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As to the emplacement mechanism of nappes, an eye-opening (for me) paper that I recommend to anyone trying to understand this process is Pfiffner, O.A., S.Ellis, and C.Beaumont. 2000. Collision tectonics in the Swiss Alps: Insight from geodynamic modeling. Tectonics 19(6): 1065-1094. The model for the emplacement of the Alpine nappes is that they originate within the subduction zone as long imbricated slices off the down-going Eurpoean foreland, and that they get stacked up in the orogen and uplifted through a combination of underplating of the nappe slices and erosion off the top. The numerical model, while is cannot reproduce the details of this process, shows quite clearly how material gets sheared off the down-going plate and piled up in, and uplifted through the orogen. It even gives suggestions of how large fold nappes could develop, although again, the model is coarser in scale than the details that we can now observe of the uplifted nappes. Thus the major displacement at the base of the nappes occurs at the subduction interface, although subsequent deformation occurs within the orogenic pile during uplift due to underplating of new nappes, and presumably due to some extent to gravitational collapse of the orogenic pile. Such a sequence can account for either thrust or fold nappes, presumably depending on the rheology of the material when it is transferred from the down-going to the over-riding plate; it can account for the large extent of nappes both parallel and perpendicular to the shear direction; it can account for the large displacements at the base of the nappes; and it can account for subsequent deformation of the nappes that has been mentioned by several contributors to this discussion. I think this is a very insightful paper and highly recommend it if readers have not already seen it. Dr. Pfiffner was kind enough to let us reproduce a couple of figures from that paper in the 2nd edition of our Structural Geology text (Twiss & Moores, 2007, W.H.Freeman & Co; Figures 20.20 A&B, 20.34). Cheers, rob twiss