Editing Rift

{{Short description|Geological linear zone where the lithosphere is being pulled apart}}
{{Redirect|Chasm}}
{{About|the geological concept}}
{{Distinguish|rift zone}}

[[File:Rift segmentation.png|thumb|240px|Block view of a rift formed of three segments, showing the location of the accommodation zones between them at changes in fault location or polarity (dip direction)]]

[[File:GulfofSuezRift.png|200px|thumb|[[Gulf of Suez Rift]] showing main [[extensional fault]]s]]

In [[geology]], a '''rift''' is a linear zone where the [[lithosphere]] is being pulled apart<ref>[http://ethiopianrift.igg.cnr.it/rift%20valley%20significance.htm Rift valley: definition and geologic significance], Giacomo Corti, The Ethiopian Rift Valley</ref><ref>[http://www.mantleplumes.org/VM_DecompressMelt.html Decompressional Melting During Extension of Continental Lithosphere], Jolante van Wijk, MantlePlumes.org</ref> and is an example of [[extensional tectonics]].<ref>[http://www.le.ac.uk/geology/art/gl209/lecture4/lecture4.html Plate Tectonics: Lecture 2], Geology Department at University of Leicester</ref> Typical rift features are a central linear [[Fault (geology)|downfaulted]] depression, called a [[graben]], or more commonly a [[half-graben]] with normal faulting and rift-flank uplifts mainly on one side.<ref name="Leeder_&_Gawthorpe_1987">{{Cite book |last1=Leeder |first1=M.R. |title=Continental Extensional Tectonics |last2=Gawthorpe |first2=R.L. |isbn=9780632016051 |editor-last1=Coward |editor-first1=M.P. |series=Geological Society, Special Publications |volume=28 |date=1987 |pages=139–152 |chapter=Sedimentary models for extensional tilt-block/half-graben basins  |editor-last2=Dewey |editor-first2=J.F. |editor-last3=Hancock |editor-first3=P.L. |chapter-url=https://pages.uoregon.edu/rdorsey/BasinAnalysis/BasinPapers/Leeder_&_Gawthorpe_1987.pdf}}</ref> Where rifts remain above sea level they form a [[rift valley]], which may be filled by water forming a [[rift lake]]. The axis of the rift area may contain [[volcanic rocks]], and active [[volcanism]] is a part of many, but not all, active rift systems.

Major rifts occur along the central axis of most [[mid-ocean ridge]]s, where new [[oceanic crust]] and lithosphere is created along a [[divergent boundary]] between two [[tectonic plate]]s.

''Failed rifts'' are the result of continental rifting that failed to continue to the point of break-up.  Typically the transition from rifting to spreading develops at a [[triple junction]] where three converging rifts meet over a [[Hotspot (geology)|hotspot]].  Two of these evolve to the point of seafloor spreading, while the third ultimately fails, becoming an [[aulacogen]].

==Geometry==
[[image:FlankMalawi.png|thumb|left|Topographic profile of the [[Malawi Lake]]]]
Most rifts consist of a series of separate segments that together form the linear zone characteristic of rifts. The individual rift segments have a dominantly half-graben geometry, controlled by a single basin-bounding fault. Segment lengths vary between rifts, depending on the elastic thickness of the lithosphere. 

Areas of thick colder lithosphere, such as the Baikal Rift, have segment lengths in excess of 80&nbsp;km, while in areas of warmer thin lithosphere, segment lengths may be less than 30&nbsp;km.<ref name="Ebinger">{{cite journal|last=Ebinger|first=C.J. |author2=Jackson J.A. |author3=Foster A.N.  |author4=Hayward N.J.|year=1999|title=Extensional basin geometry and the elastic lithosphere|journal=[[Philosophical Transactions of the Royal Society A]]|volume=357|pages=741–765|issue=1753|doi=10.1098/rsta.1999.0351|bibcode=1999RSPTA.357..741E |s2cid=91719117 }}</ref> Along the axis of the rift the position, and in some cases the polarity (the dip direction), of the main rift bounding fault changes from segment to segment. Segment boundaries often have a more complex structure and generally cross the rift axis at a high angle. These segment boundary zones accommodate the differences in fault displacement between the segments and are therefore known as accommodation zones.

