In the light of new data such as seismicity (Sue and Tricart, 2003, Delacou et al., 2004), Geodesy (Sue et al., 2000, Calais et al., 2001) and geological observations (Lazarre et al., 1996, Bistacchi and Massironi, 2000, Sue and Tricart, 2002, Agard et al., 2003, Sue and Tricart, 2003, Champagnac et al., 2004, Grosjean et al., 2004, Malusa, 2004, Tricart et al., 2004, Schwartz et al., 2005), a stimulating discussion has started on the significance of extensional structures developed in the Western Alpine belt since the Neogene. In accordance with the present day stress field (Calais et al., 2001, Delacou et al. 2004), brittle extension perpendicular to the mountain belt is recorded (Tricart et al., 2001, Tricart et al., 2004, Sue and Tricart, 2003, Malusa, 2004, Agard et al., 2002, Champagnac et al., 2004, Grosjean et al., 2004, Schwartz et al., 2005). An older stress state is characterized by extension direction parallel to the orogenic alpine belt and shortening direction vertical or perpendicular to the belt (Bistaccchi et Massironi, 2000, Champagnac et al., submitted). Ages and duration of the two brittle tectonic phases are poorly documented. Time constraints on the brittle tectonic evolution of the alpine belt is an important challenge in order (1) to better understand how orogen parallel extension switched to extension perpendicular to the mountain belt and (2) to highlight the relationships between the brittle tectonic phases and the formation of the internal basement domes.

In this paper, we present new brittle microtectonic data from the High Maurienne valley near Modane between the basement domes of Vanoise and Ambin (Figure 1). We then discuss these data with respect to available geochronological data, brittle micro-tectonics dataset and mapping of the main faults of the Vanoise area. Finally, we propose an alternative model for the formation of the basement domes of the internal Alps.