POSTPLIOCENE STRESS-FIELD AND PRESENT-DAY STRAINS IN THE SOUTHWESTERNALPS

A detailed analysis of 16 new earthquake mechanisms, followed by a syn thesis of all available focal solutions in the southwestern Alps, allo ws to characterize the various present-day deformation regimes within the pre-Triassic basement of this region. We define 4 areas of deforma tion, 3 of which are mainly compressional and one of which is extensio nal (Haute-Ubaye region). The use of microtectonic data, which provide s information about stress state in the sedimentary cover, and the ana lysis of P-axes and T-axes directions of the focal solutions, allow to discuss the present-day stress field in this region. We show the vari ation of stress field directions which are NE-SW in the Brianconnais u nits, N-S in the Digne region and NW-SE in the south south-east of the Argentera massif and in the Ligurian basin. On the basis of these ana lyses we discuss the present-day kinematic relationships between the w estern Alps and the Ligurian Basin, in the framework of the NW-SE conv ergence between the European and African plates. We then examine wheth er or not previously published late Tertiary geodynamic models are sti ll appropriate for the present-day tectonics in the southwestern Alps.