High pressure (HP) to ultra high pressure (UHP) rocks in modern orogenic belts have been interpreted as part of subducted oceanic or continental plates (Ernst 1999, Eide and Liou 2000, Guillot et al. 2003). The exhumation of subducted lithosphere requires a mechanically weak zone at the interface between the subduction plane and the rigid overlying mantle peridotites with a viscosity greater than 1020 Pa.s (Hirth and Kohlstedt 2003). At shallow depths (<40-50 km) blueschists are exhumed in the accretionary wedge along the interface between the subducting plate and the overriding plate (Platt 1993). Hydrated sediments at the base of the accretionary wedge can develop a corner flow due to their low viscosity (<1019 Pa.s; Cloos and Schreeve 1988), which facilitates the exhumation of blueschists during active subduction of the oceanic plate (Platt 1986). This is the case for the Franciscan complex in California (Platt 1986, Cloos and Schreve 1988), the northern Carribean complex in Cuba and the Dominican Republic (Goncalvez et al. 2000, Garcia-Casco et al. 2002) and the External Piemont zone (Schistes Lustrés units) in the Western Alps (Schwartz 2001, Agard et al. 2002).

At greater depth, an accretionary wedge is pinched out and the abundance of sediments significantly decreases, yet the exhumation of eclogitic rocks is rapid (Duchêne et al. 1997). Therefore, a different mechanism is necessary for the exhumation of eclogitic rocks. Proposed processes for the exhumation of HP to UHP rocks vary depending on the tectonic settings. They may be classified into three end-members: (1) Partial melt provides a low viscosity layer in high temperature continental subduction, such as western Norway (Labrousse et al. 2002) and Dabie Shan in China (Hacker et al. 2000); (2) Serpentinites and/or phengite-bearing rocks play the role of mechanically weak layer in cool continental subduction, such as the Tso Morari in Himalaya and Dora Maira in the Alps (Guillot et al. 2001, de Sigoyer et al. 2004; Chopin and Schertl 1999), their exhumation likely assisted by the low density of eclogitized continental rocks (Chemenda et al. 1996), or (3) exhumation involves eclogitized oceanic or arc-related mafic rocks, a phenomenon only documented in few places, such as the Alpine belt of Cuba (Zaza zone) (Auzende et al. 2002, Garcia-Casco et al. 2002) and the Kohistan arc in Pakistan (Le Fort et al. 1997). Rare occurrences of eclogitized mafic rocks are not surprising considering the high density of eclogitized mafic rocks (3500 kg.m-3) compared to the mantle peridotites (3300 kg.m-3). The negative density difference requires boundary forces to compensate the gravity force.

Eclogites are commonly accompanied by serpentinites. Some serpentinites are considered to have been exposed on the oceanic floor before the subduction (Scambelluri et al. 1995, Li et al. 2004). The second type of serpentinite is hydrated peridotites at the base of mantle wedges (Guillot et al. 2000, 2001). These serpentinites may replace the role of hydrated sediments at greater depths and act as the lubricant and producing a return flow for the exhumation of eclogitic rocks. This proposal is supported by the systematic occurrence of serpentinites spatially associated with eclogitic rocks derived from oceanic crust in the Alps (Hermann et al. 2000, Schwartz et al. 2000, Li et al. 2004), in Cuba (Auzende et al. 2002, Garcia-Casco et al. 2002 ) and in Kohistan (Le Fort et al. 1997).

The aim of this paper is to describe the occurrence of serpentinites and eclogites in the Monviso massif (Western Alps), the origin of serpentinites, and the geometry and mechanisms for the exhumation of the eclogitized oceanic crust during active subduction.