Continental subduction in the Western Alps: major issues.

Continental subduction is hereafter defined as continental, dominantly crustal, material recording high-pressure metamorphism (in most cases at blueschist- and eclogite-facies P-T conditions), without any prejudice upon pre-orogenic position and syn-orogenic burial mechanism. The main questions related to continental subduction in the Western Alps may be summarised as follows:

• In most models, continental subduction is succeeding oceanic subduction, the buoyant continental crust being dragged down by the subducting slab of oceanic lithosphere. Subduction erosion has also been proposed as a major mechanism (e.g. for the Sesia-Dent Blanche nappes). Do these models apply in the Western Alps, in the lack of a ‘true’ subduction zone?
• Which parts of the continental crust are subducted? Is it the entire palaeomargin, or just parts of it, for example extensional allochthons? What is the role of the inherited structures associated with the rifting history of the palaeomargin?
• What is the age of the HP/UHP metamorphism? Is it the same along as well as across the belt, or is this metamorphism diachronous? In the latter case, does this reflect the progressive burial of a single continental plate, or are there several continental domains separated by oceanic domains?
• What are the mechanisms for the exhumation of the HP-UHP rocks? Is erosion the driving force for exhumation? What is the record of erosion in the nearby sedimentary basins? Erosion may have been combined to other tectonic processes, like buoyant uprise of the HP-UHP bodies, associated or not with plate divergence, slab roll-back, … Collision may have reworked most of the evidence, but a careful analysis of the field evidence provides major clues.