Editing Triple junction

{{Short description|Meeting point of three tectonic plates}}
{{For|the solar cell configuration|Multi-junction solar cell}}
{{Distinguish|triple point|triple divide}}

[[File:Triplice giunzione RTF casuale.png|thumb|Simplified map of a ridge (R)–trench (T)–transform fault (F) triple junction of tectonic plates A, B, and C, with arrows indicating direction of plate movement]]
A '''triple junction''' is the point where the boundaries of three [[list of tectonic plates|tectonic plates]] meet. At the triple junction each of the three boundaries will be one of three types – a [[mid-ocean ridge|ridge]] (R), [[oceanic trench|trench]] (T) or [[transform fault]] (F) – and triple junctions can be described according to the types of plate margin that meet at them (e.g. fault–fault–trench, ridge–ridge–ridge, or abbreviated F-F-T, R-R-R). Of the ten possible types of triple junctions only a few are [[Triple junction stability|stable]] through time (''stable'' in this context means that the geometrical configuration of the triple junction will not change through geologic time). The meeting of four or more plates is also theoretically possible, but junctions will only exist instantaneously.<ref name="Fowler2005">{{cite book |author1=C. M. R. Fowler |author2=Connie May Fowler |author3=Clarence Mary R. Fowler |title=The Solid Earth: An Introduction to Global Geophysics |url=https://books.google.com/books?id=PifkAotvTroC&pg=PA26|year=2005 |publisher=Cambridge University Press |isbn=978-0-521-58409-8 |page=26}}</ref>

==History==
The first scientific paper detailing the triple-junction concept was published in 1969 by [[Dan McKenzie (professor)|Dan McKenzie]] and [[W. Jason Morgan]].<ref>{{cite journal|title=Evolution of Triple Junctions|first1=D. P.|last1=McKenzie|first2=W. J.|last2=Morgan|journal=[[Nature (journal)|Nature]]|volume=224|issue=5215|pages=125–133|date=11 October 1969|doi=10.1038/224125a0|bibcode = 1969Natur.224..125M |s2cid=4151329}}</ref> The term had traditionally been used for the intersection of three divergent boundaries or spreading ridges. These three divergent boundaries ideally meet at near 120° angles.

In [[plate tectonics]] theory during the breakup of a continent, three divergent boundaries form, radiating out from a central point (the triple junction). One of these divergent plate boundaries fails (see [[aulacogen]]) and the other two continue spreading to form an ocean. The [[Seafloor spreading|opening]] of the south [[Atlantic Ocean]] started at the south of the [[South America]]n and [[Africa]]n continents, reaching a triple junction in the present [[Gulf of Guinea]], from where it continued to the west. The NE-trending [[Benue Trough]] is the failed arm of this junction.<ref name=Petters1978>{{cite journal
 |jstor=30061985
 |title=Stratigraphic Evolution of the Benue Trough and Its Implications for the Upper Cretaceous Paleogeography of West Africa
 |author=S. W. Petters
 |journal=The Journal of Geology
 |volume=86| issue =  3 
 |date=May 1978
 |pages=311–322|doi=10.1086/649693
 |bibcode=1978JG.....86..311P|s2cid=129346979
 }}</ref>

In the years since, the term triple-junction has come to refer to any point where three tectonic plates meet.

==Interpretation==
The properties of triple junctions are most easily understood from the purely kinematic point of view where the plates are rigid and moving over the surface of the Earth. No knowledge of the Earth's interior or the geological details of the crust are then needed. Another useful simplification is that the kinematics of triple junctions on a flat Earth are essentially the same as those on the surface of a sphere. On a sphere, plate motions are described as relative rotations about [[Euler pole]]s (see [[Plate reconstruction]]), and the relative motion at every point along a plate boundary can be calculated from this rotation. But the area around a triple junction is small enough (relative to the size of the sphere) and (usually) far enough from the pole of rotation, that the relative motion across a boundary can be assumed to be constant along that boundary. Thus, analysis of triple junctions can usually be done on a flat surface with motions defined by vectors.

