Etude d'une résurgence active dans la caldera de Siwi (Tanna, Vanuatu) : le système Yenkahe-Yasur

Resurgence, defined as the post-collapse uplift of the caldera floor, is widespread phenomenon worldwide but it is still poorly understood. This work is a multidisciplinary study of a resurgent dome: the Yenkahe dome, located inside the Siwi caldera, in Vanuatu. The relevance of this dome is multiple: firstly, resurgence is currently active, secondly it is fast, so the associated structures are well-preserved, and lastly, the Yenkahe dome presents the originality to be associated with a volcanic cone which has been permanently active for at least several hundred years: the Yasur volcano. A primary tectonic study based on field observations, satellite images and available low-resolution digital elevation models brought the evidence of a two-stage (at least) dome growth history. The first stage is a vertical growth that produced the longitudinal graben on top of the dome. It was followed, in the second stage, by an eastward displacement of the deformation source, generating an uplift of the eastern dome relative to the western dome. The Yenkahe structural map was then refined through the computation of a high-resolution photogrammetric digital surface model (DSM). Besides the great number of details allowing precise fault mapping, this DSM highlights the presence of numerous destabilization scars associated with the evolution of the dome. In the future, similar destabilization events may produce tsunamis reaching inhabited areas in a couple of minutes. The characteristics of the long-term deformation source of the Yenkahe dome, and of other resurgent domes with a similar morphology, were investigated by analogue modeling (intrusion of silicone putty in a sand-plaster mixture). The results of the study show the width of the graben created by an elongated intrusion source mostly depends on the depth of this source. The source of the Yenkahe dome, presumably a magmatic intrusion, would be approximately one-kilometer deep. A second study, based on a punching process in a numerically modeled elastoplastic medium, shows a different internal structure for the dome. However, it confirms the order of magnitude obtained for the source depth (1-2 km), making this result more robust. The presumed magmatic source may be connected to the Yasurvolcano, exhibiting an open conduit activity for several hundred years, which would imply an incremental growth governed by transient over-pressurizing events at depth (such as magma injections). Lastly, a panel of geophysical methods were carried out within the caldera (magnetics, gravimetry, electrical methods, etc.). They revealed that the post-collapse history of Siwi involved, not only resurgence tectonics, but also the emplacement of lava fields and of several volcanic edifices. They also demonstrate the presence of an extended hydrothermal system, particularly deep (more than 300 m) and active on the eastern side of the dome. The associated extended alteration may favor the destabilization of the resurgent edifice.La résurgence, définie comme le soulèvement du plancher des calderas postérieurement à leur effondrement, est largement répandue mais encore très mal comprise. L'objectif de cette thèse est l'étude pluridisciplinaire d'un dôme résurgent: le Yenkahe, au sein de la caldera de Siwi au Vanuatu. L'intérêt de ce dôme est multiple. D'une part, la résurgence est active, d'autre part, elle est très rapide donc elle est associée à des structures relativement préservées. Enfin le Yenkahe présente l'originalité d'être associé à un cône volcanique en éruption permanente depuis au moins plusieurs centaines d'années: le Yasur. Une première étude tectonique basée sur des observations de terrain, complétées par l'exploitation d'images satellites et de modèles numériques de terrain à basse résolution existants, a permis de mettre en évidence une histoire en deux temps (au moins) de la croissance du dôme résurgent. Une première phase de croissance verticale engendrant un graben longitudinal aurait été suivie par une seconde phase de déplacement vers l'est de la source de la déformation, occasionnant un soulèvement de la partie est du dôme par rapport à la partie ouest. Ces grands traits structuraux ont pu être raffinés, par la suite, par le calcul d'un modèle numérique de surface à haute résolution issu de la photogrammétrie. Outre le haut niveau de détails permettant une cartographie plus fine des failles, ce modèle a apporté la mise en évidence de nombreuses traces de déstabilisations sur les flancs du Yenkahe. Dans l'avenir, de telles déstabilisations pourraient engendrer des tsunamis atteignant des zones habitées en quelques minutes. Les caractéristiques de la source de déformation à long terme du Yenkahe, et d'autres dômes résurgents présentant une morphologie similaire, ont fait l'objet d'une étude en modélisation analogique (intrusion de silicone dans un mélange de sable-plâtre). Les résultats de cette étude montrent que la taille du graben engendré en surface par l'intrusion d'une source allongée dépend surtout de la profondeur de cette source. On tire de cette étude que l'intrusion associée au Yenkahe, supposée magmatique, serait située aux alentours d'un kilomètre sous la surface.Une seconde étude en modélisation numérique, basée sur un processus de poinçonnement dans un milieu élastoplastique, met en avant une géométrie interne de dôme différente mais confirme l'ordre de grandeur obtenu pour la profondeur de source (1-2 km), rendant le résultat plus robuste. Cette source magmatique est peut-être connectée au Yasur, qui montre une activité de dégazage en conduit ouvert depuis plusieurs centaines d'années. Ceci qui impliquerait un mode de croissance incrémental gouverné par des événements de surpression transitoires (tels que des injections magmatiques). Enfin, un ensemble de méthodes géophysiques appliquées à l'étude de la caldera de Siwi (gravimétrie, mesures magnétiques et électriques, etc.) révèle que l'histoire post-effondrement de cette caldera comporte, outre la résurgence tectonique, des événements effusifs et la construction de plusieurs édifices volcaniques successifs. Ces méthodes mettent également en évidence la présence d'un système hydrothermal très étendu, particulièrement profond (plus de 300 m) et actif à l'est du dôme. L'altération hydrothermale associée pourrait favoriser des déstabilisations de l'édifice résurgent

