measurements in the profile of a folded layer (fig. 39A): orthogonal thickness, t a , and thickness parallel to the axial surface, T a , where a refers to the dip of a folded surface relative to the axial trace. Of more practical use in subsurface geology is the isochore thickness, IT a , which differs slightly from the T a thickness in all folds except true similar folds (fig. 39A). All three thickness measurements are equal along the axial trace in the profile plane. On the basis of the relation between the limb dip, or, and t' = ta/to or T' = T a /To, all classes of folds can be precisely classified (figs. 398 and 39C). Class 1B (parallel folds) and Class 2 (similar folds) are the most widely known classes, but are fairly rare in nature (Hudleston, 1973; Powell, 1974; Gray, 1979,
1981; Orozco and Galvez, 1979). Each half fold (quarter wavelength) can be plotted on these graphs, as depicted by curves It and It for the fold illustrated in figure 39A.
Certain fold profiles are characteristic of certain models of formation. Thus the mechanics of fold formation can be inferred from fold morphology in some cases (Ramsay, 1967, p. 366-372, 391-415). For example, consider folds E and m, which represent experimentally modeled domes analog ous to gneiss domes (stiff, buoyant layer) and salt domes (soft, buoyant layer), respectively (fig. 40). The t'/cr and T/a plots for each fold are shown in figure 39. They indicate that both are Class 1C folds. Accepting the consensus that Class 1C folds form
Figure 38. Percentage thickness change (Δh%) around selected diapirs and pillows in the East Texas Basin. Diagrammatic insets show methods of calculating Δh%.