Post-depositional fracturing and subsidence of pumice flow deposits: Lascar Volcano, Chile

Unconsolidated pyroclastic flow deposits of the 1993 eruption of Lascar Volcano, Chile, have, with time, become increasingly dissected by a network of deeply penetrating fractures. The fracture network comprises orthogonal sets of decimeter-wide linear voids that form a pseudo-polygonal grid visible on the deposit surface. In this work, we combine shallow surface geophysical imaging tools with remote sensing observations and direct field measurements of the deposit to investigate these fractures and their underlying causal mechanisms. Based on ground penetrating radar images, the fractures are observed to have propagated to depths of up to 10 m. In addition, orbiting radar interferometry shows that deposit subsidence of up to 1 cm/year occurred between 1993 and 1996 with continued subsidence occurring at a slower rate thereafter. In situ measurements show that 1 m below the surface, the 1993 deposits remain 5°C to 15°C hotter, 18 years after emplacement, than adjacent deposits. Based on the observed subsidence as well as estimated cooling rates, the fractures are inferred to be the combined result of deaeration, thermal contraction, and sedimentary compaction in the months to years following deposition. Significant environmental factors, including regional earthquakes in 1995 and 2007, accelerated settling at punctuated moments in time. The spatially variable fracture pattern relates to surface slope and lithofacies variations as well as substrate lithology. Similar fractures have been reported in other ignimbrites but are generally exposed only in cross section and are often attributed to formation by external forces. Here we suggest that such interpretations should be invoked with caution, and deformation including post-emplacement subsidence and fracturing of loosely packed ash-rich deposits in the months to years postemplacement is a process inherent in the settling of pyroclastic material

Comparative analysis of the impact of geological activity on astronomical sites of the Canary Islands, Hawaii and Chile

An analysis of the impact of seismic and volcanic activity was carried out at selected astronomical sites, namely the observatories of El Teide (Tenerife, Canary Islands), Roque de los Muchachos (La Palma, Canary Islands), Mauna Kea (Hawaii) and Paranal (Chile) and the candidate site of Cerro Ventarrones (Chile). Hazard associated to volcanic activity is low or negligible at all sites, whereas seismic hazard is very high in Chile and Hawaii. The lowest geological hazard in both seismic and volcanic activity was found at Roque de los Muchachos observatory, in the island of La Palma.Comment: 12 pages and 11 figure

Géodynamique andine : résumés étendus = Andean geodynamics : extended abstracts = Geodinamica andina : resumenes expandidos

Dans la région étudiée, le très volumineux volcanisme ignimbritique Oligocène supérieur-Miocène inférieur, est daté entre 25 et 19 Ma. A l'ouest (Formation Oxaya) les ignimbrites sont distales et peu déformées. A l'est (Formation Lupica) elles sont interstratifiées avec des laves et des sédiments alluviaux et lacustres, et sont fortement déformées. Les grandes différences dans la stratigraphie et le degré de déformation de ces séries contemporaines reflètent une différence dans l'environnement de dépôt et dans la contrainte tectonique postérieurement au dépôt. La forte affinité shoshonitique, en plus de l'abundance de la sanidine, suggèrent une origine d'arrière-arc pour les ignimbrites. (Résumé d'auteur