Emplacement mechanisms of contrasting debris avalanches at Volcán Mombacho (Nicaragua), provided by structural and facies analysis - Université Clermont Auvergne Accéder directement au contenu
Article Dans Une Revue Bulletin of Voclanology Année : 2008

Emplacement mechanisms of contrasting debris avalanches at Volcán Mombacho (Nicaragua), provided by structural and facies analysis

Martin Pilato
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Résumé

We study the lithology, structure, and emplacement of two debris-avalanche deposits (DADs) with contrasting origins and materials from the Quaternary– Holocene Mombacho Volcano, Nicaragua. A clear comparison is possible because both DADs were emplaced onto similar nearly flat (3° slope) topography with no apparent barrier to transport. This lack of confinement allows us to study, in nature, the perfect case scenario of a freely spreading avalanche. In addition, there is good evidence that no substratum was incorporated in the events during flow, so facies changes are related only to internal dynamics. Mombacho shows evidence of at least three large flank collapses, producing the two well-preserved debris avalanches of this study; one on its northern flank, “Las Isletas,” directed northeast, and the other on its southern flank, “El Crater,” directed south. Other southeastern features indicate that the debris-avalanche corresponding to the third collapse (La Danta) occurred before Las Isletas and El Crater events. The materials involved in each event were similar, except in their alteration state and in the amount of substrata initially included in the collapse. While “El Crater” avalanche shows no signs of substratum involvement and has characteristics of a hydrothermal weakening-related collapse, the “Las Isletas” avalanche involves significant substratum and was generated by gravity spreading-related failure. The latter avalanche may have interacted with Lake Nicaragua during transport, in which case its run-out could have been modified. Through a detailed morphological and structural description of the Mombacho avalanches, we provide two contrasting examples of non-eruptive volcanic flank collapse. We show that, remarkably, even with two distinct collapse mechanisms, the debris avalanches developed the same gross stratigraphy of a coarse layer above a fine layer. This fine layer provided a low friction basal slide layer. Whereas DAD layering and the run-outs are roughly similar, the distribution of structures is different and related to lithology: Las Isletas has clear proximal faults replaced distally by inter-hummock depressions where basal unit zones are exhumed, whereas El Crater has faults throughout, but the basal layer is hidden in the distal zone. Hummocky forms depend on material type, with steep hummocks being formed of coherent lava units, and low hummocks by matrix-rich units. In both avalanches, extensional structures predominate; the upper layers exclusively underwent longitudinal and lateral extension. This is consistent with evidence of only small amounts of blockto- block interactions during bulk horizontal spreading. The base of the moving mass accommodated transport by large amounts of simple shear. We suggest that contractional structures and inter-block collisions seen in many other avalanches are artifacts related to topographic confinement.

Dates et versions

hal-00338760 , version 1 (14-11-2008)

Identifiants

Citer

Thomas Shea, Benjamin van Wyk de Vries, Martin Pilato. Emplacement mechanisms of contrasting debris avalanches at Volcán Mombacho (Nicaragua), provided by structural and facies analysis. Bulletin of Voclanology, 2008, 70 (8), pp.899-921. ⟨10.1007/s00445-007-0177-7⟩. ⟨hal-00338760⟩
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