L'analyse microstructurale a permis de démontrer que depuis le Pliocène la série du Cézallier est soumise à un régime tectonique distensif caractérisé par un axe d'extension N-S et que l'hydrothermalisme est synchrone de cette tectonique distensive. L'analyse de la genèse des conduits révèle que la répartition spatiale des fractures naturelles ouvertes qui drainent les fluides est déterminée par l'état de contrainte qui règne dans le bâti.
Mots-clés : Extension tectonique, Pliocène, Quaternaire, Fracture ouverte, Contrainte, Contrôle tectonique, Hydrodynamique, Eau thermale, Puy-de-Dôme, Cantal, Haute-Loire (Cézallier).
Based upon the Chassole cored borehole, the fractures pattern of the Cezallier orthogneissic basement has been studied from a geometrical (relations between fractures), kinematic and chronological point of view, with an emphasis on brittle faults and apertures. Stress tensors have been computed.
These studies show that during the Plioquaternary period, the Cezallier domain is a distensive area in a general compressive context. The average minor principal stress during this period is N-S.
Normal faults apertures, relationships between hydrothermal minerals deposited at different times together with the chronology of these deposits reveal the following features :
- calcite and siderite infillings are contemporaneous with the Plioquaternary NS distensive phase;
- the underground liquid flows are directly governed by the opened fractures pattern;
- the fractures apertures are directly related to the NS distensive phase;
- the widest and longest apertures are associated with the fractures which lie parallel to the tensile σ 1-σ 2 plane. The strike of these fractures varies from N 60 °E to N 130 °E. We consider that they constitute the major drains for underground flow.
Dernière mise à jour le 08.02.2019