Structure of an active volcano associated with a resurgent block inferred from thermal mapping: The Yasur–Yenkahe volcanic complex (Vanuatu)

International audienceSubsurface thermal measurements provide a valuable tool to map hydrothermal-fluid release zones in active volcanic areas. On the Yasur–Yenkahe volcanic complex (Tanna Island, Vanuatu archipelago), fumaroles and hot springs abound, signs of upraising heat fluxes associated to a well-developed hydrothermal activity. Combination of high resolution mapping of ground thermal anomalies with geomorphological analysis allows the characterization of the structural relationships between the active Yasur volcano and the Yenkahe resurgent block.A complex system of heat release and hydrothermal fluid circulation below the Yasur–Yenkahe complex is evidenced. Circulation, though propagating vertically as a whole, is funneled by stratification. Thus, the main thermal fluid released is almost exclusively concentrated along structural limits that break the seals induced by the stratified nature of the ground. Three types of medium/high temperature anomalies have been evidenced: (1) broad hydrothermalized areas linked with planar stratification that favor lateral spreading, (2) linear segments that represent active faults, and (3) arcuate segments related to paleo-crater rims. The limit between the Yasur volcano and the Yenkahe resurgent block is characterized by an active fault system accommodating both the rapid uplift of the Yenkahe block and the overloading induced by the volcano weight. In such a setting, faults converge below the cone of Yasur, which acts as a focus for the faults. Evidence of such structures, sometimes hidden in the landscape but detected by thermal measurements, is critical for risk assessment of flank landslides

Fluid circulation pattern inside La Soufrière volcano (Guadeloupe) inferred from combined electrical resistivity tomography, self-potential, soil temperature and diffuse degassing measurements

International audienceAfter a drastic decline in 1983, hydrothermal activity at La Soufrière lava dome (Guadeloupe, Lesser Antilles) has been progressively increasing in the summit area since 1992, raising the threat of a renewed eruptive activity. To better constrain the geometry of the hydrothermal system, an extensive high-resolution self-potential survey was performed on the dome and three multi-method profiles combining electrical resistivity tomography, self-potential, ground thermometry and soil CO 2 diffuse degassing measurements were carried out to cover its southern periphery in January 2011. Results indicate that hydrothermal ascending flows are currently restricted to a proximal area including the dome and its very vicinity. The extension of hydrothermal alteration inferred from electrical resistivity tomography reflects the presence of a heat source just below the dome. A first-order correction of topography-related self-potential variations allows the identification of major hydrothermal fluid circulations pathways, as well as significant meteoric infiltration zones. Local shifting of hydrothermal fluids towards the dome periphery is favored by the presence of major axes. The regional La Ty fault appears as the major axis draining large volumes of hydrothermal and magmatic fluids. However hydrothermal activity remains confined inside a collapse structure surrounding the dome, that formed in the last 9000 years as a result of recurrent edifice collapses, the latest occurring at the onset of the 1530 AD eruption. The combination of these qualitative results with structural analysis leads to a synthetic model of magmatic and hydrothermal fluids circulation inside the dome, which may be useful for the assessment of potential hazards associated with a renewal of fluid pres-surization, and a possibly associated partial flank-failure

Estimation of magma depth for resurgent domes: An experimental approach

International audiencePost-collapse resurgence is a phenomenon affecting many calderas. Attributed to a renewed magma rise, the process is still poorly understood and the associated source parameters remain poorly constrained. A set of experiments has been conducted to gain insight into the structural evolution of caldera resurgent domes. A sand–plaster mixture was chosen as an analogue for the brittle pile of volcanic rocks, and silicone putty simulates the ductile behavior of the intruding magma. Resurgence is driven by the vertical uplift of the silicone, with variable shape and depth. Similarity conditions are achieved through eight dimensionless numbers, which are of the same order of magnitude in both nature and experiments. Results show that extension due to doming is, in many cases, accommodated by one axial graben. Opposite master faults of this graben intersect at depth at the junction with the rising viscous silicone. The simplicity of the geometry of the whole analogue system provides equations which allow the estimation of the silicone depth from surface parameters. These equations are then used in some field examples to assess the magma depth beneath natural resurgent domes

