Widespread extension in the core of the western Alps revealed by earthquake analysis

The western Alps are an active collision belt whose current stress field is inhomogeneous [Muller et al., 1992]. We report new seismological data whic h significantly improve our knowledge of this stress field. About 1600 eart hquakes which occurred in the western Alps during the last 10 years were pr ecisely located, and 79 new focal solutions were computed. The analysis of this database shows that widespread extension affects all the internal zone s of the belt. To better constrain the associated stress regime, six stress tensors were computed using the Gephart and Forsyth [1984] method. They sh ow that the current tectonics of the western Alps are contrasted with close variation in the stress regime (transpression to the front of the belt con trasting with extension in the care of the belt). The extensional direction is radial to the arcuate geometry of the belt and bounded outboard by the former thrust of the internal zones onto the external zone, suggesting exte nsional reactivation of this inherited crustal discontinuity. Such widespre ad extension within the inner part of an actually ongoing collision belt ca nnot be explained by simple collision-related tectonics. We propose that in trabelt buoyancy forces, such as those produced by a slab retreat or slab b reak-off, interfere with the boundary forces driven by the ongoing Europe-A frica convergence.