Incorporation processes in volcanic rockslide-debris avalanches from field observations: implications on emplacement mechanisms

Rockslide-debris avalanches associated with volcanic sector collapses are highly erosive phenomena. The amountof incorporated material is hard to estimate but the few data available suggest that it can easily reach 10 vol.% ofthe deposit. There are two major consequences of substratum incorporation on flow behaviour: 1) loss of kineticenergy by friction and 2) gain of potential energy as the volume increase. The efficiency of the erosive processeswill greatly influence flow mobility. We present field observations on several debris avalanche deposits (DADs) inEcuador (Chimborazo and Imbabura volcanoes), Chile (Taapaca volcanoes), and France (Monts Dore volcano) toillustrate how rockslide-debris avalanches incorporate substratum.The most common process of substratum incorporation is piece-by-piece erosion. This appears as basal shearingfeatures and has been found in most of the deposits. Nevertheless, the size of these erosion features varies greatlyfrom place to place. We found very large sections of basal contact at Chimborazo DAD. Planar contacts on epi-clastic, ash fall and fluviatile deposits in the distal region generally present minor erosion features (from severalcentimetres to few decimetres-long). Where the pre-avalanche topography is more rough, the shearing features aremuch larger (up to metres-long) and show evidence of the impact of the flow. We found pebbles from the substra-tum reduced to powder in the Monts Dore debris avalanche deposit about 35 km from the source. In this deposit,the presence of large megablocks (> 50 m-wide) in the mixed facies induces erosion of large blocks (> 5 m-wide)of unconsolidated conglomerate. Such erosive mechanisms appear to be energy-consuming.However, piecemeal erosion is not the unique incorporation process. We also found in most of the deposits signifi-cant evidence of substratum fluidisation. The best example of substratum fluidisation was encountered at one site atChimborazo DAD where a large amount (several tens of m3) of the Chalupas unconsolidated ignimbrite is injected40 m-up into the deposit body. Fluidised sand and pumice injections (several centimetres-wide and metres-long)are also observed at Imbabura, Monts Dore and Taapaca DADs. Such an erosive mechanism appears much moreenergy-saving than piecemeal erosion.In conclusion we show that incorporation processes are complex and controlled by the substratum nature andtopography. It is important to note that low density and fine grained substratum layers such as unconsolidated ign-imbrites, ash and pumice fall, and sand deposit can be fluidised by the overriding rockslide-debris avalanche. Weexpect that such substratum can enhance the flow mobility.