DAMAGE AND STRAIN LOCALIZATION AROUND A PRESSURIZED SHALLOW-LEVEL MAGMA RESERVOIR

International audienceStructures developing above long‐term growing shallow‐level magma reservoirs, such as resurgent domes, may contain information on the reservoir itself. To understand the formation of such tectonic features, we have investigated the deformation process around a shallow pressurized magma reservoir embedded in a damaging elastic volcanic edifice. Our model allows evidencing the effect of the progressive damage in producing the fault pattern associated to tectonic surface deformation. Damage is first isotropic around the cavity and constitutes a damaged zone. Then the free‐surface effect appears and an anisotropic shear strain develops from the boundary of the damaged zone; it localizes on reverse faults that propagate upward to the surface. When the surface deformation is sufficient, normal faulting appears. Finally, the complete structure shows an undeformed wedge above the damaged zone. This structure is similar to those found by analogue modelling and from field geologic observations. From this model, we found a relation to estimate the reservoir radius and depth from the graben and dome widths. From limit analysis, we deduced an analytical expression of the magma reservoir pressure which provides a better understanding of the magma pressure build‐up during doming. The dip of reverse faults limiting the dome can be inferred from the minimal pressure required to rupture the crust around the reservoir. Finally, the magma reservoir overpressure, the dip of the faults, the reservoir depth and the damaged zone radius is inferred from three parameters: the ratio ρR computed from the dome and graben widths, the cohesion and the friction angle

A structural outline of the Yenkahe volcanic resurgent dome (Tanna Island, Vanuatu Arc, South Pacific)

International audienceA structural study has been conducted on the resurgent Yenkahe dome (5 km long by 3 km wide) located in the heart of the Siwi caldera of Tanna Island (Vanuatu arc, south Pacific). This spectacular resurgent dome hosts a small caldera and a very active strombolian cinder cone - the Yasur volcano - in the west and exhibits an intriguing graben in its central part. Detailed mapping and structural observations make it possible to unravel the volcano-tectonic history of the dome. It is shown that, following the early formation of a resurgent dome in the west, a complex collapse (caldera plus graben) occurred and this was associated with the recent uplift of the eastern part of the present dome. Eastward migration of the underlying magma related to regional tectonics is proposed to explain this evolution

DAMAGE AND STRAIN LOCALIZATION AROUND A PRESSURIZED SHALLOW-LEVEL MAGMA RESERVOIR

International audienceStructures developing above long‐term growing shallow‐level magma reservoirs, such as resurgent domes, may contain information on the reservoir itself. To understand the formation of such tectonic features, we have investigated the deformation process around a shallow pressurized magma reservoir embedded in a damaging elastic volcanic edifice. Our model allows evidencing the effect of the progressive damage in producing the fault pattern associated to tectonic surface deformation. Damage is first isotropic around the cavity and constitutes a damaged zone. Then the free‐surface effect appears and an anisotropic shear strain develops from the boundary of the damaged zone; it localizes on reverse faults that propagate upward to the surface. When the surface deformation is sufficient, normal faulting appears. Finally, the complete structure shows an undeformed wedge above the damaged zone. This structure is similar to those found by analogue modelling and from field geologic observations. From this model, we found a relation to estimate the reservoir radius and depth from the graben and dome widths. From limit analysis, we deduced an analytical expression of the magma reservoir pressure which provides a better understanding of the magma pressure build‐up during doming. The dip of reverse faults limiting the dome can be inferred from the minimal pressure required to rupture the crust around the reservoir. Finally, the magma reservoir overpressure, the dip of the faults, the reservoir depth and the damaged zone radius is inferred from three parameters: the ratio ρR computed from the dome and graben widths, the cohesion and the friction angle

Constraints on the source of resurgent doming inferred from analogue and numerical modeling – Implications on the current feeding system of the Yenkahe dome-Yasur volcano complex (Vanuatu).

