A gravitational spreading origin for the Socompa debris avalanche B. van Wyk de Vries a, * , S. Self ²,1 , P.W. Francis c,2 , L. Keszthelyi d a Magmas et Volcans (UMR 6524), Observatoire du Physique du Globe, Universite  Blaise Pascal, 5 rue Kessler, 63038 Clermont-Ferrand, France b SOEST, University of Hawai'i, 2525 Correa Road, Honolulu, HI 96822 USA c Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK d Planetary Image Research Laboratory, Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA Received 21 January 2000; accepted 29 July 2000 Abstract 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 gravita- tional 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 edi®ce, truncating two spreading-related anticlines, which collapsed in the event. Outcrops near the volcano preserve evidence of edi®ce 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 effec- tively liquidised, and were ejected from beneath the edi®ce. 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 edi®ce 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. Identi®cation of spreading at other sites could therefore be a ®rst step towards assessment of the potential for this style of catastrophic sector collapse. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Socompa; sector; collapse; spreading; debris; avalanche 1. Introduction 1.1. Description of Socompa volcano and its avalanche Socompa Volcano (Fig. 1) has one of the most voluminous terrestrial sector collapses and avalanche deposits and is remarkably well preserved by the hyper-arid climate of the Atacama Desert of northern Chile (Fig. 2). The deposit's freshness provides an Journal of Volcanology and Geothermal Research 105 (2001) 225±247 0377-0273/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S0377-0273(00)00252-3 www.elsevier.nl/locate/jvolgeores * Corresponding author. Tel: 133-04-73-346732; fax: 133-04- 73-346744. E-mail addresses: vanwyk@opgc.univ-bpclermont.fr (B. van Wyk de Vries), self@lani.soest.hawaii.edu (S. Self), lpk@jupiter.lpl.arizona.edu (L. Keszthelyi). 1 Tel.: 11-808-956-5996; fax: 11-808-956-2538. ² P.W. Francis died on 30 October 1999.