Inverted graben

When the stress path of the rock section, loaded by burial and tectonic stress, intersects the faulting instability envelope earlier than the folding instability envelope due to decreased cohesion and friction along pre-existing rifting-related normal faults, the deformation results in their inversion (Figs. 5.1 and 5.2). Owing to the amount of stress transfer through the pre-existing normal fault into the footwall, inverted grabens can form either by the inversion of normal faults combined with footwall-edge shortcut faults, in efficient transfer cases or by the inversion of normal faults alone, in less efficient transfer cases. An efficient transfer case occurs when the friction along a pre-existing normal fault is higher. Less efficient transfer cases occur when this friction is low. In these cases the stress is funnelled mostly through the hanging wall. A softer pre-existing graben fill would also have a tendency to avoid buttressing against the footwall edge, which further enhances the chance of avoiding the propagation of short-cut reverse faults through the footwall edge.

Examples of inverted grabens and half-grabens come from the North Sea (e.g. Badley et al., 1989; Figs. 5.3 and 5.4; Nalpas et al., 1995), the north Pyrenees (Hayward and Graham, 1989; Fig. 5.5), the Walls basin, British Isles (Coward et al., 1989; Fig. 5.6), Salta province in northern Argentina (Lowell, 1995), the Gippsland basin, Australia (Davis, 1983), southern Altiplano, Bolivia (Lowell, 1995), and Dneper-Donetsk, Russia (Ulmishek et al., 1994).

Inverted structures (Fig. 5.1) are characterized by different structural features recorded by syn-rift, postrift and syn-inversion sedimentary sections. The syn-rift sedimentary sequence usually thickens towards the controlling normal fault.