Editing Transform fault

{{Short description|Plate boundary where the motion is predominantly horizontal}}
[[File:Continental-continental conservative plate boundary opposite directions.svg|thumb|Diagram showing a transform fault with two plates moving in opposite directions]]
[[Image:Transform fault-1.svg|thumb|Transform fault (the red lines)]]
A '''transform fault''' or ''' transform boundary''', is a [[fault (geology)|fault]] along a [[plate boundary]] where the [[motion (physics)|motion]] is predominantly [[Horizontal plane|horizontal]].<ref name="MooresTwiss">{{Cite book |url=https://books.google.com/books?id=tvgRBAAAQBAJ&pg=PA130|title=Tectonics |last1=Moores E.M. |last2=Twiss R.J. |date=2014 |publisher=Waveland Press |isbn=978-1-4786-2660-2|page=130}}</ref> It ends abruptly where it connects to another plate boundary, either another transform, a spreading ridge, or a [[subduction|subduction zone]].<ref name=Kerey>{{cite book|last=Kearey|first=K. A.|title=Global Tectonics|year=2007|publisher=John Wiley & Sons|location=Hoboken, NJ, USA|pages=84–90}}</ref> A transform fault is a special case of a ''[[strike-slip fault]]'' that also forms a plate boundary.

Most such faults are found in [[oceanic crust]], where they accommodate the lateral offset between segments of [[Divergent boundary|divergent boundaries]], forming a [[zigzag]] pattern. This results from oblique [[seafloor spreading]] where the direction of motion is not perpendicular to the trend of the overall divergent boundary. A smaller number of such faults are found on land, although these are generally better-known, such as the [[San Andreas Fault]] and [[North Anatolian Fault]].

==Nomenclature==
Transform boundaries are also known as '''conservative plate boundaries''' because they involve no addition or loss of [[lithosphere]] at the Earth's surface.<ref name="BGS">{{Cite web |url=https://www.bgs.ac.uk/discoveringGeology/hazards/earthquakes/plateTectonics.html |title=Plate Tectonics |publisher=British Geological Survey|date=2020 |access-date=16 February 2020}}</ref>

== Background ==
Geophysicist and geologist [[John Tuzo Wilson]] recognized that the offsets of [[oceanic ridges]] by faults do not follow the classical pattern of an offset fence or geological marker in [[Harry Fielding Reid|Reid's]] [[Elastic-rebound theory|rebound theory of faulting]],<ref>Reid, H.F., (1910). The Mechanics of the Earthquake. in The California Earthquake of April 18, 1906, Report of the State Earthquake Investigation Commission, Carnegie Institution of Washington, Washington D.C.</ref> from which the sense of slip is derived.  The new class of faults,<ref name=Wilson>{{Cite journal|first=J.T. |last=Wilson |date=24 July 1965 |title=A new class of faults and their bearing on continental drift|journal=Nature |volume=207 |issue=4995 |pages=343–347 |doi=10.1038/207343a0 |bibcode = 1965Natur.207..343W |s2cid=4294401 }}</ref> called transform faults, produce slip in the opposite direction from what one would surmise from the standard interpretation of an offset geological feature.  Slip along transform faults does not increase the distance between the ridges it separates; the distance remains constant in [[earthquakes]] because the ridges are spreading centers.  This [[hypothesis]] was confirmed in a study of the fault plane solutions that showed the slip on transform faults points in the opposite direction than classical interpretation would suggest.<ref>Sykes, L.R. (1967). Mechanism of earthquakes and nature of faulting on the mid-oceanic ridges, Journal of Geophysical Research, 72, 5–27.</ref>

==Difference between transform and transcurrent faults==
 {{multiple image
 | direction = vertical
 | width = 250
 | footer =
 | image1 = Transform fault.svg
 | alt1 =
 | caption1 = Transform fault
 | image2 = Transcurrent NEW.svg
 | alt2 =
 | caption2 = Transcurrent fault
 }}
Transform faults are closely related to transcurrent faults and are commonly confused. Both types of fault are strike-slip or side-to-side in movement; nevertheless, transform faults always end at a junction with another plate boundary, while transcurrent faults may die out without a junction with another fault. Finally, transform faults form a tectonic plate boundary, while transcurrent faults do not.

