Widespread post-nappe normal faulting in the Internal Western Alps:a new constraint on arc dynamics

Abstract

Abstract: Field data collected in the Briançonnais and Piémont zones at the centre of the western Alpine arc demonstrate a high density of normal faults. Normal displacements are pervasive from the millimetric to the kilometric scales. They occur both parallel and at high angles to the arc, overprinting all the Alpine folds and thrusts and the associated schistosities. The orientation of fault striae implies that the pile of metamorphic nappes SE of the Pelvoux massif has undergone a general extension. Extension tends to be multidirectional in the Briançonnais zone. Orogen-parallel extension prevails in most of the Piémont Schistes lustrés. To the west, in Briançonnais zone, brittle extension is associated with a reversal of movement along the Briançonnais Frontal Thrust that started before the end of the Oligocene. In the Piémont Schistes lustrés, the onset of normal faulting occurred early in the Miocene and continued the ductile extension that accompanied the exhumation of HP–LT metamorphic thrust sheets during the Palaeogene. In both zones, extension remains currently active, resulting in the regional seismicity. Since more than 20 Ma ago, this normal faulting in the internal arc coexists with an outward propagation of folds and thrusts in the external arc. We discuss its possible significance in the context to the dynamic evolution of the Alps.