Neogene to ongoing normal faulting in the inner western Alps: A major evolution of the late alpine tectonics

[1] Widespread brittle extension occurs in the internal zones of the southwestern Alps, forming a dense fault network, which overprints the compressional structures. The extension has been followed by transcurrent motions. In terms of paleostress this extension is radial to the belt close to the Crustal Penninic Front and multidirectional in the eastern part of the internal zones. The paleostress field of the strike-slip phase is coherent with the extensional one and compatible with dextral shear along longitudinal faults. Globally, we propose that the internal zones of the southwestern Alps underwent a single Neogene transtensive tectonic regime. To strengthen our structural results, the current tectonics has been analyzed using seismotectonic tools. This approach allowed the recognition of several active faults. Moreover, the ongoing tectonics is coherent with the observed brittle deformation. The fault network controls the present-day seismicity. The combined seismotectonic and structural approaches give coherency and continuity from the Neogene to ongoing extensional tectonics in the southwestern Alps. In the framework of the whole western Alps a synthesis of our results with neotectonic- related data allowed us to provide an accurate map of the Neogene to present kinematics of the belt. Extension, which appears as a major feature of the internal zones, has been coeval with the propagation of thrusts in the outer zones during Neogene times. The geodynamic processes, which rule this major tectonic evolution of the belt, remain a matter of debate.