Soil gas profiles as a tool to characterise active tectonic areas: the Jaut Pass example (Pyrenees, France)

A new method to investigate active tectonic structures, using soil gas composition at faults, suggests relevant information about regional stress conditions which can be obtained rapidly and at relatively low cost. In 1995, we carried out geochemical profiles around the epicentre of the M=5.1 1980 earthquake near Arudy in the French Pyrenees, where the presence of minor fractures is evident in the field, and confirmed using satellite SPOT imagery. Fractures are conduits facilitating fluid migrations in the crust, and are also pathways for the release of deep-seated gases to the atmosphere. In order to investigate the implication of these fractures in the present deformation, i.e. if they are connected to the Hercynian substratum at a depth of about 1800 m, soil gases were measured along four traverses crossing the observed structures. Gases determined were 222Rn, CO2 and 4He, each of them for their characteristic source: 222Rn has essentially a shallow origin due to its short half-life, whereas CO2 is the major soil gas component with a mainly surficial biogenic source. However, there could additionally be CO2 from crustal or mantle degassing, which would also be the principal sources of He. Data analysis clearly reveals anomalous values for each gas at specific positions along the traverses. Two sets of fractures corresponding to different observed trends are distinguished: the one characterised by He anomalies accompanied by other gases, and the second with few identified He anomalies. The agreement between the geochemical data and the field observations leads us to propose a deformation model for the area studied, analogous to a pull-apart system located in a right lateral shear zone.