Renewed definition of nappe boundaries within the Gran Paradiso (Western Alps): implications for its tectonic evolution

1 p.The Gran Paradiso massif essentially consists of coarse-grained orthogneisses, derived from Permian granitoids (Bertrand et al., 2005; Ring et al., 2005), and various types of paragneisses. Detailed mapping of the northern part of the Gran Paradiso in the Cogne and Valsavarenche valleys has revealed the occurrence of three types of monocyclic sequences. The first one, previously identified by Compagnoni et al. (1974), consists of monogenic (quartz) metaconglomerates with interbedded graphite-bearing micaschists. The second one displays graphite-bearing, polygenic metaconglomerates of probable Carboniferous age. A third sequence is characterized by ankerite-bearing quartzites and micaschists, associated to some metabasites, and assumed to be Permian in age. The monocyclic sequences are used as tools for deciphering the 3D internal structure of the Gran Paradiso massif. The lowert tectonic element, the Money Unit, contains the first and second types of monocyclic sequences, which are folded together during the second episode of deformation (D2). The upper tectonic element, the Gran Paradiso Unit, can be divided into two subunits by the third monocyclic sequence. In conjunction with an improved understanding of the kinematic (Le Bayon & Ballèvre, 2006) and metamorphic (Le Bayon et al., 2006) evolution of the Gran Paradiso, this will allow to draw a clearer picture of the evolution of the Western Alps, from the subduction of the continental palaeomargin to the stacking of crustal slices during collision