PALEOSTRESS 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 (sigma(1)) in the same area, and examines the contrast between unperturbed areas (stable direction of s igma(1)) and perturbed areas (changing sigma(1) direction). We studied mesoscale structures on a 1000 m(2) continuous limestone exposure nea r a regional scale strike-slip fault. Local sigma(1) directions were d educed from a high concentration of minor strike-slip faults, extensio n fractures and stylolites formed during the Pyrenean shortening in La nguedoc, 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 st ylolites 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 tem poral changes in the orientation and intensity of the stress field pro duced by the nearby regional strike-slip faults. Our study suggests th at the origin of stress deviations or superimpositions cannot be expla ined by random measurements of sigma(1). It is essential to be able to synthesize the fault pattern and the stress trajectories which it det ermines, and to do this, a very high density or a selection of data fr om mesoscale structures is needed.