==Stability==
[[File:Points triples entre plaques continentales.svg|thumb|upright=1.85|Main [[list of tectonic plates|tectonic plate]] boundaries – ridge (red), trench (green), transform fault (black) – and corresponding triple junctions (yellow dots)]]
Triple junctions may be described and their stability assessed without use of the geological details but simply by defining the properties of the [[Mid-ocean ridge|ridges]], [[oceanic trench|trenches]] and [[transform faults]] involved, making some simplifying assumptions and applying simple velocity calculations. This assessment can generalise to most actual triple junction settings provided the assumptions and definitions broadly apply to the real Earth.

A stable junction is one at which the geometry of the junction is retained with time as the plates involved move. This places restrictions on relative velocities and plate boundary orientation. An unstable triple junction will change with time, either to become another form of triple junction (RRF junctions easily evolve to FFR junctions), will change geometry or are simply not feasible (as in the case of FFF junctions). The inherent instability of an FFF junction is believed to have caused the formation of the [[Pacific plate]] about 190 million years ago.<ref>{{Cite journal|last1=Boschman|first1=Lydian M.|last2=Hinsbergen|first2=Douwe J. J. van|date=2016-07-01|title=On the enigmatic birth of the Pacific plate within the Panthalassa Ocean|journal=Science Advances|volume=2|issue=7|pages=e1600022|doi=10.1126/sciadv.1600022|pmid=29713683|pmc=5919776|bibcode=2016SciA....2E0022B|issn=2375-2548|doi-access=free}}</ref>

By assuming that plates are rigid and that the Earth is spherical, [[Leonhard Euler]]'s theorem of [[Euler's rotation theorem|motion on a sphere]] can be used to reduce the stability assessment to determining boundaries and relative motions of the interacting plates. The rigid assumption holds very well in the case of [[oceanic crust]], and the radius of the Earth at the equator and poles only varies by a factor of roughly one part in 300 so the Earth approximates very well to a sphere.

[[Dan McKenzie (geophysicist)|McKenzie]] and [[W. Jason Morgan|Morgan]]<ref name="auto">Evolution of Triple Junctions, McKenzie, D. P., and Morgan, W. J., Nature, 224, 125 (1969)</ref> first analysed the stability of triple junctions using these assumptions with the additional assumption that the Euler poles describing the motions of the plates were such that they approximated to straight line motion on a flat surface. This simplification applies when the Euler poles are distant from the triple junction concerned. The definitions they used for R, T and F are as follows:

* R – structures that produce [[lithosphere]] symmetrically and perpendicular to the relative velocity of the plates on either side (this does not always apply, for example in the [[Gulf of Aden]]).
* T – structures that consume lithosphere from one side only. The relative velocity vector can be oblique to the plate boundary.
* F – [[active fault]]s parallel to the slip vector.

=== Stability criteria ===
{{Unreferenced section|date=August 2021}}

For a triple junction between the plates A, B and C to exist, the following condition must be satisfied:

:<sub>A</sub>v<sub>B</sub> + <sub>B</sub>v<sub>C</sub> + <sub>C</sub>v<sub>A</sub> = 0

where <sub>A</sub>v<sub>B</sub> is the relative motion of B with respect to A.

This condition can be represented in velocity space by constructing a velocity triangle ABC where the lengths AB, BC and CA are proportional to the velocities <sub>A</sub>v<sub>B</sub>, <sub>B</sub>v<sub>C</sub> and <sub>C</sub>v<sub>A</sub> respectively.

Further conditions must also be met for the triple junction to exist stably – the plates must move in a way that leaves their individual geometries unchanged. Alternatively the triple junction must move in such a way that it remains on all three of the plate boundaries involved.

