Fig. 2

Schematic structure of a folded mountain chain, from Heim (1918). According to Heim, M is the general level of the Earth’s surface, approximately at sea level, both before and after folding. Sch is the base of the part of the crust which will become folded—a shear zone or transition zone which will separate the folded part of the crust from underlying part which will not be affected. R is the thickness of that part of the crust which will be affected by the folding. In the Jura mountains, R is 1,000–2,000 m; in the central zone of the Alps, R is around 15 km (according to Argand, Heim notes). After the folding and the establishment of isostatic equilibrium, T is the greatest depth of the shear zone under the original surface, and H is the reconstructed height of the folded rocks above the original surface (now eroded away); hence T + H is the total folded thickness F. tF = F − R is height amplitude of the folding, or the purely tectonic fold height. A is the amount of erosion and O = H − A is the remaining orographic height of the mountain range. S = T − R is the isostatic subsidence