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Themes of research of PSN team

Hydrothermal systems in active tectonics

Our main topic, hydrothermal systems in the context of active tectonics, is developed in a special location of the planet and workplace: the Himalayas of Nepal. This vertebral backbone of our work, with which numerous teams of the institute are associated, is coordinated by Frédéric Girault.

It is around our main topic and in our main workplace that the experimental, numerical and intellectual tools of the team are developed. The Himalayas of Nepal, indeed, propose a unique location of the planet where hydrothermal circulations, through networks of major crustal faults, provide pathways for massive emissions of carbon dioxide, produced by metamorphic reactions in the crust at a depth of a few kilometers where also the major Himalayan earthquakes are initiated, and this remarkable carbon dioxide, in addition, is characterized by a high concentration of radioactive radon gas. These emissions of carbon dioxide, with surface fluxes often comparable, and sometimes larger, than fluxes measured on the ground in active volcanic zones, have now been observed in several locations in Central Nepal, in several locations in Mid-Western Nepal, and, with smaller flux values, in several locations of Western and Far-West Nepal, which still remain largely unexplored regions. Thus, we have at our disposal surface sites which give access to vectors of internal activity of the deep Earth. Our work on the degassing Himalayan sites and their relations with the seismic activity intensified after the deadly Gorkha earthquake of 25April 2015.

Beyond the determination of the physical and chemical properties of these emissions, and their spatial mapping, the challenge for the coming years is to study the temporal variations and the understanding of the relatioship with the accumulation and release of tectonic strain. To make progress, it will be necessary to better assess the production and transport mechanisms of the carbon dioxide, through pre-existing or evolving fault networks, and to better characterize the crustal reservoirs, using geophysical observations such seismic surveys, the properties of the seismic waves, or electromagnetic soundings. In parallel, we will need to develop, through laboratory experiments and theoretical modelling, a better understanding of the effect of pressurized carbon dioxide reservoirs on various properties of the crust such as mechanical properties or electrical conductivity, and the interplay between carbon dioxide and other geological fluids.

Satellite Action 1: Geological gases in active volcanoes

This work allows us to integrate the methods and techniques developed in active volcanic zones, and, conversely, to test on such sites the concepts and techniques developed in our sites in Nepal. The site currently studied by Frédéric Girault, in collaboration in France with university Pierre et Marie Curie, and with the ISTerre team in Chambéry, is the Furnas volcano in the Azores. Other sites in the French territories are also considered. In addition, volcanic plumes are studied in collaboration with Guillaume Carazzo in IPGP.

Satellite Action 2: Modelling of gas flux and industrial applications

This action, coordinated by Frédéric Girault, aims at developing and testing general tools of numerical modelling of carbon dioxide transport and its radon signatures. This action is supported through contracts with the industry and relies on collaborations with institutes specialized in applied geophysics.

Satellite Action 3: Physics of underground cavities and application to the preservation of painted caves

This action, initiated in IPGP by Frédéric Perrier, takes place in the framework of a French group coordinated by François Bourges (GEConseil web site) who has been monitoring caves in France since 1995. This action relies on the study of concrete cases of painted caves, such as the Pech Merle cave, the Chauvet cave, or other instrumented sites, to develop a thermodynamical approach dedicated to underground sites, using the methods and concepts developed in Nepal in the main topic of the team. These underground cavities also offer a unique context to apply the methods of geomagnetism to various underground materials such deposits and speleothems (contact is Aude Isambert). In these sites, we also aim at developing, in collaboration with geomicrobiologists, novel approaches to study speleothems and the concretions found in hydrothermal systems.

Satellite Action 4: Anthropic matter fluxes and contaminated sites

This action, coordinated by Aude Isambert, aims at developing geophysical and geochemical tools to study signatures of human activities. Studies are currently in progress using sediments from the Seine river in the Paleomagnetism team of IPGP, in collaboration with Christine Franke (Mines Paris Tech). These studies potentially offer applications of the physics of natural sites applied to contaminated sites and rivers.

Satellite Action 5: Gas fluxes, aquifers and exchanges in the critical zone

This action, coordinated by Frédéric Girault, concentrates at near surface sites where we can study mechanisms of interaction between geological gases, biosphere, surface hydrology and near surface hydrogeology, using in particular a comprehensive spatial and temporal analysis of the radon signature. This work takes place in the framework of the Network of Drainage Basins, with sites currently studied in the Mont Lozère (drainage basin of the Sapine stream) or the drainage basin of Valescure in the Cévennes mountains.