==Mechanics==
Faults in general are focused areas of deformation or [[Strain (materials science)|strain]], which are the response of built-up stresses in the form of [[Compression (physics)|compression]], tension, or [[shear stress]] in rock at the surface or deep in the Earth's subsurface.  Transform faults specifically accommodate [[lateral strain]] by transferring displacement between mid-ocean ridges or subduction zones. They also act as the plane of weakness, which may result in splitting in [[rift zone]]s.{{citation needed|date=February 2020}}

==Transform faults and divergent boundaries==
Transform faults are commonly found linking segments of divergent boundaries ([[mid-oceanic ridge]]s or spreading centres).   These mid-oceanic ridges are where new seafloor is constantly created through the [[upwelling]] of new [[basaltic]] [[magma]].  With new seafloor being pushed and pulled out, the older seafloor slowly slides away from the mid-oceanic ridges toward the continents.  Although separated only by tens of kilometers, this separation between segments of the ridges causes portions of the seafloor to push past each other in opposing directions. This lateral movement of seafloors past each other is where transform faults are currently active.  
[[File:Spreading center and strips.png|thumb|upright=1.35|Spreading center and strips]]

Transform faults move differently from a strike-slip fault at the mid-oceanic ridge.   Instead of the ridges moving away from each other, as they do in other strike-slip faults, transform-fault ridges remain in the same, fixed locations, and the new ocean seafloor created at the ridges is pushed away from the ridge. Evidence of this motion can be found in paleomagnetic striping on the seafloor.

A paper written by geophysicist Taras Gerya theorizes that the creation of the transform faults between the ridges of the mid-oceanic ridge is attributed to rotated and stretched sections of the mid-oceanic ridge.<ref name=Gerya>{{cite journal|last=Gerya|first=T.|title=Dynamical Instability Produces Transform Faults at Mid-Ocean Ridges|journal=Science|year=2010|volume=329|issue=5995|pages=1047–1050| doi = 10.1126/science.1191349 |bibcode = 2010Sci...329.1047G|pmid=20798313|s2cid=10943308}}</ref> This occurs over a long period of time with the spreading center or ridge slowly deforming from a straight line to a curved line. Finally, fracturing along these planes forms transform faults. As this takes place, the fault changes from a normal fault with extensional stress to a strike-slip fault with lateral stress.<ref name=Bonatti>{{cite journal|author1=Bonatti, Enrico|author2=Crane, Kathleen|title=Oceanic Fracture Zones|journal=Scientific American|year=1984|volume=250|issue=5|pages=40–52|doi=10.1038/scientificamerican0584-40|bibcode=1984SciAm.250e..40B}}</ref> In the study done by Bonatti and Crane,{{who|date=November 2017}} [[peridotite]] and [[gabbro]] rocks were discovered in the edges of the transform ridges.  These rocks are created deep inside the Earth's mantle and then rapidly exhumed to the surface.<ref name=Bonatti/> This evidence helps to prove that new seafloor is being created at the mid-oceanic ridges and further supports the theory of plate tectonics.

Active transform faults are between two tectonic structures or faults. [[Fracture zone]]s represent the previously active transform-fault lines, which have since passed the active transform zone and are being pushed toward the continents.  These elevated ridges on the ocean floor can be traced for hundreds of miles and in some cases even from one continent across an ocean to the other continent.

==Types==
In his work on transform-fault systems, geologist Tuzo Wilson said that transform faults must be connected to other faults or tectonic-plate boundaries on both ends; because of that requirement, transform faults can grow in length, keep a constant length, or decrease in length.<ref name=Wilson/> These length changes are dependent on which type of fault or tectonic structure connect with the transform fault. Wilson described six types of transform faults:

''Growing length:''  In situations where a transform fault links a spreading center and the upper block of a subduction zone or where two upper blocks of subduction zones are linked, the transform fault itself will grow in length.<ref name=Wilson/>

[[File:Spreading to upper NEW.svg|Spreading to upper NEW]] [[File:Upper to upper.svg|Upper to upper]]

''Constant length:'' In other cases, transform faults will remain at a constant length.  This steadiness can be attributed to many different causes.  In the case of ridge-to-ridge transforms, the constancy is caused by the continuous growth by both ridges outward, canceling any change in length.  The opposite occurs when a ridge linked to a subducting plate, where all the lithosphere (new seafloor) being created by the ridge is subducted, or swallowed up, by the subduction zone.<ref name=Wilson/> Finally, when two upper subduction plates are linked there is no change in length. This is due to the plates moving parallel with each other and no new lithosphere is being created to change that length.

[[File:Spreading centers constant.svg|Spreading centers constant]] [[File:Upper to down NEW.svg|Upper to down NEW]]

Decreasing length faults: In rare cases, transform faults can shrink in length.  These occur when two descending subduction plates are linked by a transform fault.  In time as the plates are subducted, the transform fault will decrease in length until the transform fault disappears completely, leaving only two subduction zones facing in opposite directions.<ref name=Wilson/>

[[File:Down to down NEW.svg|Down to down NEW]] [[File:Spreading to Down NEW.svg|Spreading to Down NEW]]

== Examples ==
[[File:Tectonic plates (2022).svg|thumb|upright=1.8|Map of Earth's principal plates (transform boundaries shown as yellow or green lines)]]
The most prominent examples of the mid-oceanic ridge transform zones are in the [[Atlantic Ocean]] between [[South America]] and [[Africa]].  Known as the St. Paul, [[Romanche Trench|Romanche]], Chain, and Ascension fracture zones, these areas have deep, easily identifiable transform faults and ridges.  Other locations include: the East Pacific Ridge located in the South Eastern [[Pacific Ocean]], which meets up with [[San Andreas Fault]] to the North.

