Approche hydrogéochimique et structurale des circulations dans un réservoir du domaine alpin (massif d'Allevard, France) Hydrogeochimical and structural investigations of groundwater flows in an alpin reservoir (massif of Allevard, France)

Résumé

Les caractéristiques hydrogéochimiques des réservoirs profonds du domaine alpin occidental sont abordées par le biais du système hydrominéral d'Allevard (France). Le volume des données et la variabilité des caractéristiques qui en découle, permettent de considérer cette source comme un exemple représentatif des sources thermominérales du domaine alpin occidental. L'étude combinée de ces données relatives à l'hydrochimie, à la géochimie isotopique, à la présence de gaz et d'éléments traces, confrontées au contexte fissural et structural des réservoirs, permet une approche cohérente des circulations profondes.

On propose en conclusion un modèle qui pourrait être généralisé aux systèmes thermominéraux situés dans un contexte structural comparable (front de chevauchement des massifs cristallins externes; front de chevauchement briançonnais) et présentant un faciès semblable (eaux chloro-sulfatées). Ce modèle tient compte de mélanges entre eaux d'origine superficielle et/ou semi-profondes et d'eaux d'origine profonde.

Abstract

Investigations of the hydrogeochemical and structural characteristics of groundwater flows in an Alpine hydrothermal reservoir (Allevard Massif, French Western Alps) are carried out in order to define the origin and genesis of thermal and mineral waters.

The geological formations constituting the Allevard Massif consist of a Hercynian basement and a continuous sedimentary cover from Carboniferous to Middle Jurassic. The basement is composed mainly of micaschists, the sedimentary cover of Paleozoic clastic deposits : Triassic formations (evaporites, dolomites, limestones); Liassic calcareous marls, schistics marls and Limestones. A groundwater outlet (Allevard Spring) from this reservoir is located in the calcareous terrains of Lias.

The basement-cover contact is a major N40 subvertical fault. Faulting tectonics and subsequent rock fracturing is important for the hydrogeology of the area under study since the geological formations are initially impermeable (except for the Triassic terrains). Observations of underground galleries in the neighbourhood of the spring showed the presence of water outflows located on N40 strike-slip faults affecting both the basement and the Liassic terrains. The role of faulting tectonics in groundwater occurrence is indeed clearly emphasized by the position of the Allevard Spring at the intersection of two major faults with N60 and N140 trends.

Available data cover nearly a hundred years (1888-1930) and emphasize the great variability of the chemical features of the spring water. This is related to the tact that the water outflowing from the spring is a mixture in varying proportions of waters from different sources. Combined interpretation of the structural characteristics, isotopical (³H; ¹⁸O; ²H; ³⁴S) and chemico-physical (major and trace elements, gases, temperature, ...) data provides the basis of a working assumption on a flow and mixing model in which three different hydrogeological subsystems were distinguished : a surface subsystem comprising cold infiltration waters of a bicarbonate-calcium and a sulfate-calcium type; a subsurface subsystem constituted by the fissured Liassic calcareous terrains comprising part of the infiltrated waters which intermingle with much warmer waters of a sodium-chloride type originating from a subsystem located at a great depth. By means of geochemical indicators, a depth of nearly 4000 m has been estimated.

Such a model, in agreement with geological evidence and reported by some recent works showing that halite deposits are probable at great depths, could be generalized to other hydrothermal systems occurring within the framework of the French Western Alps in a comparable structural context (thrust front of the external metamorphic massifs; "Briançonnais" thrust front).