Decollement
Encyclopedia
Décollement 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.
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.
depends on relative motion. A décollement horizon can either form due to high compressibility between bodies (usually in lithologies
such as marl
s, shale
s and evaporite
s), 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.
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.
. 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.
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.
evaporite
s 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.
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.
Jura mountains
The Jura Mountains are a small mountain range located north of the Alps, separating the Rhine and Rhone rivers and forming part of the watershed of each...
, 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
Fold (geology)
The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in...
and overthrusting
Obduction
Obduction is the overthrusting of continental crust by oceanic crust or mantle rocks at a convergent plate boundary. It can occur during an orogeny, or mountain-building episode....
. In an extensional setting décollements can be formed in a number of different ways.
Origin
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 nappeNappe
In geology, a nappe is a large sheetlike body of rock that has been moved more than or 5 km from its original position. Nappes form during continental plate collisions, when folds are sheared so much that they fold back over on themselves and break apart. The resulting structure is a...
rooting in the faraway Swiss Alps
Swiss Alps
The Swiss Alps are the portion of the Alps mountain range that lies within Switzerland. Because of their central position within the entire Alpine range, they are also known as the Central Alps....
. The décollement hypothesis of Buxtorf, although new for the Jura, was not novel at all for the Alps. Marcel Alexandre Bertrand
Marcel Alexandre Bertrand
Marcel Alexandre Bertrand was a French geologist who was born in Paris. He was a student at the École Polytechnique, and beginning in 1869 he attended the Ecole des Mines de Paris. Beginning in 1877 he performed geological mapping studies of Provence, Jura Mountains and the Alps...
published a paper in 1884 that delt with Alpine nappism
Geology of the Alps
The Alps form part of a Tertiary orogenic belt of mountain chains, called the Alpide belt, that stretches through southern Europe and Asia from the Atlantic all the way to the Himalayas. This belt of mountain chains was formed during the Alpine orogeny. A gap in these mountain chains in central...
, décollement, thin-skinned tectonics
Thin-skinned deformation
Thin-skinned deformation is a style of deformation in plate tectonics at a convergent boundary which occurs with shallow thrust faults that only involves cover rocks , and not deeper basement rocks....
was implied in that paper but the actual term was not used until Buxtorf's 1907 publication.
Formation
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 shearShear stress
A shear stress, denoted \tau\, , is defined as the component of stress coplanar with a material cross section. Shear stress arises from the force vector component parallel to the cross section...
depends on relative motion. A décollement horizon can either form due to high compressibility between bodies (usually in lithologies
Lithology
The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples or with low magnification microscopy, such as colour, texture, grain size, or composition. It may be either a detailed description of these characteristics or be a summary of...
such as marl
Marl
Marl or marlstone is a calcium carbonate or lime-rich mud or mudstone which contains variable amounts of clays and aragonite. Marl was originally an old term loosely applied to a variety of materials, most of which occur as loose, earthy deposits consisting chiefly of an intimate mixture of clay...
s, shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
s and evaporite
Evaporite
Evaporite is a name for a water-soluble mineral sediment that result from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporate deposits, marine which can also be described as ocean deposits, and non-marine which are found in standing bodies of...
s), 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
Basement Rock
Basement or Basement Rock music was a sub-genre coined in 2006 in an article by music magazine TGR. This was first in relation to the existence of underground record label Criminal Records but more for the independent bands they represent. The roots of the sub-genre are noted to be as far back as...
-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
Nappe
In geology, a nappe is a large sheetlike body of rock that has been moved more than or 5 km from its original position. Nappes form during continental plate collisions, when folds are sheared so much that they fold back over on themselves and break apart. The resulting structure is a...
. 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
Thick-skinned deformation
Thick-skinned deformation is a geological term which refers to crustal shortening that involves basement rocks and deep-seated faults. Crustal shortening occurs when the region is undergoing horizontal compression. This occurs in orogenesis, or mountain building, during which the crust is shortened...
as well.
Compressional setting
In a fold-thrust belt a décollement is the lowest detachment and this type of structure can form in the foreland basinForeland basin
A foreland basin is a depression that develops adjacent and parallel to a mountain belt. Foreland basins form because the immense mass created by crustal thickening associated with the evolution of a mountain belt causes the lithosphere to bend, by a process known as lithospheric flexure...
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 wedgeAccretionary wedge
An accretionary wedge or accretionary prism is formed from sediments that are accreted onto the non-subducting tectonic plate at a convergent plate boundary...
. 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.Extensional setting
Décollements that occur in an extensional setting are typically acccompanied by tectonic denudationDenudation
In geology, denudation is the long-term sum of processes that cause the wearing away of the earth’s surface leading to a reduction in elevation and relief of landforms and landscapes...
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 complexMetamorphic core complexMetamorphic core complexes are exposures of deep crust exhumed in association with largely amagmatic extension. They form, and are exhumed, through relatively fast transport of middle and lower continental crust to the Earth's surface...
. 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 isostaticallyIsostasyIsostasy is a term used in geology to refer to the state of gravitational equilibrium between the earth's lithosphere and asthenosphere such that the tectonic plates "float" at an elevation which depends on their thickness and density. This concept is invoked to explain how different topographic...
and isothermally. Décollement develops as antiform that migrates toward shallower depths.
Jura Décollement
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 TriassicTriassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
evaporite
Evaporite
Evaporite is a name for a water-soluble mineral sediment that result from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporate deposits, marine which can also be described as ocean deposits, and non-marine which are found in standing bodies of...
s 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
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...
and Cenozoic
Cenozoic
The Cenozoic era is the current and most recent of the three Phanerozoic geological eras and covers the period from 65.5 mya to the present. The era began in the wake of the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs and...
cover of the fold-and-thrust belt and the adjacent Molasse Basin
Molasse basin
The Molasse basin is a foreland basin north of the Alps, that formed during the Oligocene and Miocene epochs. The basin formed due to the flexure of the European plate under the weight of the orogenic wedge of the Alps that was forming to the south....
have been deformed over the weak basal décollement and displaced by some 20 km and more toward the northwest.
Appalachian-Ouachita Décollement
The Appalachian-Ouachita orogenOuachita orogeny
The Ouachita orogeny was a mountain building event that resulted in the folding and faulting of strata currently exposed in the Ouachita Mountains...
along the eastern and southern margin of North America includes a late Paleozoic
Paleozoic
The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...
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
Facies
In geology, facies are a body of rock with specified characteristics. Ideally, a facies is a distinctive rock unit that forms under certain conditions of sedimentation, reflecting a particular process or environment....
. The basal décollement of the fold thrust belt varies in stratigraphic level both along strike
Strike and dip
Strike and dip refer to the orientation or attitude of a geologic feature. The strike line of a bed, fault, or other planar feature is a line representing the intersection of that feature with a horizontal plane. On a geologic map, this is represented with a short straight line segment oriented...
and across strike. The geometry of the décollement also reflects the shapes of promontories and embayments of the late Precambrian-early Paleozoic rifted margin.