Editing Thrust fault (section)

==Thrust geometry and nomenclature==
[[File:Faultbendfold.png|thumb|200px|Diagram of the evolution of a fault-bend fold or 'ramp anticline' above a thrust ramp, the ramp links [[decollement]]s at the top of the green and yellow layers]]
[[File:Fault-propagation fold.gif|thumb|150px|Diagram of the evolution of a fault propagation fold]]
[[File:Duplex1.png|thumb|180px|Development of thrust duplex by progressive failure of ramp footwall]]
[[File:Antiformal stack.jpg|thumb|140px|Antiformal stack of thrust imbricates proved by drilling, Brooks Range Foothills, Alaska]]

===Reverse faults===
A thrust fault is a type of [[Fault (geology)#Dip-slip faults|reverse fault]] that has a [[strike and dip|dip]] of 45 degrees or less.<ref name="USGSGloss">{{cite web | url=https://earthquake.usgs.gov/learn/glossary/?term=dip%20slip | title=dip slip | publisher=United States Geological Survey | work=Earthquake Glossary | access-date=5 December 2017}}</ref><ref name="UCSB">{{cite web | url=http://scienceline.ucsb.edu/getkey.php?key=2845 | title=How are reverse faults different than thrust faults? In what way are they similar? | publisher=[[University of California, Santa Barbara]] | work=UCSB Science Line | date=13 February 2012 | access-date=5 December 2017}}</ref>

If the angle of the fault plane is lower (often less than 15 degrees from the horizontal<ref name="Crosby1967">{{cite journal | url=http://archives.datapages.com/data/cspg/data/015/015003/0219.htm | title=High Angle Dips at Erosional Edge of Overthrust Faults | author=Crosby, G. W. | journal=Bulletin of Canadian Petroleum Geology | year=1967 | volume=15 | issue=3 | pages=219–229}}</ref>) and the displacement of the overlying block is large (often in the kilometer range) the fault is called an ''overthrust'' or ''overthrust fault''.<ref name="Glossary2005">{{cite book | title=Glossary of Geology | publisher=American Geological Institute | editor-last1=Neuendorf | editor-first1=K. K. E. | editor-last2=Mehl Jr. | editor-first2=J. P. | editor-last3=Jackson | editor-first3=J. A.| year=2005 | location=Alexandria, Virginia | page=462| edition=5th }}</ref> Erosion can remove part of the overlying block, creating a ''fenster'' (or ''[[Window (geology)|window]]'') – when the underlying block is exposed only in a relatively small area. When erosion removes most of the overlying block, leaving island-like remnants resting on the lower block, the remnants are called ''klippen'' (singular ''[[klippe]]'').

===Blind thrust faults===
{{Main|Blind thrust earthquake}}
If the fault plane terminates before it reaches the Earth's surface, it is called a ''blind thrust'' fault. Because of the lack of surface evidence, blind thrust faults are difficult to detect until rupture. The destructive [[1994 Northridge earthquake|1994 earthquake in Northridge, Los Angeles, California]], was caused by a previously undiscovered blind thrust fault.

Because of their low [[strike and dip|dip]], thrusts are also difficult to appreciate in mapping, where lithological offsets are generally subtle and stratigraphic repetition is difficult to detect, especially in [[peneplain]] areas.

===Fault-bend folds===
Thrust faults, particularly those involved in [[Thrust tectonics#Thin-skinned deformation|thin-skinned]] style of deformation, have a so-called ''ramp-flat'' geometry. Thrusts mainly propagate along zones of weakness within a sedimentary sequence, such as [[mudstone]]s or [[Rock salt|halite]] layers; these parts of the thrust are called ''[[decollement]]s''. If the effectiveness of the decollement becomes reduced, the thrust will tend to cut up the section to a higher stratigraphic level until it reaches another effective decollement where it can continue as bedding parallel flat. The part of the thrust linking the two flats is known as a ''ramp'' and typically forms at an angle of about 15°–30° to the bedding. Continued displacement on a thrust over a ramp produces a characteristic fold geometry known as a ''ramp anticline'' or, more generally, as a ''fault-bend fold''.

===Fault-propagation folds===
Fault-propagation folds form at the tip of a thrust fault where propagation along the decollement has ceased, but displacement on the thrust behind the fault tip continues. The formation of an asymmetric anticline-syncline fold pair accommodates the continuing displacement. As displacement continues, the thrust tip starts to propagate along the axis of the syncline. Such structures are also known as ''tip-line folds''. Eventually, the propagating thrust tip may reach another effective decollement layer, and a composite fold structure will develop with fault-bending and fault-propagation folds' characteristics.

===Thrust duplex===
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Duplexes occur where two decollement levels are close to each other within a sedimentary sequence, such as the top and base of a relatively strong [[sandstone]] layer bounded by two relatively weak mudstone layers. When a thrust that has propagated along the lower detachment, known as the ''floor thrust'', cuts up to the upper detachment, known as the ''roof thrust'', it forms a ramp within the stronger layer. With continued displacement on the thrust, higher stresses are developed in the footwall of the ramp due to the bend on the fault. This may cause renewed propagation along the floor thrust until it again cuts up to join the roof thrust. Further displacement then takes place via the newly created ramp. This process may repeat many times, forming a series of fault-bounded thrust slices known as ''imbricates'' or [[Horse (geology)|horses]], each with the geometry of a fault-bend fold of small displacement. The final result is typically a lozenge-shaped duplex.

Most duplexes have only small displacements on the bounding faults between the horses, which dip away from the foreland. Occasionally, the displacement on the individual horses is more significant, such that each horse lies more or less vertically above the other; this is known as an ''antiformal stack'' or '''imbricate stack'''. If the individual displacements are still greater, the horses have a foreland dip.

Duplexing is a very efficient mechanism of accommodating the shortening of the crust by thickening the section rather than by folding and deformation.<ref>{{cite journal|last1=Moore|first1=Thomas E.|last2=Potter|first2=Christopher J.|title=Structural Plays in Ellesmerian Sequence and Correlative Strata of the National Petroleum Reserve, Alaska|journal=U.S. Geological Survey Open File Report|series=Open-File Report |volume=03-253|year=2003|doi=10.3133/ofr03253 |url=https://pubs.usgs.gov/of/2003/0253/pdf/of03-253.pdf|access-date=5 July 2022}}</ref>