Resumo

ROVERATO, Matteo  e  CAPRA, Lucia. Microtextural characteristics as indication of transport and emplacement of two debris avalanche deposits in the Colima volcano (Mexico). Rev. mex. cienc. geol [online]. 2013, vol.30, n.3, pp.512-525. ISSN 2007-2902.

During its history Colima volcano has experienced numerous partial edifice sector collapses with the emplacement of debris avalanche deposits (DAD) of contrasting volume, morphology and texture. A previous detailed stratigraphic study in the southeastern sector of the volcano described by Roverato et al. (2011, J. Volc. Geoth. Res., 207, 33-46) allowed the recognition of two debris avalanche deposits named San Marcos DAD (SM-DAD V= ~1.3 km³) and Tonila DAD (T-DAD V= ~1 km³). Radiocarbon dates of organic material, directly associated with these deposits, gave ages of >28 kyr cal. BP for the SM-DAD and ~15 kyr cal. BP for the T-DAD. The San Marcos collapse was promoted by ongoing volcano-tectonic deformation. The failure event triggered a "dry" debris avalanche (water content <10%). In contrast, the Tonila failure occurred in "wet" paleoclimate conditions during a period characterized by high humidity; the fact that the volcanic system was partially water-saturated was an important factor in the volcanic instability and transportation processes. This work sheds light on the transport and emplacement mechanisms of debris avalanches based on a detailed granulometric and microtextural analysis of the studied deposits. SM-DAD and T-DAD had a massive dynamic behavior during their emplacement, characterized by a lack of selective depositional process. The SM-DAD originatedfrom a "dry" inertial granularflow in which high-energy grain-grain interaction prevailed. Abundantfines and fluids within the mass before the collapse could have enhanced its mobility. In fact, the water content in the flowing mass promoted positive pore-water pressure that reduced the frictional energy dissipation enhancing flow mobility. Both DADs show partially broken crystals/particles, percussion marks and fractures of different intensity that are the result of punctual, rapid and high-energy impacts. In general, the observed microscopic features suggest that the particle-particle interaction occurred in a collisional regime characterized by grains subjected to short, high velocity impacts, while evidence of frictional interaction is less common than that of the collisional type.

Palavras-chave : debris avalanche deposit; microtextures; scanning electron microscopy; emplacement mechanisms; Colima Volcano; Mexico.