The lithostratigraphy in this chapter will be interpreted as mechanical stratigraphy, which determines the nature of the deformational response of the shortened rock package to applied stresses. Each mechanical stratigraphy is characterized by particular faulting and folding strengths. When the rock section is loaded by orogenic stress, strengths control whether the section deforms by:

1. fold-first deformation (e.g. Heim, 1919; Wiltschko and Eastman, 1983; Fisher et al., 1992; Woodward, 1992; Fischer and Anastasio, 1994; Morley, 1994);

2. fault-first deformation (Rich, 1934; Fox, 1959; Royse et al., 1975; Suppe, 1983; Medwedeff, 1989; Hedlund et al., 1994; Jamison and Pope, 1996); or

3. contemporaneous fold–fault deformation (e.g. Dahlstrom, 1970; Brown and Spang, 1978; Williams and Chapman, 1983; Chester and Chester, 1990; Suppe and Medwedeff, 1990; Alonso and Teixell, 1992; Couzens and Dunne, 1994; Tavarnelli, 1994).

The mechanical stratigraphy controls the energy balance, described in Chapter 3, because it affects the propagation of new faults, the reactivation of preexisting faults, the gravity forces of the uplifting part of the thrust sheet and the internal deformation of the thrust sheet.

Role of mechanical stratigraphy in thrust sheet initiation

As described in Chapter 3, the mechanism of thrust sheet detachment is controlled by the competition of faulting and folding strengths during the orogenic stress buildup. The strength limit, which is reached first along the stress path of the autochthonous rock section, determines the structural type of the future thrust sheet.

If the critical strength for folding is the lower of the two critical strengths, detachment folds will form.