International audienceResurgence, defined as the post-collapse long-term uplift of a caldera floor, is commonly attributed to a renewed rise of magma. The Yenkahe dome (Vanuatu) exhibits a common morphology - elongated with a graben on top - among resurgent domes, and is also one of the most active structures of the kind. In this study, we performed a joint analysis based on analogue and finite element numerical models to (1) constrain the width and depth of the long-term deformation intrusive source of the Yenkahe dome and (2) discuss the close association between the Yenkahe dome and the active Yasur cone. We consider the resurgent deformation at the surface to be driven by the uplift of a magma reservoir roof in depth. As the edifice deformation response depends on the medium and the source properties, the mechanical behavior of the upper crust and the nature of the source are modeled using two very different sets of hypotheses. Analogue modeling uses silicone putty, an analogue for a large viscous magma body, intruding a sand-plaster mixture reproducing a Mohr-Coulomb behavior for the crust. Numerical models consider the vertical displacement of a rigid indenter, allowing the conservation of a flat-shaped roof, into an elastoplastic material. Numerical and analogue models show different resurgent dome structures at depth but similar dome and graben morphologies in the surface. Inverse faults - or equivalent shearing zones - delimiting the dome provide an explanation for the confined nature of resurgent doming and the persistent volcanic activity on the dome border represented by the Yasur volcano. Analogue and numerical models together provide an estimation range of 1-1.8 km for the intrusive deformation source depth, and 1.3-2 km for its width. The proposed association between the Yenkahe dome and the Yasur volcano is compatible with such a shallow depth of the magma reservoir, and argues for a discontinuous resurgence process. (C) 2015 Elsevier B.V. All rights reserved

Localization of magma injections, hydrothermal alteration, and deformation in a volcanic detachment (Piton des Neiges, La Réunion)

(IF 2,217)International audienceThis contribution aims at understanding how magmatism, hydrothermal alteration, and deformation may have interacted to localize a detachment (a low-angle normal fault) in a basaltic volcano. Piton des Neiges, an inactive volcano of La Réunion Island, has been deeply cut by erosion, allowing its inner structure to be investigated. The deepest unit observed in the edifice is a kilometer-scale plutonic complex, the top of which is intruded by multiple sills. This zone of repeated sill intrusions has been interpreted as a detachment because it displays evidence of hydrothermal alteration in the greenschist facies linked to a brittle-ductile shear deformation. Deformation begins with cataclasis and is followed by mylonitization and chlorite crystallization, then by hydrofracturing and pumpellyite crystallization. Subsequent and post-deformation calcite crystallization occurs in voids such as fractures and vacuoles. Aluminium substitutions in chlorite suggest that the syn-deformation hydrothermal alteration did not exceed 250 °C and peaked in the deformation zone. Comparison of bulk-rock major element analyses of fresh, altered and deformed rocks shows that the zone of sill intrusion and deformation localized increased concentrations of P and K otherwise depleted in the footwall and hangingwall rocks, suggesting that the detachment acted as a trap for fluids.In contradiction with proposed models of volcano spreading, it is apparent that the portion of Piton des Neiges accessible to observation did not deform by creep of a large hydrothermal system or a plutonic complex below its solidus. Instead, the interface between the already cooled plutonic complex and the host rock acted as a brittle failure zone and was repeatedly intruded by magma injections. This localized heat source promoted hydrothermal alteration and low temperature creep in and around the discontinuity. The same process of magmatism-related weakening might occur on active volcanoes; it may, for instance, explain the slow post-eruptive deformation of the Eastern flank of Piton de La Fournaise (the active volcano of La Réunion) observed since the April 2007 eruption

Influence of rainfalls on heat and steam fluxes of fumarolic zones: Six months records along the Ty fault (Soufrière of Guadeloupe, Lesser Antilles)

International audienceFumarolic zones are permeable areas where both steam and heat are expelled to the atmosphere. Surface fluxes and flows, which are representative of the intensity of the hydrothermal circulation in depth, can be monitored by thermometers, thermal infrared cameras, spectrometers, or condensers. However, the superficial activity of fumarolic zones can be modified by the meteorological conditions, in particular the rainfalls, which might result in erroneous estimations. From this perspective, we developed a set of physical equations to quantify the effects of rainfalls on the thermal behavior of fumarolic zones. Results were faced to continuous measurements achieved at the Ty fault fumarolic zone (La Soufrière volcano, Guadeloupe, Lesser Antilles) during six months in 2010, using six vertical series of thermometers measuring the heat transfer in the ground and one condenser measuring the rising steam flux. Results demonstrate that in the absence of rainfalls, heat and steam flux reach an equilibrium that is representa tive of the geothermal flux in depth. Conversely, after the rainfalls, the cooling of the ground provokes a deepening of the condensation level. The related soil temperature drop can be estimated by computing the heat required to warm the infiltrated water up to boiling temperature while the recovery rate is directly linked to the geothermal flux. Our observations allow defining in which conditions flux are at steady state, but also to build a first order numer ical model allowing estimating both the physical parameters of the ground (thermal conductivity, precipitation efficiency coefficient and surface flux constant) and the long term thermal behavior of the hydrothermal system. In particular, our results predict that the hydrothermal activity must vanish on the zones where the geothermal flux drops under a certain threshold (60 W/m 2 at La Soufrière). The existence of this limit may have strong implications for the precipitation rate of minerals and the possible reactivation of the fumarolic zones during volcanic crises