Large-scale scenarios

Large-scale anisotropies influence the evolving structural style of a thrustbelt by affecting the lithospheric deformation, which includes both elastic and inelastic components (Karner et al., 1993). The elastic deformation is represented by lithospheric flexure. The inelastic deformation is represented by brittle deformation and ductile creep. The lithospheric deformation at a specific depth depends on the local rock strength. The rock strength varies with depth in the lithosphere in response to changes in ambient temperature, stress, strain and compositional variations (e.g. Goetze and Evans, 1979; Brace and Kohlstedt, 1980; Kirby, 1983).

At shallow depths in the lithosphere, the rock strength depends mainly on the confining pressure and it yields by frictional sliding on fracture surfaces (Brace and Kohlstedt, 1980). Such a brittle deformation of the upper lithosphere is influenced by the distribution of pre-existing faults and fractures in the crust and their orientation with respect to the applied tectonic force. Confirmation of such an influence is provided by rock mechanic tests (e.g., Donath, 1961; Turner and Weiss, 1963). The critical calculations based on hand specimens (e.g. Jaeger, 1960), on the oceanic lithosphere of the Central Indian Ocean (Karner et al., 1993), the continental lithosphere of the Rocky Mountain foreland (Allmendinger et al., 1982) and on the Pyrenean foreland (Desegaulx et al., 1991) also demonstrate the influence.

Deeper within the lithosphere, the rock strength would be expected to decrease with depth as increasing temperature promotes thermally activated creep processes (Goetze and Evans, 1979; Goetze, 1978). Even relatively small applied forces would be expected to cause the top and bottom of the lithosphere to yield, thereby reducing the thickness of the strong core region of the lithosphere.