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L'évolution de la majorité des volcans basaltiques est marquée par des phénomènes récurrents d'instabilité latérale. De nombreux facteurs d'instabilité, impliqués dans des déstabilisations à long terme ou des effondrements de flancs instantanés, ont été recensés depuis l'événement majeur qui a frappé le Mont Saint-Helens en 1980. Cependant, le rôle de ces facteurs sur la stabilité mécanique des édifices est mal contraint dans la mesure où les glissements de flancs résultent en général de plusieurs causes simultanées. Notre étude se concentre sur une comparaison des caractéristiques morphologiques et structurales de deux systèmes basaltiques, La Réunion (Océan Indien, France) et Stromboli (Mer Tyrrhénienne, Italie). Nous avons montré que, bien qu'ayant des volumes et des contextes géodynamiques très différents, les systèmes sont tous deux caractérisés par une activité intrusive intense le long de rift zones et ont subi des déstabilisations latérales récurrentes durant leur évolution. Parmi les facteurs d'instabilité, les exemples de La Réunion et de Stromboli soulignent l'influence majeure des complexes intrusifs dans la croissance et le démantèlement des volcans, comme le prouvent les études de terrain et la surveillance des ces volcans actifs. Les modèles classiques considèrent que le processus d'instabilité latérale en domaine volcanique résulte de la mise en place d'une ou plusieurs intrusions verticales, entrainant des mouvements de flancs le long d'une surface de glissement pré-existante. De nouvelles données de terrain obtenues au Piton des Neiges (La Réunion), ainsi que des données de littérature sur d'autres édifices, ont permis de mettre en évidence le rôle des intrusions sub-horizontales dans les déstabilisations de flancs et de caractériser la géométrie des intrusions sub-verticales et sub-horizontales au sein des volcans basaltiques. Cette étude compare les résultats de la modélisation numérique des champs de déplacements de surface crées par la mise en place d'intrusions magmatiques à faible / fort pendage dans les édifices basaltiques, grâce à une méthode d'éléments frontières mixte (Mixed Boundary Element Method), dans le but de déterminer le comportement mécanique d'un édifice soumis à des injections magmatiques sous différents champs de contraintes. Les résultats de cette étude montrent qu'un champ de contraintes anisotrope favorise le glissement le long des intrusions, généré par la contrainte cisaillante co-intrusive, à l'origine de déplacements à l'échelle du flanc de l'édifice. Ces déplacements de grande ampleur, préférentiellement liés à des intrusions subhorizontales, peuvent probablement déclencher des grands glissements latéraux si leur amplitude dépasse le seuil de stabilité de l'édifice. L'application des résultats théoriques à des exemples réels de déformations enregistrées sur des volcans basaltiques (dont La Réunion et Stromboli, au cours de leurs crises éruptives de 2007) révèle que le modèle de déstabilisation associée à des intrusions sub-verticales est un mécanisme pouvant générer des effondrements de flancs sur des petits édifices à fortes pentes comme Stromboli. De plus, nos données de terrain et les résultats de modélisation confirment l'importance des intrusions sub-horizontales dans l'évolution morpho-structurale des grands édifices basaltiques à faibles pentes comme le Piton de la Fournaise (La Réunion), l'Etna ou le Kilauea, et plus particulièrement dans les instabilités de flancs pouvant causer des tsunamis dévastateurs. / Most basaltic volcanoes are affected by recurrent lateral instabilities during their evolution. Numerous factors have been shown to be involved in the process of flank destabilization occurring over long periods of time or by instantaneous failures. However, the role of these factors on the mechanical behaviour and stability of volcanic edifices is poorly-constrained as lateral failure usually results from the combined effects of several parameters. Our study focuses on the morphological and structural comparison of two end-member basaltic systems, La Reunion (Indian ocean, France) and Stromboli (southern Tyrrhenian sea, Italy). We showed that despite major differences on their volumes and geodynamic settings, both systems present some similarities as they are characterized by an intense intrusive activity along well-developed rift zones and recurrent phenomena of flank collapse during their evolution. Among the factors of instability, the examples of la Reunion and Stromboli evidence the major contribution of intrusive complexes to volcano growth and destruction as attested by field observations and the monitoring of these active volcanoes. Classical models consider the relationship between vertical intrusions of magma and flank movements along a preexisting sliding surface. A set of published and new field data from Piton des Neiges volcano (La Reunion) allowed us to recognize the role of subhorizontal intrusions in the process of flank instability and to characterize the geometry of both subvertical and subhorizontal intrusions within basaltic edifices. This study compares the results of numerical modelling of the displacements associated with high-angle and low-angle intrusions within basaltic volcanoes. We use a Mixed Boundary Element Method to investigate the mechanical response of an edifice to the injection of magmatic intrusions in different stress fields. Our results indicate that the anisotropy of the stress field favours the slip along the intrusions due to cointrusive shear stress, generating flank-scale displacements of the edifice, especially in the case of subhorizontal intrusions, capable of triggering large-scale flank collapses on basaltic volcanoes. Applications of our theoretical results to real cases of flank displacements on basaltic volcanoes (such as the 2007 eruptive crisis at La Reunion and Stromboli) revealed that the previous model of subvertical intrusions-related collapse is a likely mechanism affecting small-scale steeply-sloping basaltic volcanoes like Stromboli. Furthermore, our field study combined to modelling results confirms the importance of shallow-dipping intrusions in the morpho-structural evolution of large gently-sloping basaltic volcanoes like Piton de la Fournaise, Etna and Kilauea, with particular regards to flank instability, which can cause catastrophic tsunamis.