[[Dan McKenzie (geophysicist)|McKenzie]] and [[W. Jason Morgan|Morgan]]<ref name="auto"/> demonstrated that these criteria can be represented on the same velocity space diagrams in the following way. The lines ab, bc and ca join points in velocity space which will leave the geometry of AB, BC and CA unchanged. These lines are the same as those that join points in velocity space at which an observer could move at the given velocity and still remain on the plate boundary. When these are drawn onto the diagram containing the velocity triangle these lines must be able to meet at a single point, for the triple junction to exist stably.

These lines necessarily are parallel to the plate boundaries as to remain on the plate boundaries the observer must either move along the plate boundary or remain stationary on it.

* For a [[Mid-ocean ridge|ridge]] the line constructed must be the perpendicular bisector of the relative motion vector as to remain in the middle of the ridge an observer would have to move at half the relative speeds of the plates either side but could also move in a perpendicular direction along the plate boundary.
* For a [[transform fault]] the line must be parallel to the relative motion vector as all of the motion is parallel to the boundary direction and so the line ab must lie along AB for a transform fault separating the plates A and B.
* For an observer to remain on a [[oceanic trench|trench]] boundary they must walk along the strike of the trench but remaining on the overriding plate. Therefore, the line constructed will lie parallel to the plate boundary but passing through the point in velocity space occupied by the overriding plate.

The point at which these lines meet, J, gives the overall motion of the triple junction with respect to the Earth.

Using these criteria it can easily be shown why the FFF triple junction is not stable: the only case in which three lines lying along the sides of a triangle can meet at a point is the trivial case in which the triangle has sides lengths zero, corresponding to zero relative motion between the plates. As faults are required to be active for the purpose of this assessment, an FFF junction can never be stable.

==Types==
McKenzie and Morgan determined that there were 16 types of triple junction theoretically possible, though several of these are speculative and have not necessarily been seen on Earth. These junctions were classified firstly by the types of plate boundaries meeting – for example RRR, TTR, RRT, FFT etc. – and secondly by the relative motion directions of the plates involved. Some configurations such as RRR can only have one set of relative motions whereas TTT junctions may be classified into TTT(a) and TTT(b). These differences in motion direction affect the stability criteria.

McKenzie and Morgan claimed that of these 16 types, 14 were stable with FFF and RRF configurations unstable, however, York<ref>{{cite journal | last=York | first=Derek | title=Evolution of Triple Junctions | journal=Nature | volume=244 | issue=5415 | year=1973 | issn=0028-0836 | doi=10.1038/244341a0 | pages=341–342| bibcode=1973Natur.244..341Y | s2cid=4202607 }}</ref> later showed that the RRF configuration could be stable under certain conditions.

===Ridge–ridge–ridge junctions===
[[Image:Tectonical map of East Africa.png|thumb|alt=A map of the Afar triangle, showing the East of Africa and the three ridges passing through the Red Sea, the Gulf of Aden and the East African Rift Valley.|A map of the [[Afar Depression|Afar triangle]] in East Africa, an example of an RRR junction and the only triple junction on Earth that can be seen above sea level.]]

An RRR junction is always stable using these definitions and therefore very common on Earth, though in a geological sense ridge spreading is usually discontinued in one direction leaving a [[aulacogen|failed rift zone]]. There are many examples of these present both now and in the geological past such as the South Atlantic opening with ridges spreading North and South to form the [[Mid-Atlantic Ridge]], and an associated [[aulacogen]], the [[Benue Trough]], in the [[Niger Delta]] region of Africa. RRR junctions are also common as rifting along three fractures at 120° is the best way to relieve stresses from uplift at the surface of a sphere; on Earth, stresses similar to these are believed to be caused by the mantle [[Hotspot (geology)|hotspots]] thought to initiate rifting in continents.

The stability of RRR junctions is demonstrated below – as the perpendicular bisectors of the sides of a triangle always meet at a single point, the lines ab, bc and ca can always be made to meet regardless of relative velocities.

