard. The prediction of such events poses special problems as they may not be associated with any normal volcanic (i.e. magmatic related) phenomena. In contrast structural analysis of volcanoes shows that there are features that develop progressively and gradually before collapse that can be used to predict failure sites. Such feature can provide the data for installing appropriate monitoring techniques. When the failure happens, the resulting avalanche generally has a very long run out, that is often related to the height of fall, or the sliding masses centre of gravity. Closer analysis for several cases show that this relationship hold only approximately and varies according to the material composing the avalanche, topography and the stuff over which the avalanche moves. The problem facing modellers or such phenomena is that there are many processes possible to explain the mobility (Acoustic fluidisation, rock fracturing stress release, water pore pressure etc...), but none can provide a universal model, so far. Some models can predict very well the run out and morphology of the avalanche, but do not indicate well what physical process is at work to provide the low resistance needed for long runout. Answers to these problems may lie in the structural analysis of debris avalanches deposits. In fact they are stuffed with faults, fractures, shear zones, and rich in textural information that can be used to determine the kinematic and can lead to separating out possible processes though textural analysis of clasts, or granulometry. Examples I'll provide are the Socompa debris avalanche deposit in Chile, which is covered in a dense fault network, Mombacho (Nicaragua), Chimborazo (Ecuador) and Perrier (Auvergne). These show that faults are produced throughout the emplacement of an avalanche, and thus the structures can be used to determine the early development of the mass. In all cases, they indicate a vertical velocity profile or a slower top layer, with extrusion of material from the lower part. This indicates that basal friction must be very low, but that the skin of the avalanche provides a major retarding stress.