Editing Fault (geology) (section)

==Fault types==

Faults are mainly classified in terms of the angle that the fault plane makes with the Earth's surface, known as the [[Strike and dip|dip]], and the direction of slip along the fault plane.<ref>{{cite web|title=What is a fault and what are the different types?|url=https://www.usgs.gov/faqs/what-a-fault-and-what-are-different-types?qt-news_science_products=0#qt-news_science_products|website=USGS: Science for a Changing World|access-date=13 October 2021}}</ref>
Based on the direction of slip, faults can be categorized as:
* ''strike-slip'', where the offset is predominantly horizontal, parallel to the fault trace;
* ''dip-slip'', offset is predominantly vertical and/or perpendicular to the fault trace; or
* ''oblique-slip'', combining strike-slip and dip-slip.

===Strike-slip faults===
{{further|Strike-slip tectonics}}
[[File:strike slip fault.png|thumb|Schematic illustration of the two strike-slip fault types, as seen from above]]
In a '''strike-slip fault''' (also known as a ''wrench fault'', ''tear fault'' or ''transcurrent fault''),<ref>{{cite dictionary|editor-last=Allaby|editor-first=Michael|entry=Strike-Slip Fault|dictionary=A Dictionary of Geology and Earth Sciences|edition=4th|publisher=Oxford University Press|year=2015|isbn=978-0-19-965306-5|chapter-url=http://www.oxfordreference.com/view/10.1093/acref/9780199653065.001.0001/acref-9780199653065-e-8171}}</ref> the fault surface (plane) is usually near vertical, and the footwall moves laterally either left or right with very little vertical motion. Strike-slip faults with left-lateral motion are also known as ''sinistral'' faults and those with right-lateral motion as ''dextral'' faults.<ref>{{cite book|last=Park|first=R.G.|title=Foundation of Structural Geology|edition=3|page=11|year=2004|publisher=Routledge|isbn=978-0-7487-5802-9|url=https://books.google.com/books?id=ycASqdxSG3YC&pg=PA11}}</ref> Each is defined by the direction of movement of the ground as would be seen by an observer on the opposite side of the fault.

A special class of strike-slip fault is the ''[[transform fault]]'' when it forms a [[plate tectonics|plate]] boundary. This class is related to an offset in a [[spreading center]], such as a [[mid-ocean ridge]], or, less common, within continental [[lithosphere]], such as the [[Dead Sea Transform]] in the [[Middle East]] or the [[Alpine Fault]] in New Zealand. Transform faults are also referred to as "conservative" plate boundaries since the lithosphere is neither created nor destroyed.

===Dip-slip faults===
[[File:Nor rev.png|thumb|Vertical [[Cross section (geology)|cross-sectional]] view, along a plane perpendicular to the ''fault plane'', illustrating normal and reverse dip-slip faults]]

'''Dip-slip faults''' can be either ''normal'' ("[[extensional fault|extensional]]") or ''reverse''. The terminology of "normal" and "reverse" comes from [[coal mining]] in England, where normal faults are the most common.<ref name="Peacock_etal_2000">{{cite journal|last1=Peacock |first1=D. C. P.|last2=Knipe|first2=R. J. |last3=Sanderson|first3=D. J.|year=2000|title=Glossary of normal faults|journal=Journal of Structural Geology|volume=22|issue=3 |page=298|doi=10.1016/S0191-8141(00)80102-9 |bibcode=2000JSG....22..291P}}</ref>

With the passage of time, a regional reversal between [[Tension (geology)|tensional]] and [[Compression (geology)|compressional]] [[Stress (mechanics)|stresses]] (or vice-versa) might occur, and faults may be reactivated with their relative block movement inverted in opposite directions to the original movement (fault inversion). In such a way, a normal fault may therefore become a reverse fault and vice versa.

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====Normal faults====
[[File:Normal fault diagram.gif|thumb|upright=0.6|left|Normal fault diagram]]
In a normal fault, the hanging wall moves downward, relative to the footwall. The [[Strike and dip|dip]] of most normal faults is at least 60 degrees but some normal faults dip at less than 45 degrees.<ref name="Oskin_2019">{{cite web|author=Oskin, Michael E. |url=https://geo.libretexts.org/Courses/University_of_California_Davis/GEL_101%3A_Structural_Geology_(Oskin)/Part_I%3A_Brittle_Deformation_and_Mechanics/2A%3A_Normal_Faults|title=Normal Faults |work=LibreTexts|date=3 June 2019|access-date=6 April 2022}}</ref>
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=====Basin and range topography=====
[[File:Fault-Horst-Graben.svg|thumb|upright=1.35|Diagram illustrating the structural relationship between grabens and horsts.]]
A downthrown block between two normal faults dipping towards each other is a [[graben]]. A block stranded between two grabens, and therefore two normal faults dipping away from each other, is a [[Horst (geology)|horst]]. A sequence of grabens and horsts on the surface of the Earth produces a characteristic ''[[basin and range topography]]''.
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=====Listric faults=====
A listric fault is a type of normal fault that has a concave-upward shape with the upper section near Earth's surface being steeper, becoming more horizontal with increased depth.  Normal faults can evolve into listric faults with the fault plane curving into the Earth.  They can also form where the hanging wall is absent (such as on a cliff), where the footwall may slump in a manner that creates multiple listric faults.
<gallery>
File:Listric fault diagram.jpg|Cross-section diagram of a listric fault
File:Listric walls in a cliff wall.jpg|Cross-section diagram of multiple listric faults in a cliff wall
</gallery>

=====Detachment faults=====
The fault panes of listric faults can further flatten and evolve into a horizontal or near-horizontal plane, where slip progresses horizontally along a [[decollement]]. [[Extensional tectonics|Extensional]] decollements can grow to great dimensions and form [[detachment fault]]s, which are low-angle normal faults with regional [[tectonics|tectonic]] significance.

