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

Abstract

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.

Extensional neotectonics around the bend of the Western/Central Alps: an overview

“…The depth of the hypocenters and the geometry of the CPF and thus the 3D-geometry of the Briançonnais zone, suggest that the seismicity along these three active faults is restricted to the Briançonnais units; in others words, seismicity remains located above the CPF as already shown to the North along the ECORS profile (see cross-section Fig. 2 a, Sue and Tricart 2003). Focal mechanisms indicate mainly normal faulting, with focal planes compatible with the directions of the seismic alignments.…”

“…In the Briançonnais zone, just east of the CPF, seismicity occurs in NNW-SSE elongated clusters, following the arcuate shape of the CPF. This seismic activity correlates with the fault network mapped at the surface, and particularly with a family of longitudinal faults described by Sue and Tricart (2003). Three major faults observed in the field seem to be seismically active: the high-durance fault (HDF), the Serennes-Bersézio fault (SBF), and the East-Briançonnais fault (EBF) (Fig.…”

“…2). At first, extension was considered a local phenomenon, resulting from secondary tectonic complexities within a collisional system; however, with increasing data density extension became a first-order characteristic of Alpine neotectonics Sue and Tricart 2003;Delacou et al 2004). Indeed, the Western Alps reflect two contrasting tectonic regimes, with a more or less important extension in the core of the belt and localized compression in outer areas.…”

“…The seismicity localized to the SE of the Pelvoux massif, along the southern part of the so-called Briançon-nais seismic arc, is limited to the upper crust with hypocenters not deeper than some 15 km (Rothé 1941;Sue and Tricart 2003) (Fig. 3).…”

Quantification of strain rate in the Western Alps using geodesy: comparisons with seismotectonics

“…5). This transcurrent dextral fault system runs from the Aiguilles Rouges strike-slip fault, possibly connected north-eastward with the dextral seismic alignment of the northern Valais (Deichmann et al 2002), towards the southern front of the Belledonne massif (Thouvenot et al 2003) up to the High-Durance and Bersézio/Argentera faults Southeastward (Sue et al 1999;Sue & Tricart 2003).…”