Exemple d'approche pluridisciplinaire dans la caractérisation d'eaux thermales carbo-gazeuses A pluridisciplinary approach to carbonic and groundwater survey

Résumé

Une approche pluridisciplinaire est menée à propos d'eaux souterraines carbo-gazeuses : au niveau des relations entre tectonique active et hydrothermalisme; sur les origines des composants aqueux et gazeux (CO₂) par analyse de compositions isotopiques (²H, ¹⁸O, ³H et ¹³C) et mises en équation des équilibres carboniques.

Trois exemples sont traités dans te Sud-Est de la France : le premier en région de socle métamorphique (émergence thermale chaude); les deux autres en domaine de couverture carbonatée épaisse (source karstique littorale froide et aquifère karstique peu profond).

La complémentarité des informations acquises permet de préciser d'une part, le rôle de certaines directions fissurales dans les cheminements souterrains par rapport au contexte sismotectonique régional; d'autre part, la genèse des eaux et leur âge relatif; enfin l'origine du CO₂ qui peut se révéler mixte (biogénique-mantellique) ou infracrustal.

Se dégage de la sorte une méthodologie d'étude de ces eaux particulières qui mérite d'être plus largement développée.

Mots-clés:

• Géothermalisme,
• carbo-gazeux,
• sismotectonique,
• tectonique cassante,
• néotectonique,
• hydrochimie isotopique,
• équilibres carboniques,
• France

Abstract

Comparison of fissural, isotopic and hydrochemical data constitutes an approach to geothermal phenomena which has several consequences. The examples which are given are taken from a present regional tectonic context which is characteristic of a drawing close of the African and the European plates. Its maximum horizontal stress is of a N-S SE trend. Consequently the fissural trends EW to ENE - NSW existing on the three areas under survey would play no major role in groundwater flows and rising of deep C.O. Such a significant role would be probably played by the joining of NS to NW trends, which at present act in extension (N-S fractures) or with transverse movements.

They are found either at the gazeous and carbonic outlets (bare basement) or in the neighbourhood of these outlets (less than 300 m distant) located in the covering (thickness 300 m). It can thus be assumed that the fissuration is at present continuous between the covering and the basement in the N-S direction. This assumption is in agreement with the present stress field and the associated deformations. So all these data make it possible to emphasize on one hand the role of some fractural directions in groundwater flow according to the regional sismotectonic context, on the other the origins of the waters as well as their relative ages and lastly the origin of the CO₂ which can be mixed (biogenic and from the mantle) or infracrusted. Such an approach has several consequences connected with :

1) geothermal phenomena : the water temperature and the survey of the hydrothermal flows can be considered as a preliminary survey for deep local hydraulic investigation,

2) seismic risks : the fact that the dissolved gas (i.e. CO₂) derives its origin from the mantle is an indicator of such risks,

3) The survey of deep geological accidents in particular within deep sedimentary caver : a methodology for the study of this particular type of waters which can efficiently be applied elsewhere is thus created.

Keywords:

• Carbonic and gazeous,
• geothermal,
• neotectonics,
• isotopic,
• hydrochemistry,
• carbonic equilibrium,
• France