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Décollement

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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.[1][2]

Décollement(fromFrenchdécoller'to detach from') is a gliding plane between two rock masses, also known as a basal detachment fault. Décollements are adeformationalstructure, resulting in independent styles of deformation in the rocks above and below the fault. They are associated with both compressional settings (involvingfoldingandoverthrusting[3]) and extensional settings.

Origin

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The term was first used by geologists studying the structure of the SwissJura Mountains,[4]coined in 1907 by A. Buxtorf, who released a paper that theorized that the Jura is the frontal part of a décollement at the base of anappe,rooted in the farawaySwiss Alps.[5][6]Marcel Alexandre Bertrandpublished a paper in 1884 that dealt withAlpine nappism.Thin-skinned tectonicswas implied in that paper but the actual term was not used until Buxtorf's 1907 publication.[4][5]

Formation

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Décollements are caused by surface forces, which 'push' atconverging plate boundaries,facilitated by body forces[7](gravity sliding). Mechanically weak layers instrataallow the development of stepped thrusts (either over- or underthrusts),[8]which originate atsubductionzones and emerge deep in theforeland.Rock bodies with differinglithologieshave different characteristics of tectonic deformation. They can behave in a brittle manner above the décollement surface, with intense ductile deformation below the décollement surface.[9]Décollement horizons may be at depths as great as 10 km[10]and form due to high compressibility between differing rock bodies or along planes of high pore pressures.[11]

Typically, the basal detachment of the foreland part of a fold-thrust belt lies in a weak shale or evaporite at or near thebasement.[1]Rocks above the décollement areallochthonous,rocks below areautochthonous.[1]If material is transported along a décollement greater than 2 km, it may be considered anappe.[5]The faulting and folding that occurs with a regional basal detachment may be referred to as "thin-skinned tectonics",[1]but décollements occur in 'thick-skinned' deformational regimes as well.[12]

Compressional setting

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In afold-thrust belt,the décollement is the lowest detachment[1](see Fig 1.) and forms in theforeland basinof asubductionzone.[1]A fold-thrust belt may contain other detachments above the décollement—an imbricate fan of thrust faults andduplexesas well as other detachment horizons. In compressional settings, the layer directly above the décollement will develop more intense deformation than other layers, and weaker deformation below the décollement.[13]

Effect of friction

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Décollements are responsible forduplex formation,the geometry of which greatly influences the dynamics of thethrust wedge.[14]The amount of friction along the décollement affects the shape of the wedge; a low-angle slope reflects a low-friction décollement, whereas a higher-angle slope reflects a higher-friction basal detachment.[2]

Types of folding

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Two different types of folding may occur at a décollement. Concentric folding is identified by uniform bed thickness throughout the fold, and is necessarily accompanied by detachment or a décollement as part of the deformation that occurs with a thrust fault.[15]Disharmonic folding does not have uniform bed thickness throughout the fold.[16]

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. Ahalf grabenforms, 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.[9][17]

Extensional setting

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Décollements in extensional settings are accompanied by tectonicdenudationand high cooling rates.[5]They can form by several methods:

  1. The megalandslide model predicts extension with normal faults near the original fault source and shortening further away from the source.[18]
  2. Thein situmodel predicts numerous normal faults overlying one large décollement.[18]
  3. The rooted, low anglenormal faultmodel predicts that the décollement is created when two thin sheets of rock decouple at depth. Near the thickest part of theupper plate,extensional faulting may be negligible or absent, but as the upper plate thins, it loses its ability to remain coherent and may behave as a thin-skinned extensional terrane.[18]
  4. Décollements can form from high angle normal faults.[9][18]Uplift in a second stage of extension allows the exhumation of ametamorphic core complex(see Fig. 2). Ahalf grabenforms, 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 décollement. Then, the upper crust is thinned above the décollement by normal faulting. New high-angle faults control the propagation of the décollement and help crustal exhumation. Finally, major and rapid horizontal extension lifts the terrainisostaticallyand isothermally. A décollement develops as an antiform that migrates toward shallower depths.[9]

Examples

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Jura Décollement

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Located in theJura Mountains,north of the Alps, it was originally thought to be a folded décollement nappe.[5][6]The thin-skinned nappe was sheared off on 1000 meter-thick deposits ofTriassicevaporites.[5][19][20]The frontal basal detachment of the Jura fold-and-thrust belt forms the most external limit of the Alpine orogenic wedge with the youngest fold-and-thrust activity.[21]TheMesozoicandCenozoiccover of the fold-and-thrust belt and the adjacentMolasse Basinhave been deformed over the weak basal décollement and displaced by some 20 km and more toward the northwest.[19]

Appalachian-Ouachita Décollement

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TheAppalachian-Ouachita orogenalong the southeastern margin of the North American craton includes a latePaleozoicfold-thrust belt with a thin-skinned flat-and-ramp geometry, related to lateral and vertical variations in rock lithologies. The décollement surface varies along and acrossstrike.Promontories and embayments in the late Precambrian-early Paleozoicrifted marginare preserved in the décollement geometry.[22]

