Orogen-parallel brittle extension as a major tectonic imprint in the Neogene evolution of the south-western Alpine arc

Abstract

We present a new analysis of the late Alpine brittle deformation in the southern branch of the western Alpine arc, focusing on the stack of internal metamorphic nappes east of the Argentera external crystalline massif. The regional-scale fault network is dominated by a NW–SE-striking right-lateral fault system that follows the general curvature of the arc and controls the overall morphology of the area. A second fault set strikes N–S and is mainly represented by normal faults which accommodate orogen-parallel extension. Structural analysis and paleostress tensors derived from inversion of fault-slip data reveal a complex pattern of deformation involving extensional and strike-slip deformation events. The orogen-parallel extension previously described in the internal zones at the east of the Pelvoux massif is confirmed further south, and we show that it is combined with right-lateral strike-slip deformation that increases in intensity towards the south-west. The stability of the minimum stress axis (σ3) direction suggests that extensional and transcurrent regimes are contemporaneous and highlights regional interferences between inner brittle extension, parallel to the strike of the belt, and the strike-slip strain field driven by the counterclockwise rotation of the Apulia–Adria plate. The curved geometry of the belt constrains the direction of extension and the coupling relationship between the internal and external Alpine zones. We propose that the Neogene tectonic history is a result of the unique curved tectonic architecture of the south-western Alps with respect to the rotation of the Apulia–Adria indenting plate. The southern tip of the western Alpine arc represents a transitional zone between extension in the inner chain and strike-slip/compression in the outer parts.