Décollement (dé-collé-ment) is a gliding plane between two rock masses. In French, "décoller" means "to detach from" or "to rip off" and was first used by geologists studying the structure of the Swiss Jura Mountains,
 but is also known as a detachment zone. This is a structure of strata owing to deformation, resulting in independent styles of deformation in the rocks above and below. In a compressional setting it is associated with folding and overthrusting. In an extensional setting décollements can be formed in a number of different ways.
The term came into use in 1907 when A. Buxtorf released his paper that theorised that the Jura is the frontal part of a décollement nappe rooting in the faraway Swiss Alps. The décollement hypothesis of Buxtorf, although new for the Jura, was not novel at all for the Alps. Marcel Alexandre Bertrand published a paper in 1884 that delt with Alpine nappism, décollement, thin-skinned tectonics was implied in that paper but the actual term was not used until Buxtorf's 1907 publication.
Décollement is facilitated by body forces and this has lead many researchers to equate décollement with gravity sliding. However, there is overwhelming evidence that much décollement is due to the surface forces that arise (push) at converging plate boundaries: Lubricating layers seem to be weak enough to permit development of stepped thrusts that originate at subduction zones and emerge far in the foreland. Sometimes an issue is made of whether this is overthrusting or underthrusting, but that is irrelevant mechanically because resistance to shear depends on relative motion. A décollement horizon can either form due to high compressibility between bodies (usually in lithologies such as marls, shales and evaporites), or can form along planes of high pore pressures. The depths of these structures range from a few to over 10 km Two layers separated by the décollement layer can have different characteristics of tectonic deformation, they can act as a boundary between a brittle, (slip along the décollement), domain above and a zone with intense ductile deformation (flowing of solid rock) below the detachment surface.
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 rock-cover contact. Rocks above the detachment are allochthonous in that they have been transported relative to their origianl location. If the distance traveled is greater than 2 km then the slab of rock is considered a nappe. In contrast, rocks that lie below the detachment are autochthonous, in that they have not been transported by fault slip and thus lie in their original position. Geologists sometimes refer to the style of deformation in which faulting and folding occur only above a regional basal detachment as thin-skinned tectonics, but décollements can take place in thick-skinned deformation as well.
In a fold-thrust belt a décollement is the lowest detachment and this type of structure can form in the foreland basin of a Subduction zone. The belt may contain higher level detachments, an imbricate fan (see Fig. 1) of thrust faults and duplexes, as well as several detachment horizons, but they all lie above the décollement. In the condition of tectonic compression, the layer above the décollement layer is apt to develop more intense deformation than other layers, and the deformation in the layer under the décollement layer is weaker.
Effect of friction
Décollements are responsible for duplex formation, which evolution and geometry vary in styles and greatly influence the dynamics of the thrust wedge. The amount of friction that occurs along the décollement has an effect on the shape of the wedge. The resulting low-angle slope of the frontal part of the wedge reflects the low-friction upper décollement, whereas higher slope angles are a consequence of the higher basal friction décollement.
Types of folding
There are two different ideas on the type of folding that occurs with décollements in a fold-thrust belt. Concentric folding, (unform bed thickness), is necessarily accompanied by detachment or décollement as part of thrust-fault deformation. Another author notes that most folding is disharmonic, that is, the from of such folds is not uniform throughout the stratigraphic column.
Décollements that occur in an extensional setting are typically acccompanied by tectonic denudation and high cooling rates. Four ways of décollements occuring in an extensional setting are a megalandslide, in situ ductile stretching or intrusion and rooted, low angle normal faults, high angle normal faults.
- The megalandslide model shows extension with normal faults near the original source and shortening further away from the source.
- The in situ model shows numerous normal faults overlaying one large décollement.
- The rooted, low angle normal fault model shows the décollement as a narrow zone of decoupling between two thin sheets of rock. Towards the thick end of the upper plate, extensional faulting may be negligible or absent, but towards the thin end it loses its ability to remain coherent and becomes a thin-skinned extensinal fault terrane.
- Décollements can form from high angle normal faults by being uplifted in a second stage of extension, which allows the exhumation of a metamorphic core complex. The stages are (as seen in Fig. 2), a half graben forms, stress orientation is not perturbed, because of high fault friction. Next, elevated pore pressure (Pp) leads to low effective friction that forces σ1 to be fault parallel in footwall. Low-angle fault forms and is ready to act as décollement. Then, the upper crust is thinned above décollement by normal faulting. New high-angle faults control décollement propagation and help crustal exhumation. Finally, major and rapid horizontal extension raises isostatically and isothermally. Décollement develops as antiform that migrates toward shallower depths.
This structure is located in the Jura Mountains that run just north of the Alps and originally theorized as a folded décollement nappe by A. Buxtorf in 1907. The thin-skinned nappe was sheared off on Triassic evaporites that were deposited up to 1 km thick. The frontal (northwest and west) basal thrust of the Jura décollement fold-and-thrust belt forms the most external limit of the Alpine orogenic wedge with the youngest fold-and-thrust activity. 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.
The Appalachian-Ouachita orogen along the eastern and southern margin of North America includes a late Paleozoic external fold-thrust belt in which the thrust faults exhibit a characteristic thin-skinned flat-and ramp geometry. The geometry of the basal décollement is related to lateral and vertical variations in stratigraphic facies. The basal décollement of the fold thrust belt varies in stratigraphic level both along strike and across strike. The geometry of the décollement also reflects the shapes of promontories and embayments of the late Precambrian-early Paleozoic rifted margin.
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