The large scale stress patterns observed in intraplate area is generally considered to result from far-field boundary forces that drive plate tectonics. However, no present day deformation has been detected in the Paris Basin, yet significant deviatoric stresses are measured in limestone formations observed above soft argillite layers encountered in this region. Further, the pore pressure measured in the argillite is larger than that measured in the surrounding permeable zones. These observations suggest a presently active source of stress in this sedimentary system. We propose that this stress is not related to tectonics but to pressure solution effects activated by pore pressure transients. These transients develop in the natural fracture system that affects limestone formations and are linked to climatic variations. They involve periods that range from thousands to hundreds of thousands years. This mechanism generates time-dependent shear stresses in soft formations and explains overpressures observed in the very low permeability argillite layer. Effects of pressure solution may be modeled by different visco-elastic behaviors for the various formations. It outlines the influence of time dependent material properties on the present day stress field. These results imply that an understanding of the viscoelastic properties of sedimentary formations is necessary for extrapolating measured surface deformations to basement rocks in domains of very slow tectonics.
The French agency responsible for the management of nuclear waste (ANDRA) has conducted very detailed stress measurements near Bure, a small village located in the eastern sedimentary Paris Basin (Cornet and Rockel, 2012; Wildeveau at al., 2007). A set of 42 hydraulic tests have been conducted at depths ranging from 375 to 703 meters below ground surface, in 7 different boreholes located at the tips of a roughly triangular area with sides equal to about 10 km (figure 1). They have been used to characterize the regional stress field within the Callovo-Oxfordian argillite as well as in the underlying and overlying limestone formations. In addition, orientations of borehole breakout observed in eight different boreholes distributed throughout the area of interest have been determined. Finally the horizontal deformation of a 500 m deep, vertical shaft with a 4.5 m diameter has been monitored for three different levels in the Oxfordian limestone.
