Structural analysis of the early stages of catastrophic stratovolcano flank-collapse using analogue models

Many major volcanic flank collapses involve the failure of low-angle strata in or under the edifice. Such failures produce voluminous, destructive debris avalanches that are a major volcanic hazard. At Socompa, Las Isletas-Mombacho and Parinacota volcanoes, field studies have shown that during catastrophic flank collapse a significant segment of their substrata was detached and expelled from beneath the volcanic edifice and formed a mobile basal layer on which the sliding flanks were transported. Previous studies have proposed that gravitational flank spreading was likely involved in the onset of sudden substrata failure. The early stages of this particular type of flank collapse can be modelled under laboratory conditions using analogue models. This allows us to study the development of structures accommodating early deformation of the sliding flank during catastrophic collapse. In the experiments, the detached substratum segment (low-viscosity basal layer) was modelled with a silicone layer, and the overlying stratovolcano with a layered sand cone. The first structure developed in the models is a graben rooted in the low-viscosity basal layer. This graben forms the limits of the future avalanche-amphitheatre and divides the sliding flank into a 'toreva' domain (upper sliding flank) and a 'hummock' domain (lower sliding flank). These domains display distinctive structural patterns and kinetic behaviour. Normal faults develop in the toreva domain and inside the graben, while the hummock domain is characterised by transtensional structures. The hummock domain also over-thrusts the lower amphitheatre sides, which allows subsequent sideways avalanche spreading. Measurements show that horizontal speeds of the hummock domain are always higher than that of the toreva domain during model collapse. The main role played by the low-viscosity basal layer during this type of collapse is to control the size, shape and structural complexity of the sliding flank; it also transmits mass and momentum from the toreva to the hummock domain.