CHARACTERIZATION OF STRESS PERTURBATIONS NEAR MAJOR FAULT ZONES - INSIGHTS FROM 2-D DISTINCT-ELEMENT NUMERICAL MODELING AND FIELD STUDIES (JURA MOUNTAINS)

We consider and discuss the presence of discontinuities in the crust a s a major source of stress perturbations. Based on 2-D distinct-elemen t modelling, we reconstruct the local stress field around a vertical d iscontinuity in various geological contexts. The resulting stress dist ribution reveals that major directional stress changes occur near the tips of the discontinuity so that stress deviations can reach values a s large as 50 degrees. We establish simple relationships controlling s tress changes around a pre-existing fault zone as a function of(1) the remote differential stress magnitude, (sigma(1) - sigma(3)), (2) the friction coefficient on the discontinuity, and (3) the strike of the d iscontinuity relative to the far-field stress. As a geological example , we present the Morez Fault Zone in the internal Jura. Paleostress re construction in forty-two sites indicates that the trends of the Mio-P liocene compression are N110 degrees on average near the fault, wherea s they are N130 degrees in the surrounding areas. A comparison between the results of the tectonic study and those of theoretical modelling suggests that the 20 degrees counterclockwise deviation is directly re lated to the reactivation of this large weak zone. We thus evaluate th e role of mechanical decoupling along pre-existing zones of weakness, especially with consideration to the accommodation of the Alpine defor mation in the Jura belt. (C) 1997 Elsevier Science Ltd.