Tectonic evolution and related fracturing in the Causses Basin (Aveyron, France) : the Tournemire area example

The Institute for Nuclear Safety and Protection (IPSN) launched the « Tournemire » program, in 1988. One of its aims is to understand and characterize the fluid transfer processes in argillaceous rocks. They are interesting rocks for the long-term storage of nuclear waste. To this purpose, the IPSN installed an experimental site in a tunnel, which gives access to a 250 m-thick Toarcian and Domerian shale unit near Tournemire (Aveyron, France) (fig. 1). The fluids, in this type of rock with very low intrinsic permeability, 10−14 m/s [Boisson et al., 1998], used to flow (calcite crystallizations in fractures), and still flow, principally in the fractures [Barbreau et Boisson, 1993 ; Boisson, 1995] formed during the tectonic history of the formation. In order to constrain the history of fluid flow in the formation, it was necessary to characterize the tectonic fracturing and to identify the tectonic events responsible, on the one hand, for the apparition of the fractures and, on the other hand, for their eventual reactivation. The method used was a microtectonic and kinematic analysis. The studied area belongs to the western border of the Causses basin, a Permian-Mesozoic basin trending N-S. The slightly monoclinal series in this area range from the Trias, discordant westward on the Permian formations of the St-Affrique basin, to the lower Kimmerigian locally present on the Larzac plateau (fig. 1). The upper Liassic shales (Domerian, Toarcian) are between two limestone and dolomite formations. Two major (regional-scale) ESE-WNW reverse faults, the Cernon fault and the St-Jean-d’Alcapies fault, cross the area. Their offsets may reach several hundred meters. These two faults limit an ESE-WNW trending block where the experimental site is located. The tectonic fractures in the area result from two main tectonic phases. Phase 1, extensional, occurred during the Mesozoic and comprises three episodes (fig. 2). The first episode, characterised by an E-W extension (fig. 3), did not produce significant structures in the Toarcian shales. The second episode, with a NW-SE extension direction (fig. 4 and fig. 5), occurred during the diagenesis of the shales. It led to the development of calcareous nodules. These nodules are considered to be « mode I » fractures formed in association with fluid expulsion during the sediment compaction (fig. 4). The last episode has a N-S direction, (fig. 7) and is probably late Jurassic in age [Macquar, 1973 ; Blès et al., 1989 ; Martin et Bergerat, 1996]. It produced E-W conjugate normal faults (fig. 6) and two perpendicular sets of extensional joints trending E-W and N-S. The second major tectonic phase corresponds to the « pyrenean » compression. The σ1 directions vary from NE-SW to NW-SE, with two major pulses striking N020-N030 and N160-N170 (fig. 2, fig. 9 and fig. 10). The N020-N030 direction corresponds to the paroxysm of the « pyrenean » phase, dated as late Middle Eocene [Arthaud et Laurent., 1995]. It reactivated major faults and formed associated folds (fig. 8). Numerous fractures due to the N160-170 compressional event are concentrated principally in the center of the block bordered by the ESE-WNW major faults (fig. 2). Chronological criteria indicate that the direction of compression rotated counter-clockwise during the « pyrenean » compressional phase (fig. 11). A third compression direction (N130) has been evidenced, for example, by N030 trending tension gashes cross-cut by N130 trending gashes (fig. 12). The significance of this last tectonic event is unclear. It is mainly observed in the west drift of the experimental site (fig. 1C), and could result of the re-orientation of the stresses at this site close to an important shear zone. Three sets of joints, trending N020, N160 and N090 (fig. 13 and fig. 14) have been recognized. The joints are classically extensional fractures that propagate perpendicular to the minimum principal stress σ3 [Endelger, 1985 ; Pollard et Aydin, 1988 ; Rives, 1992]. In strike-slip regimes, such fractures strike parallel to the maximum principal stress σ1. The average N020, N160 and N090 joints thus very probably are created respectively during the N020 pyrenean strike-slip event, N160 strike-slip event and N-S Mesozoic extension. The established chronology between the different compressional episodes involves the reactivation of the N020 and N160 fractures may have caused only senestral strike slip. However, the presence of dextral strike slip on some vertical planes trending N-S, not associated with conjugate planes but with E-W vertical planes indicates their origin is not related to the « Pyrenean » phase. Consequently, we assumed that a set of N-S joints created during the extensive phase, in the same time as the E-W joints. An elasticity theory model gives an account of field observations on this type of fractures. The model proposed by Caputo [1995] describes the geometry of networks, of joints as a function of the tectonic regime (fig. 15). Two coeval sets of joints form under the same tectonic event. For an extensive stress state, the two sets are orthogonal to each other. Under strike slip regimes, two orthogonal sets form but one of the two sets forms horizontally (parallel to the bedding planes when the stratification is horizontal). The mechanism involves a stress permutation between σ3 and σ2 in the vicinity of the first fracture zone at the moment of failure. The network of orthogonal joints (N-S and E-W) appeared under the N-S extensive event. We show two sets of joints with the same orientation formed at two different ages (fig. 16). Their differentiation was possible with the chronology of the deformation, which was determined by the microtectonic analysis. The pre-existing fractures, originated before the « pyrenean » phase, necessarily influenced the expression and the distribution of the fractures associated with the « pyrenean » phase. These pre-existing fractures must be taken into account to understand and constrain the fluid circulations in the Toarcian shales.