Transform faults are not limited to oceanic crust and spreading centers; many of them are on [[continental margin]]s.  The best example is the [[San Andreas Fault]] on the Pacific coast of the United States.  The San Andreas Fault links the [[East Pacific Rise]] off the West coast of Mexico (Gulf of California) to the [[Mendocino triple junction]] (Part of the [[Juan de Fuca plate]]) off the coast of the [[Northwestern United States]], making it a ridge-to-transform-style fault.<ref name=Wilson/> The formation of the San Andreas Fault system occurred fairly recently during the [[Oligocene]] Period between 34 million and 24 million years ago.<ref name=Atwater>{{cite journal|last=Atwater|first=Tanya|author-link=Tanya Atwater|title= Implications of Plate Tectonics for the Cenozoic Tectonic Evolution of Western North America |journal=Bulletin of the Geological Society of America|year=1970|volume=81|issue=12|pages=3513–3536|doi=10.1130/0016-7606(1970)81[3513:ioptft]2.0.co;2|bibcode=1970GSAB...81.3513A}}</ref> During this period, the [[Farallon plate]], followed by the Pacific plate, collided into the [[North American plate]].<ref name=Atwater/> The collision led to the subduction of the Farallon plate underneath the North American plate.  Once the spreading center separating the Pacific and the Farallon plates was subducted beneath the North American plate, the San Andreas Continental Transform-Fault system was created.<ref name=Atwater/>

[[Image:Alpine Fault SRTM.jpg|thumb|upright=1.8|The [[Southern Alps]] rise dramatically beside the [[Alpine Fault]] on [[New Zealand]]'s [[West Coast, New Zealand|West Coast]]. About 500 kilometres (300&nbsp;mi) long; northwest at top.]]
In [[New Zealand]], the [[South Island]]'s [[Alpine Fault]] is a transform fault for much of its length. This has resulted in the folded land of the [[Southland Syncline]] being split into an eastern and western section several hundred kilometres apart. The majority of the syncline is found in [[Southland, New Zealand|Southland]] and [[The Catlins]] in the island's southeast, but a smaller section is also present in the [[Tasman District]] in the island's northwest.

Another example is the Húsavík‐Flatey fault. This oceanic transform fault is nearly completely submerged, but ~10 km is exposed in northern Iceland, near the town of [[Húsavík]]. There, it manifests as a series of [[Half-graben|half-grabens]] and sharp [[Fault scarp|fault scarps]]. Since oceanic transform faults are often difficult to research because of their submerged nature, this fault represents a rare opportunity for research. Scientists inspected [[Holocene]] earthquake activity by looking cross sections of the fault, and found the approximate earthquake frequency in the region to be 600 ± 200 years.<ref>{{Cite journal |last=Matrau |first=Rémi |last2=Klinger |first2=Yann |last3=Thorðarson |first3=Thorvaldur |last4=Guðmundsdóttir |first4=Esther R. |last5=Avşar |first5=Ulas |last6=Parisi |first6=Laura |last7=Fittipaldi |first7=Margherita |last8=Jónsson |first8=Sigurjón |date=2024-03-28 |title=Evidence for Holocene Earthquakes along the Húsavík‐Flatey Fault in North Iceland: Implications for the Seismic Behavior of Oceanic Transform Faults |url=https://pubs.geoscienceworld.org/ssa/bssa/article-abstract/114/4/1966/637048/Evidence-for-Holocene-Earthquakes-along-the?redirectedFrom=fulltext |journal=Bulletin of the Seismological Society of America |volume=114 |issue=4 |pages=1966–1993 |doi=10.1785/0120230119 |issn=0037-1106|url-access=subscription }}</ref> 

Other examples include:
* [[Middle East]]'s [[Dead Sea Transform]] Fault
* [[Pakistan]]'s [[Chaman Fault]]
* [[Turkey]]'s [[North Anatolian Fault]]
* [[North America]]'s [[Queen Charlotte Fault]]
* [[Myanmar]]'s [[Sagaing Fault]]

== See also ==
* {{annotated link|Fracture zone}}
* {{annotated link|Leaky transform fault}}
* {{annotated link|List of tectonic plate interactions}}
* {{annotated link|Plate tectonics}}
* {{annotated link|Strike-slip tectonics}}
* {{annotated link|Structural geology}}

==References==
{{Reflist}}

{{physical oceanography|expanded=other}}

[[Category:Structural geology]]
[[Category:Plate tectonics]]
[[Category:Faults (geology)]]
[[Category:Strike-slip earthquakes]]