Most basaltic volcanoes are affected by recurrent lateral instabilities during their evolution. Numerous factors have been shown to be involved in the process of flank destabilization occurring over long periods of time or by instantaneous failures. However, the role of these factors on the mechanical behaviour and stability of volcanic edifices is poorly-constrained as lateral failure usually results from the combined effects of several parameters. Our study focuses on the morphological and structural comparison of two end-member basaltic systems, La Reunion (Indian ocean, France) and Stromboli (southern Tyrrhenian sea, Italy). We showed that despite major differences on their volumes and geodynamic settings, both systems present some similarities as they are characterized by an intense intrusive activity along well-developed rift zones and recurrent phenomena of flank collapse during their evolution. Among the factors of instability, the examples of la Reunion and Stromboli evidence the major contribution of intrusive complexes to volcano growth and destruction as attested by field observations and the monitoring of these active volcanoes. Classical models consider the relationship between vertical intrusions of magma and flank movements along a preexisting sliding surface. A set of published and new field data from Piton des Neiges volcano (La Reunion) allowed us to recognize the role of subhorizontal intrusions in the process of flank instability and to characterize the geometry of both subvertical and subhorizontal intrusions within basaltic edifices. This study compares the results of numerical modelling of the displacements associated with high-angle and low-angle intrusions within basaltic volcanoes. We use a Mixed Boundary Element Method to investigate the mechanical response of an edifice to the injection of magmatic intrusions in different stress fields. Our results indicate that the anisotropy of the stress field favours the slip along the intrusions due to cointrusive shear stress, generating flank-scale displacements of the edifice, especially in the case of subhorizontal intrusions, capable of triggering large-scale flank collapses on basaltic volcanoes. Applications of our theoretical results to real cases of flank displacements on basaltic volcanoes (such as the 2007 eruptive crisis at La Reunion and Stromboli) revealed that the previous model of subvertical intrusions-related collapse is a likely mechanism affecting small-scale steeply-sloping basaltic volcanoes like Stromboli. Furthermore, our field study combined to modelling results confirms the importance of shallow-dipping intrusions in the morpho-structural evolution of large gently-sloping basaltic volcanoes like Piton de la Fournaise, Etna and Kilauea, with particular regards to flank instability, which can cause catastrophic tsunamis.

On the volcanic ocean island Tenerife in the Canary archipelago, several giant flank collapses haveoccurred over the last one million years. The link between the lateral collapses and the resultingvolcanism in the rift zones is not fully understood and was investigated within the framework of thisproject. A field survey along the north eastern rift zone on Tenerife was performed, where dyke intrusionstructural data was collected to analyse the exact distribution of dykes in the area. To determine themagma ascent rate, diffusion analysis of major elements in olivine crystals was conducted incollaboration with ETH Zürich. Additionally, analogue gelatine model experiments were performed toimprove the understanding of the processes that reconfigure linear rift zones to triaxial or multiaxial riftzones. Injections of fluid into the gelatine ridges produced fractures which mainly were linear andfollowed the along strike direction. Position A’ in the centre of the unstable section, produced the mostdiverse injection patterns and had a roughly 30 % higher acceleration compared to position A, positionedat the border of the unstable section. From the results of the diffusion modelling of Mg in olivinephenocrysts an ascent rate of the magma was calculated to an average ascent velocity of 10.4–14.0 km/h.This is extremely fast and suggests that the Orotava collapse is strongly connected to the formation ofankaramite dykes in the area as a result of sudden decrease in the lithostatic pressure. / På vulkanön Teneriffa i den kanariska övärlden har flera jättelika skred skett. Kopplingen mellan dessaskred och vulkanismen i sprickzonerna på ön har undersökts inom ramen för detta arbete. Enfältundersökning genomfördes längs den nordöstra sprickzonen på Teneriffa, där data ommagmagångarnas orientering och läge insamlades. För att bestämma hur snabbt magman steg till ytan,genomfördes diffusionsanalys av Mg i olivinkristaller från ankaramitgångar i samarbete med ETH iZürich. Utöver det utfördes även experiment med analoga gelatinmodeller för att förbättra förståelsenför de processer som förändrar linjära riftzoner till treaxliga eller fleraxliga riftzoner. Injiceringen avvätskan i gelatinmodellerna producerade sprickor som i huvudsak var linjära och följde strykriktningenpå riftzonen. Position A’, i mitten av den instabila sektorn i modellen gav högst variation avinjiceringarna och hade omkring 30 % högre acceleration jämfört med position A placerad i kanten avden instabila sektorn. Baserat på resultatet av diffusionsmodelleringen kunde en uppstigningshastighetför magman beräknas till en medelhastighet på 10,4–14,0 km/h. Detta är extremt snabbt och visar attkollapsen av Orotava är starkt kopplad till bildningen av ankaramitgångar i området som ett resultat aven plötslig minskning av det litostatiska trycket.