Palaeostress superimposition deduced from mesoscale structures in limestone: the Matelles exposure, Languedoc, France

This study investigates the reasons for the superimposition of several maximum principal stress directions (σ1) in the same area, and examines the contrast between unperturbed areas (stable direction of σ1) and perturbed areas (changing σ1 direction). We studied mesoscale structures on a 1000 m2 continuous limestone exposure near a regional scale strike-slip fault. Local σ1 directions were deduced from a high concentration of minor strike-slip faults, extension fractures and stylolites formed during the Pyrenean shortening in Languedoc, France. Most of the stylolites were formed in a stress field which was homogeneous on the exposure scale. This was followed by the reactivation of pre-existing extension fractures as strike-slip faults whose activity determined stress perturbations. A very heterogeneous stress field was produced leading to the formation of new localized stylolites and extension fractures, especially at fault terminations and at oversteps. Thus the final pattern shows the superimposition of all these structures. Reactivation of structures was caused by slight temporal changes in the orientation and intensity of the stress field produced by the nearby regional strike-slip faults. Our study suggests that the origin of stress deviations or superimpositions cannot be explained by random measurements of σ1. It is essential to be able to synthesize the fault pattern and the stress trajectories which it determines, and to do this, a very high density or a selection of data from mesoscale structures is needed.