References

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  1. ^abcdefVan Der Pluijm, Ben A. (2004).Earth Structure.New York, NY: W.W. Norton. p. 457.ISBN978-0-393-92467-1.
  2. ^abMalavieille, Jacques(2010). "Impact of erosion, sedimentation, and structural heritage on the structure and kinematics of orogenic wedges: Analog models and case studies".GSA Today:4.doi:10.1130/GSATG48A.1.
  3. ^Bates, Robert L.; Julia A. Jackson (1984).Dictionary of Geological Terms(Third ed.). New York: Anchor Books. p. 129.ISBN978-0-385-18101-3.
  4. ^abBertrand, M.(1884). "Rapports de structure des Alpes de Glaris et du bassin houiller du Nord".Bulletin de la Société Géologique de France.3rd series.12:318–330.
  5. ^abcdefH.P. Laubscher, Basel (1988). "Décollement in the Alpine system: an overview".Geologische Rundschau.77(1): 1–9.Bibcode:1988GeoRu..77....1L.doi:10.1007/BF01848672.S2CID128758221.
  6. ^abBuxtorf, A. (1907). "Zur Tektonik des Kettenjura".Berichte über die Versammlungen des Oberrheinischen Geologischen Verein:29–38.
  7. ^Hubbert, M. K.; Rubey, W. W. (1959). "Role of fluid pressure in mechanics of overthrust faulting, 1. Mechanics of fluid-filled porous solids and its application to overthrust faulting".Geological Society of America Bulletin.70(2): 115–166.Bibcode:1959GSAB...70..115K.doi:10.1130/0016-7606(1959)70[115:ROFPIM]2.0.CO;2.
  8. ^Laubscher, H. P. (1987)."Décollement".Structural Geology and Tectonics.Encyclopedia of Earth Science. p.187.doi:10.1007/3-540-31080-0_27.ISBN978-0-442-28125-0.
  9. ^abcdChery, Jean (2001). "Core complex mechanics: From the Gulf of Corinth to the Snake Range".Geology.29(5): 439–442.Bibcode:2001Geo....29..439C.doi:10.1130/0091-7613(2001)029<0439:CCMFTG>2.0.CO;2.
  10. ^McBride, John H.; Pugin, J.M.; Hatcher Jr., D. (2007).Scale independence of décollement thrusting.Geological Society of America Memoirs. Vol. 200. pp. 109–126.doi:10.1130/2007.1200(07).ISBN978-0-8137-1200-0.
  11. ^Ramsay, J, 1967,Folding and Fracturing of Rocks,McGraw-HillISBN978-0-07-051170-5
  12. ^Bigi, Sabina; Doglioni, Carlo (2002)."Thrust vs Normal Fault Decollements in The Central Appennines"(PDF).Bollettino della Società Geologica Italiana.1:161–166. Archived fromthe original(PDF)on 2012-04-25.Retrieved2011-11-17.
  13. ^LiangJie, Tang; Yang KeMing; Jin WenZheng; LÜ ZhiZhou; Yu YiXin (2008). "Multi-level decollement zones and detachment deformation of Longmenshan thrust belt, Sichuan Basin, southwest China".Science in China Series D: Earth Sciences.51(suppl. 2): 32–43.Bibcode:2008ScChD..51S..32T.doi:10.1007/s11430-008-6014-9.S2CID129914584.
  14. ^Konstantinovskaya, E.; J. Malavieille (April 20, 2011). "Thrust wedges with décollement:levels and syntectonic erosion: A view from analog models".Tectonophysics.502(3–4): 336–350.Bibcode:2011Tectp.502..336K.doi:10.1016/j.tecto.2011.01.020.
  15. ^Dahlstrom, C.D.A. (1969). "The upper detachment in concentric folding".Bulletin of Canadian Petroleum Geology.17(3): 326–347.
  16. ^Billings, M.P. (1954).Structural Geology(2nd ed.). New York: Prentice-Hall. p. 514.
  17. ^Warren, John K. (2006). "Salt tectonics".Evaporites: Sediments, Resources and Hydrocarbons.pp.375–415.doi:10.1007/3-540-32344-9_6.ISBN978-3-540-26011-0.
  18. ^abcdWernicke, Brian (25 June 1981). "Low-angle normal faults in the Basin and Range Province: nappe tectonics in an extending orogen".Nature.291(5817): 645–646.Bibcode:1981Natur.291..645W.doi:10.1038/291645a0.S2CID4269466.
  19. ^abSommaruga, A. (1998). "Décollement tectonics in the Jura foreland fold-and-thrust belt".Marine and Petroleum Geology.16(2): 111–134.doi:10.1016/S0264-8172(98)00068-3.
  20. ^Laubscher, Hans (2008)."The Grenchenberg conundrum in the Swiss Jura: a case for the centenary of the thin-skin décollement nappe model (Buxtorf 1907)".Swiss Journal of Geosciences.101:41–60.doi:10.1007/s00015-008-1248-2.S2CID129277771.
  21. ^Mosar, Jon (1999)."Present-day and future tectonic underplating in the western Swiss"(PDF).Earth and Planetary Science Letters.39(3): 143.Bibcode:1999E&PSL.173..143M.doi:10.1016/S0012-821X(99)00238-1.Archived fromthe original(PDF)on 2009-09-16.
  22. ^Thomas, William A. (1988). "Stratigraphic framework of the geometry of the basal decollement of the Appalachian-Ouachita fold-thrust belt".Geologische Rundschau.77(1): 183–190.Bibcode:1988GeoRu..77..183T.doi:10.1007/BF01848683.S2CID128573091.
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