Microseismicity of the Bearn range: Reactivation of inversion and collision structures at the northern edge of the Iberian plate

The Bearn range, located to the north of the Axial Zone in the Western Pyrenees, is affected by numerous small-magnitude seismic events. These events overlap an area characterized by specific geological structures which are interpreted to have resulted from multistage extensional and compressional deformation. An analysis of surface geology draped over digital elevation model, together with field investigations, allow identification of two main shortening episodes with differing direction of contraction: D1 represents the inversion of the North Pyrenean Basin, whose Mesozoic infill was detached from highly extended crust and transported southward over the necking zone and the northern margin of the Iberian plate; D2 corresponds to the collision stage and is characterized in the study area by backfolding and backthrusting deformation coeval with uplift in the axial part of the chain due to thickening of the Iberian plate. The microseismicity appears to concentrate along the basal part of the inverted basin units (D1) where this initially low angle thrust has been tilted and steepened during collision (D2). We propose that local steepening of this ancient inversion structure, which should not be named North Pyrenean Fault, provided the suitable dip for extensional solicitation in association with the present uplift of the Axial Zone, whatever the driving mechanism of this uplift could be.