A gravitational spreading origin for the Socompa debris avalanche

Socompa Volcano arguably provides the world's best-exposed example of a sec tor collapse-derived debris avalanche deposit. New observations lead us to re-interpret the origin of the sector collapse. We show that it was trigger ed by failure of active thrust-anticlines in sediments and ignimbrites unde rlying the volcano. The thrust-anticlines were a result of gravitational sp reading of substrata under the volcano load. About 80% of the resulting ava lanche 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 t he event. Outcrops near the volcano preserve evidence of edifice material b eing 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 edg e. Structural relations indicate that immediately prior to collapse the sub strata disintegrated, became effectively liquidised. and were ejected from beneath the edifice. Catastrophic mobilisation of substrata probably result ed from breakdown of ignimbrite clasts and cement. It may have occurred thr ough progressive rock fracture by high shear strain during spreading. Mater ial ejected from under Socompa formed 3 layer on which volcanic edifice deb ris was transported. This interpretation of events explains the puzzling ob servation that avalanche units with the lowest gravitational potential ener gy moved the furthest. It can also account fur avalanche motion normal to t he collapse scar walls. Ignimbrites and other rock types probably capable o f similar behaviour underlie many other volcanoes. Identification of spread ing at other sites could therefore be a first step towards assessment of th e potential for this style of catastrophic sector collapse. (C) 2001 Elsevi er Science B.V. All rights reserved.