Analyses of the stress field in southeastern France from earthquake focal mechanisms

Abstract

Due to the apparent deformation field heterogeneity, the stress regimes around the Provence block, from the fronts of the Massif Central and Alpine range up to the Ligurian Sea, were not well defined. To improve the understanding of the SE France stress field, we determine new earthquake focal mechanisms and we compute the present-day stress states by inversion of the 89 available focal mechanisms around the Provence domain, including the 17 new ones calculated in the current study. This study provides evidence of 6 different deformation domains around the Provence block with different tectonic regimes. On a regional scale, we identify three zones characterised by significantly different stress regimes: a western one affected by an extensional stress (normal faulting) regime, a southeastern one characterised by a compressional stress (reverse to strike-slip faulting) regime with NNW- to WNW-trending σ1 and a northeastern one, i.e., the Digne nappe front, marked by an NE-trending compression. Note that the Digne nappe back domain is controlled by an extensional regime that is deforming the western alpine core. This extensional regime could be a response to buoyancy forces related to the Alpine high topography. The stress regimes in the southeast of the Argentera Massif and around the Durance fault are consistent with a coherent NNW-trending σ1 that implies a left-lateral component of the active reverse oblique-slip of the Moyenne Durance Fault. In the Rhone Valley, an E-trending extension characterises the tectonic regime that implies a normal component of the present-day Nîmes fault displacement. This study provides evidence for short-scale variation of the stress states that reflect abrupt change in the boundary force influences on upper crustal fragments (blocks). These spatial stress changes around the Provence block result from the coeval influence of forces applied at both its extremities, i.e., in the north-east, the Alpine front push and in the southeast, the northward African plate drift. Besides these boundary forces, the influence of the mantle plume under the Massif Central can be superimposed along the western block boundary.