===Ridge–trench–fault junctions===
RTF junctions are less common, an unstable junction of this type (an RTF(a)) is thought to have existed at roughly 12[[Mya (unit)|Ma]] at the mouth of the [[Gulf of California]] where the [[East Pacific Rise]] currently meets the [[San Andreas Fault]] zone.<ref>{{cite web |url=http://www.orfeus-eu.org/Announcements/workshop_utrecht/presentations/zhang.pdf |title=Archived copy |access-date=2009-11-21 |url-status=dead |archive-url=https://web.archive.org/web/20110727153513/http://www.orfeus-eu.org/Announcements/workshop_utrecht/presentations/zhang.pdf |archive-date=2011-07-27 }}</ref> The Guadeloupe and Farallon microplates were previously being subducted under the [[North American plate]] and the northern end of this boundary met the [[San Andreas Fault]]. Material for this subduction was provided by a ridge equivalent to the modern [[East Pacific Rise]] slightly displaced to the west of the trench. As the ridge itself was subducted an RTF triple junction momentarily existed but subduction of the ridge caused the subducted lithosphere to weaken and 'tear' from the point of the triple junction. The loss of [[slab pull]] caused by the detachment of this lithosphere ended the RTF junction giving the present day ridge – fault system. An RTF(a) is stable if ab goes through the point in velocity space C, or if ac and bc are colinear.

===Trench–trench–trench junctions===
A TTT(a) junction can be found in central Japan where the [[Eurasian plate]] overrides the [[Philippine Plate|Philippine]] and [[Pacific plate]]s, with the Philippine plate also overriding the Pacific. Here the [[Japan Trench]] effectively branches to form the Ryukyu and Bonin [[Volcanic arc|arcs]]. The stability criteria for this type of junction are either ab and ac form a straight line or that the line bc is parallel to CA.

==Examples==
[[Image:Nootka Fault.gif|right|thumb|300px|The [[Nootka Fault]] at the triple junction of the [[North American plate]], the [[Explorer plate]], and the [[Juan de Fuca plate]]]]
[[File:Americas Tectonic Plate Map - by NOAA.jpg|thumb|Chile triple juncton (CTJ)]]

