A gravitational spreading origin for the Socompa debris avalanche

Socompa Volcano arguably provides the world's best-exposed example of a sector collapse-derived debris avalanche deposit. New observations lead us to re-interpret the origin of the sector collapse. We show that it was triggered by failure of active thrust-anticlines in sediments and ignimbrites underlying the volcano. The thrust-anticlines were a result of gravitational spreading of substrata under the volcano load. About 80% of the resulting avalanche deposit is composed of substrata formerly residing under the volcano and in the anticlines. The collapse scar can be traced up to 5 km from the edifice, truncating two spreading-related anticlines, which collapsed in the event. Outcrops near the volcano preserve evidence of edifice material being carried along on top of mobilised substrata. On the north side of the scar, the avalanche motion was initially at right angles to the failure edge. Structural relations indicate that immediately prior to collapse the substrata disintegrated, became effectively liquidised, and were ejected from beneath the edifice. Catastrophic mobilisation of substrata probably resulted from breakdown of ignimbrite clasts and cement. It may have occurred through progressive rock fracture by high shear strain during spreading. Material ejected from under Socompa formed a layer on which volcanic edifice debris was transported. This interpretation of events explains the puzzling observation that avalanche units with the lowest gravitational potential energy moved the furthest. It can also account for avalanche motion normal to the collapse scar walls. Ignimbrites and other rock types probably capable of similar behaviour underlie many other volcanoes. Identification of spreading at other sites could therefore be a first step towards assessment of the potential for this style of catastrophic sector collapse.