Due to the curvature of the fault plane, the horizontal extensional displacement on a listric fault implies a geometric "gap" between the hanging and footwalls of the fault forms when the slip motion occurs. To accommodate into the geometric gap, and depending on its [[rheology]], the hanging wall might fold and slide downwards into the gap and produce [[Rollover anticlines|rollover folding]], or break into further faults and blocks which fill in the gap.  If faults form, '''imbrication fans''' or '''domino faulting''' may form.
<gallery>
File:Rollover.png|Cross-section diagram of a listric fault (red line), with a resulting rollover fold
File:Listric faults and imbrication fan.jpg|Cross-section diagram showing how an imbrication fan forms
The fault planes of multiple listric faults can flatten and connect at depth into a common [[décollement]], .
</gallery>

====Reverse faults====
[[File:Reverse fault.gif|thumb|upright=0.6|left|Reverse fault]]
A reverse fault is the opposite of a normal fault—the hanging wall moves up relative to the footwall.<br>
Reverse faults indicate compressive shortening of the crust.
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=====Thrust faults=====
[[File:Thrust with fault bend fold.svg|thumb|Cross-section diagram of a thrust fault with a fault-bend fold]]
A ''[[thrust fault]]'' has the same sense of motion as a reverse fault, but with the dip of the fault plane at less than 45°.<ref name="USGSGloss">{{cite web|url=https://earthquake.usgs.gov/learn/glossary/?term=dip%20slip|title=dip slip|work=Earthquake Glossary|publisher=United States Geological Survey|access-date=13 December 2017|archive-url=https://web.archive.org/web/20171123090300/https://earthquake.usgs.gov/learn/glossary/?term=dip%20slip|archive-date=23 November 2017|url-status=live}}</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?|work=UCSB Science Line|publisher=[[University of California, Santa Barbara]]|date=13 February 2012|access-date=13 December 2017|archive-url=https://web.archive.org/web/20171027080203/http://scienceline.ucsb.edu/getkey.php?key=2845|archive-date=27 October 2017|url-status=live}}</ref> Thrust faults typically form ramps, flats and fault-bend (hanging wall and footwall) folds.

A section of a hanging wall or foot wall where a thrust fault formed along a relatively weak bedding plane is known as a ''flat'' and a section where the thrust fault cut upward through the stratigraphic sequence is known as a ''ramp''.<ref>{{cite book|last=Park|first=R.G.|title=Foundation of Structural Geology|edition=3|page=15|year=2004|publisher=Routledge|isbn=978-0-7487-5802-9|url=https://books.google.com/books?id=ycASqdxSG3YC&pg=PA11}}</ref> Typically, thrust faults move ''within'' formations by forming flats and climbing up sections with ramps. This results in the hanging wall flat (or a portion thereof) lying atop the foot wall ramp as shown in the fault-bend fold diagram.

Thrust faults form [[nappe]]s and [[klippe]]n in the large thrust belts. [[Subduction]] zones are a special class of thrusts that form the largest faults on Earth and give rise to the largest earthquakes.

===Oblique-slip faults===
[[File:Oblique slip fault.svg|thumb|Oblique-slip fault]]

A fault which has a component of dip-slip and a component of strike-slip is termed an '''oblique-slip fault'''. Nearly all faults have some component of both dip-slip and strike-slip; hence, defining a fault as oblique requires both dip and strike components to be measurable and significant. Some oblique faults occur within [[Shear (geology)#Transtension|transtensional]] and [[Shear (geology)#Transpression|transpressional]] regimes, and others occur where the direction of extension or shortening changes during the deformation but the earlier formed faults remain active.

The ''hade'' angle is defined as the [[Angle#complementary angle|complement]] of the dip angle; it is the angle between the fault plane and a vertical plane that strikes parallel to the fault.

===Ring fault===
'''Ring faults''', also known as '''caldera faults''', are faults that occur within collapsed volcanic [[caldera]]s<ref name=":0">{{cite web|url=http://maps.unomaha.edu/Maher/geo330/melissa1.html|title=Structural Geology Notebook – Caldera Faults.|website=maps.unomaha.edu|access-date=2018-04-06|archive-url=https://web.archive.org/web/20181119172858/http://maps.unomaha.edu/Maher/geo330/melissa1.html|archive-date=2018-11-19|url-status=live}}</ref> and the sites of [[bolide]] strikes, such as the [[Chesapeake Bay impact crater]]. Ring faults are the result of a series of overlapping normal faults, forming a circular outline. Fractures created by ring faults may be filled by [[ring dike]]s.<ref name=":0" />

===Synthetic and antithetic faults===
'''''Synthetic''''' and '''''antithetic''''' are terms used to describe minor faults associated with a major fault. Synthetic faults dip in the same direction as the major fault while the antithetic faults dip in the opposite direction. These faults may be accompanied by [[rollover anticlines]] (e.g. the [[Niger Delta]] structural style).