0 Academic des sciences / Elsevier, Paris CComatCriaux / Geomaterials (Sedimentologie / Sedimentology) La fabrique interne des d6p8ts du sturzsfrom de Flims (Alpes suisses) : caractkistiques et implications sur les mkanismes de transport 7’5e fabric of the sturzstrom of Flims (SwissA@): Characteristics and implications on the transport mechanisms Jean-Luc Schneider”“, Patrick Wassmerb, Beatrice Led&et-t” a Laboratoire de sedimentologie et geodynamique (CNRS-UMR 8577), universite des sciences et technologies de Lille, bdtiment SN 5,59655 Villeneuve d’Ascq cedex, France b Cereg (CNRS-UP 2037), universite Louis-Pasteur, 3, rue de I’Argonne, 67083 Strasbourg cedex, France (Regu le 31 aout 1998, accept6 apres revision le 22 mars 1999) Abstract - The Flims sturzstrom deposit (Swiss Alps) is the largest in the Alps (12 km3, Et/L = 0.13). It consists of stratified breccias that display planar discontinuities that originated during transport. Major sliding surfaces are underlined by frictional gouge. Between these surfaces, the material is fragmented with rotated fault blocks bordered by oblique fractures that show the transport directions. The transport mechanisms of the debris avalanche were dominated by layer-parallel slip movements that favoured the nonturbulent behaviour of the granular material and the reduction of internal friction. (0 Academic des sciences / Elsevier, Paris.) sturzstrom / fabrics / syn-transport fragmentation / Grisons / Switzerland Resume - Les depots du sturzstrom de Flims (Alpes suisses) sont les plus volumineux des Alpes (12 km3, t-//L = 0,13). Ils correspondent a des breches stratifiees, qui presentent des discontinuites planaires, dont I’origine est lice au mode de transport des materiaux. Les surfaces majeures de glissement sont soulignees par des horizons fortement cataclases. Elles emballent des materiaux fragment& par des fractures obliques, dont I’orientation temoigne d’une rotation des clastes, et materialisent les directions d’ecoulement. Le transport de l’avalanche de debris est domine par des mouvements couche sur couche favorables a un comportement non turbulent de la masse granulaire et a la reduction de la friction interne. (0 Academic des sciences / Elsevier, Paris.) sfurzstrom / fabrique /fragmentation syn-transport / Grisons / Suisse Abridged version Introduction Sturzstroms (Helm, 1932) or debris avalanches are large volume (> lo6 m3> and fast moving (up to 300 kn-rh-‘1 gravity movements of masses of granular or rocky material that result from flank collapse of mountains (Hsii, 1975; Shaller, 1991; Shaller and Smith-Shaller, 1996; Coussot and Meunier, 1996) or volcanoes (Siebert, 1984; Glicken, 1991; Schneider, 1997). The deposits suggest the following attributes: (1) sturzstroms are very mobile as suggested by their low apparent coefficient of friction Et/l; (Heim, 1932), the ratio of fall height Hto horizontal runout length L; this H/L ratio is usually less than 0.2 (Shaller, Note prCsent6e par Jean Dercourt. * Correspondance et tit& a part. Jean-Luc.Schneider@univ-lillel .fr C. R. Acad. Sci. Paris, Sciences de la terre et des plan&es / Earth & Planefay Sciences 1999.328.607-613