Mechanically weak layers in strata allow the development of stepped thrusts (either over- or underthrusts),[8] which originate at subduction zones and emerge deep in the foreland.
[11] Typically, the basal detachment of the foreland part of a fold-thrust belt lies in a weak shale or evaporite at or near the basement.
[13] Décollements are responsible for duplex formation, the geometry of which greatly influences the dynamics of the thrust wedge.
[5] They can form by several methods: Located in the Jura Mountains, north of the Alps, it was originally thought to be a folded décollement nappe.
[21] The Mesozoic and Cenozoic cover of the fold-and-thrust belt and the adjacent Molasse Basin have been deformed over the weak basal décollement and displaced by some 20 km and more toward the northwest.
[19] The Appalachian-Ouachita orogen along the southeastern margin of the North American craton includes a late Paleozoic fold-thrust belt with a thin-skinned flat-and-ramp geometry, related to lateral and vertical variations in rock lithologies.
Fig. 1 Imbricate fan in a thrust system with a basal décollement. The section below the décollement is undeformed basement rock. Above the décollement, deformation has occurred due to compression. A series of branching faults terminating at depth.
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Décollement formation in an extensional setting. Décollements can form from high angle normal faults. Uplift in a second stage of extension allows the exhumation of a metamorphic core complex. A
half graben
forms, but stress orientation is not perturbed due to high fault friction. Next, elevated pore pressure (Pp) leads to low effective friction that forces σ1 to be parallel to the fault in the footwall. A low-angle fault forms and is ready to act as a décollement. Then, the upper crust is thinned above the décollement by normal faulting. New high-angle faults control propagation of the décollement and help crustal exhumation. Finally, major and rapid horizontal extension lifts the terrain isostatically and isothermally. A décollement develops as an antiform that migrates toward shallower depths.
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