*The junction of the [[Red Sea]], the [[Gulf of Aden]] and the [[East African Rift]] centered in the [[Afar Triangle]] (the [[Afar triple junction]]) is the only R-R-R triple junction above sea level.
*The [[Rodrigues triple junction]] is a R-R-R triple junction in the southern Indian Ocean, where the [[African plate|African]], the [[Indo-Australian plate|Indo-Australian]] and the [[Antarctic plate|Antarctic]] Plates meet.<ref>{{cite journal|title=Propagation of the Southwest Indian Ridge at the Rodrigues Triple Junction|first1=D.|last1=Sauter|first2=V.|last2=Mendel|first3=C.|last3=Rommeveaux-Jestin|year=1997|journal=Journal Marine Geophysical Researches|volume=19|issue=6|pages=553–567|doi=10.1023/A:1004313109111|bibcode=1997MarGR..19..553S|s2cid=127866775}}</ref>
*The [[Galapagos triple junction]] is an R-R-R triple junction where the [[Nazca plate|Nazca]], the [[Cocos plate|Cocos]], and the [[Pacific plate]]s meet. The [[East Pacific Rise]] extends north and south from this junction and the [[Cocos–Nazca spreading centre]] goes to the east. This example is made more complex by the [[Galapagos Microplate]] which is a small separate plate on the rise just to the southeast of the triple junction.
*[[Chiapas]] coast off Tapachula where [[Guatemala]], [[North America]] and [[Pacific]] join and small earthquakes occur weekly. This is pushed eastward by the Cocos plate. 
*On the west coast of North America is another unstable triple junction offshore of [[Cape Mendocino]]. To the south, the [[San Andreas Fault]], a strike-slip fault and transform plate boundary, separates the [[Pacific plate]] and the [[North American plate]]. To the north lies the [[Cascadia subduction zone]], where a section of the [[Juan de Fuca plate]] called the [[Gorda plate]] is being subducted under the [[North American plate]], forming a trench (T). Another transform fault, the [[Mendocino Fault]] (F), runs along the boundary between the Pacific plate and the Gorda plate. Where the three intersect is the seismically active, F-F-T [[Mendocino triple junction]].
*The [[Amurian plate]], the [[Okhotsk microplate]], and the [[Philippine Sea plate]] meet in Japan near [[Mount Fuji]]. (see [[Mount Fuji#Geology|Mount Fuji's Geology]])
*The [[Azores triple junction]] is a geologic triple junction where the boundaries of three tectonic plates intersect: the North American plate, the Eurasian plate and the African plate, R-R-R.<ref>{{Citation|last1=Carracedo|first1=Juan Carlos|title=North-East Atlantic Islands: The Macaronesian Archipelagos|date=2021-01-01|url=https://www.sciencedirect.com/science/article/pii/B9780081029084000278|encyclopedia=Encyclopedia of Geology (Second Edition)|pages=674–699|editor-last=Alderton|editor-first=David|place=Oxford|publisher=Academic Press|language=en|doi=10.1016/b978-0-08-102908-4.00027-8|isbn=978-0-08-102909-1|access-date=2021-03-18|last2=Troll|first2=Valentin R.|s2cid=226588940|editor2-last=Elias|editor2-first=Scott A.|url-access=subscription}}</ref>
*The [[Boso triple junction]] offshore of Japan is a T-T-T triple junction between the [[Okhotsk microplate]], [[Pacific plate]] and [[Philippine Sea plate]].
*The [[North Sea]] is located at the extinct triple junction of three former continental plates of  the [[Palaeozoic]] era: [[Avalonia]], [[Laurentia]] and [[Baltica]].<ref>{{cite journal|url=http://bullard.esc.cam.ac.uk/~basin/pubs/033.pdf|title=Subsidence analyses from the North Sea 'triple-junction'|first1=N.|last1=White|first2=D.|last2=Latin|year=1993|journal=Journal of the Geological Society|volume=150|number=3|pages=473–488|doi=10.1144/gsjgs.150.3.0473|bibcode=1993JGSoc.150..473W|s2cid=129832756|url-status=dead|archive-url=https://web.archive.org/web/20110812225718/http://bullard.esc.cam.ac.uk/~basin/pubs/033.pdf|archive-date=2011-08-12}}</ref>
*The [[South Greenland triple junction]] was an R-R-R triple junction where the Eurasian, [[Greenland plate|Greenland]] and North American plates diverged during the [[Paleogene]].<ref>{{cite journal | last1 = Oakey | first1 = Gordon N. | last2 = Stephenson | first2 = Randell | title = Crustal structure of the Innuitian region of Arctic Canada and Greenland from gravity modelling: implications for the Palaeogene Eurekan orogen | journal = [[Geophysical Journal International]] | volume = 173 | page = 1041 | publisher = [[Royal Astronomical Society]] | year = 2008 | issue = 3 | doi = 10.1111/j.1365-246X.2008.03784.x | bibcode = 2008GeoJI.173.1039O | issn = 0956-540X | url = https://research.vu.nl/ws/files/2330136/209534.pdf | doi-access = free }}</ref>
*The [[Chile triple junction]] is where the [[South American plate]], the [[Nazca plate]], and the [[Antarctic plate]] meet.

==See also==
* {{annotated link|Seafloor spreading}}

==References==
{{reflist}}
* [[Naomi Oreskes|Oreskes, Naomi]], ed., 2003, ''Plate Tectonics: an Insider's History of the Modern Theory of the Earth'', Westview Press, {{ISBN|0-8133-4132-9}}

{{Triple Junctions}}

[[Category:Triple junctions| ]]
[[Category:Plate tectonics]]