Transport and emplacement mechanisms of large volcanic debris avalanches: evidence from the northwest sector of Cantal Volcano (France)

Large volcanic debris avalanche (VDA) deposits of Miocene age occur within the stratigraphic sequence of Cantal stratovolcano (central France). The VDA of Cantal volcano were initiated by sector collapse of a cone of high elevation (up to 4500 m.a.s.l.). Two distinct superimposed VDA deposit units (Breccias I and II) resulted from at least two successive large-scale collapse events during the evolution of the volcanic complex. Breccia I, the oldest one, is very thick (up to 200 m) and heterolithologic. It is overlain by Breccia II, which is thinner (up to 50 m) and contains more heterometric clasts ranging from hectometer- to millimeter-size. A deposits are poorly sorted. Clasts are angular and highly fractured at different scales, often with jigsaw fit. In Breccia II, jigsaw megaclasts have usually subspheric shapes with size decreasing distally. Basal deposits commonly display thin inversely graded layers, reorientated clasts along ramp structures, and cataclasis of the substratum. Some units have a distal fabric in the form of a poorly developed clast imbrication. The clasts are commonly sub-rounded thereby indicating abrasion during viscous flow. ropy of magnetic susceptibility (A.M.S.) measurements indicate absence of a preferentially oriented matrix fabric. Magnetic foliation parallel to the bedding suggests that the depositional process is dominated by particle settling. The magnetic fabric is better organized in distal deposits where magnetic carriers are imbricated. Moreover, the fragmented clasts and megaclasts are not abraded, rather inflation led to isotropic spherical dispersion of clastic material (=Isotropic Dispersive Inflation) which fed the interclast matrix. This is also confirmed by the lack of rotation at the individual crystal level. ese data strongly suggest transport of VDA by liquified non-turbulent granular flows. Distally, imbricated deposits suggest more turbulent flow. Emplacement of the VDA results from progressive upward aggradation of the particles by volume reduction and deflation loss. The non-turbulent liquified stage of the VDA disappears. The VDA behaves as a sliding rigid mass (plug flow) that exerts strong friction on the basement leading to gouge formation.