The NW external Alpine Thrust Belt and its implications for the geometry of the Western Alpine Orogen

The NW external Alpine Thrust Belt contains an imbricated basement-cover sequence which is broadly continuous with the French cratonic foreland. This thrust belt developed in the late stages of Alpine orogenesis and carried the previously deformed and metamorphosed internal zones to the WNW, piggy-back fashion, onto the French Craton. Balanced cross-sections restore the thrust-belt structures to lie for over 100 km to the ESE of their present, innermost outcrop trace. The regional sole thrust commonly lies in the upper crust. To acheive a balance between different crustal levels requires a layer of lower crust, on which the external zones originally lay, to underlie the entire present outcrop width of the Alps. Therefore models of Alpine orogenesis, which require the internal zones to be underlain by their deep crustal root, are not valid and new models are required to explain the sub-surface geometry of the NW Alps. This paper stresses the importance of resolving the external parts of orogens, particularly using balanced cross-sections, before attempting to either interpret the internal parts of orogenic belts or make palaeogeographical reconstructions.