Abstract

Over the last 15 years, several dozen deep boreholes (depths in excess of 500 m) have been drilled in the Maurienne Valley as part of the survey work for the new Lyons-Turin rail link. A number of parameters have been logged in these boreholes as a matter of routine, including data on fluids, such as temperature, electrical conductivity and vertical flow velocity. Because several logging campaigns have been carried out in each borehole, it has generally been possible to separate disturbances caused by the drilling (creation of links between water-bearing fractures, establishment of transitory thermal regimes, drilling-related heating, etc.) from the natural initial thermal state. Consequently, most natural thermal state descriptions are based on reconstructions, rather than on raw data. In many cases, in particular in the Houillère Briançonnaise Zone, this natural thermal state is governed by the circulation of hot and cold fluids. We suggest that these two types of circulation actually form a single circuit, in which cold waters infiltrating in the north-south trending Evaporite Zone and in the carbonates of the Sub-Briançonnais Zone flush deep waters so that they rise preferentially along the axis of the valley in the east-west trending Houillère Zone.

Focal mechanism data and leveling comparisons show that the front of the Houillère Zone (including the evaporites and the Sub-Briançonnais carbonates), which controls infiltration into the system, is currently subject to WNW-ESE extension. The rising hot waters are controlled by the valley, which is an extensional structure perpendicular to the Houillère Zone. This interpretation is in line with the increased heat flow towards the axis of the valley, and with the occurrence of rising thermal waters further down the valley, for example at Echaillon and Les Chavannes. The interaction between these two extensional systems, which has been observed at least on a regional scale, gives a highly dynamic character to the current deformation, as is shown by the new a-seismic tectonic regime that has affected the region since 1995. Given this dynamic character, it is unlikely that a hydraulic-thermal regime has existed undisturbed for more than 15,000 years.

The example described in the present article shows how the repeated logging of thermal data in boreholes over a period of time can help delineate deep circulations. Combined with detailed information about the current deformation regime, this logging data allows a detailed picture of circulation patterns to be drawn.

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