Description du sujet de thèse

Characterization of geothermal anomalies in the faults of the axial zone of the Eastern Pyrenees (Andorra, Tech, and Alt Urgell-Cerdanya)

Contexte de travail

This thesis is part of the European cross-border project PIROS, led by the University of Barcelona. The aim is to promote research and innovation in the energy transition by assessing, integrating, and valorizing geothermal energy as a renewable energy source in the cross-border mountain areas between Spain, Andorra, and France. The PIROS project seeks to consolidate knowledge and the potential of geothermal resources in order to contribute to the widespread use of geothermal energy, energy sovereignty, and climate change mitigation in the Pyrenees. Specifically, this study will evaluate the geothermal potential of the study area by characterizing recent and current geothermal anomalies in relation to tectonic structures and hydrothermal sources in Andorra and the Tech Valley.

The thesis will be jointly supervised by the University of Montpellier and the University of Barcelona, and will be primarily based at the Géosciences Montpellier laboratory. Working meetings will be held with the University of Barcelona and the University of Paris Saclay. Field mapping and sampling will be carried out in collaboration with Spanish and Parisian PhD students, and analyses (petrography, geochemistry, geochronology) will be conducted at the University of Montpellier.

Thesis objectives :

The objective of the thesis is to determine where, when, and at what temperature hydrothermal circulations occurred in relation to tectonic activity, fault structures, and different geological reservoirs. The thesis will focus on three main hydrothermal systems associated with the faults of Andorra, Tech, and the Spanish Cerdanya, including all hydrothermal resurgence and recharge zones in the surrounding massifs. It will aim to establish how these hydrothermal systems have moved through time and space and to characterize areas with high geothermal potential. This work will be conducted in international collaboration with the University of Barcelona, the University of Paris Saclay, Andorra Recerca i Innovació, the Institut Cartogràfic de Catalunya, and the Consell Comarcal de l'Alt Urgell. The proposed thesis will be conducted at Géosciences Montpellier, in thematic continuity with three other theses focusing on the same study area:

A thesis on geological controls and fracturing of current hydrothermal fluid pathways and isotopic analyses of paleo-circulations (University of Barcelona).
A thesis on hydrological and geochemical analysis of waters to determine their origin, pathways, and reservoir temperatures (University of Paris Saclay).
A thesis on the acquisition of geophysical data for 3D modeling of the hydrothermal reservoir (University of Barcelona - ICGC).

Methods :

- Structural analysis of faults and alterations: Fine-scale structural mapping of fault zones, reservoir unit juxtapositions, and alterations will provide an initial geometric characterization of surface and subsurface features, essential for the spatio-temporal understanding of the hydrothermal system and sampling strategy. Petrological characterization will help constrain the mineralogies, parageneses, and textures of hydrothermal alteration in the collected samples.

- Geo-thermochronology and geochemistry: Due to current erosion in the mountains of the study area, low-temperature thermochronology will allow mapping of recent thermal anomalies. Geochemical analyses will characterize the processes associated with fluid-rock interactions within the faults and interpret thermochronological data in terms of thermal perturbations and tectonic control. Furthermore, geochronology will infer the timing and polyphase activity of faults and hydrothermal alterations, which is crucial for assessing the sustainability of geothermal resources. These data will be coupled and compared with current resurgence data and hydrodynamic and thermal models, allowing characterization of the differences between recent and current thermal perturbations.

- Modeling: Numerical modeling of thermochronological data (AER, QTQt-HeFTy, Pecube) will reconstruct the thermo-kinematic history of regional massifs, excluding samples affected by hydrothermalism. This part may also include hydrodynamic and thermal modeling of current flows (Comsol) based on a 3D model constrained by geophysical data in an ancillary project component. These models will be coupled and compared with mapping data, alterations, and current resurgence data, providing an evolutionary perspective on tectonic and geothermal systems.

Required Skills :

The candidate must hold a Master's degree in geosciences. The required skills are those of general geology, as the subject is multidisciplinary. A background and at least an interest in structural geology (faults and basement), geochronology, endogenous petrology, geochemistry, and geothermics are expected. Even if some of these topics are not well mastered, the candidate will be able to develop them during the thesis work.

Contraintes et risques

The first year will require numerous field missions in mountainous areas, particularly for geological mapping and rock sampling.