Accommodation zones take various forms, from a simple relay ramp at the overlap between two major faults of the same polarity, to zones of high structural complexity, particularly where the segments have opposite polarity. Accommodation zones may be located where older crustal structures intersect the rift axis. In the Gulf of Suez rift, the Zaafarana accommodation zone is located where a [[shear zone]] in the [[Arabian-Nubian Shield]] meets the rift.<ref name="Younes">{{cite journal|last=Younes|first=A.I.|author2=McClay K. |year=2002|title=Development of Accommodation Zones in the Gulf of Suez-Red Sea Rift, Egypt|journal=AAPG Bulletin|volume=86|issue=6|pages=1003–1026|doi=10.1306/61EEDC10-173E-11D7-8645000102C1865D|url=http://aapgbull.geoscienceworld.org/cgi/content/abstract/86/6/1003|access-date=29 October 2012|url-access=subscription}}</ref>

Rift flanks or shoulders are elevated areas around rifts. Rift shoulders are typically about 70&nbsp;km wide.<ref name=Greenetal2018/> Contrary to what was previously thought, elevated passive continental margins (EPCM) such as the [[Brazilian Highlands]], the [[Scandinavian Mountains#Origin|Scandinavian Mountains]] and India's [[Western Ghats]], are not rift shoulders.<ref name=Greenetal2018>{{cite journal |last1=Green |first1=Paul F. |last2=Japsen |first2=Peter |last3=Chalmers |first3=James A.|last4=Bonow |first4=Johan M.|last5=Duddy |first5=Ian R.|date=2018 |title=Post-breakup burial and exhumation of passive continental margins: Seven propositions to inform geodynamic models |journal=[[Gondwana Research]] |volume=53 |pages=58–81 |doi=10.1016/j.gr.2017.03.007 |bibcode=2018GondR..53...58G }}</ref>

==Rift development==

===Rift initiation===
The formation of rift basins and strain localization reflects rift maturity. At the onset of rifting, the upper part of the lithosphere starts to extend on a series of initially unconnected [[normal fault]]s, leading to the development of isolated basins.<ref name="Withjack">{{cite book|last=Withjack|first=M.O. |author2=Schlische R.W. |author3=Olsen P.E.|title=Sedimentation in Continental Rifts|editor=Renaut R.W. & Ashley G.M.|publisher=Society for Sedimentary Geology|year=2002|series=Special Publications|volume=73|chapter=Rift-basin structure and its influence on sedimentary systems|chapter-url=http://www.rci.rutgers.edu/~schlisch/A33_2002_SEPM_rifts.pdf|access-date=28 October 2012}}</ref> In subaerial rifts, for example, drainage at the onset of rifting is generally internal, with no element of through drainage.

===Mature rift stage===
As the rift evolves, some of the individual fault segments grow, eventually becoming linked together to form the larger bounding faults. Subsequent extension becomes concentrated on these faults. The longer faults and wider fault spacing leads to more continuous areas of fault-related [[subsidence]] along the rift axis. Significant uplift of the rift shoulders develops at this stage, strongly influencing drainage and sedimentation in the rift basins.<ref name="Withjack"/>

During the climax of lithospheric rifting, as the crust is thinned, the Earth's surface subsides and the [[Mohorovičić discontinuity|Moho]] becomes correspondingly raised. At the same time, the mantle lithosphere becomes thinned, causing a rise of the top of the asthenosphere. This brings high heat flow from the upwelling asthenosphere into the thinning lithosphere, heating the orogenic lithosphere for dehydration melting, typically causing extreme metamorphism at high thermal gradients of greater than 30&nbsp;°C. The metamorphic products are high to ultrahigh temperature granulites and their associated migmatite and granites in collisional orogens, with possible emplacement of metamorphic core complexes in continental rift zones but oceanic core complexes in spreading ridges. This leads to a kind of orogeneses in extensional settings, which is referred as to rifting orogeny.<ref>{{cite journal | last1 = Zheng | first1 = Y.-F. | last2 = Chen | first2 = R.-X. | year = 2017 | title = Regional metamorphism at extreme conditions: Implications for orogeny at convergent plate margins | journal = Journal of Asian Earth Sciences | volume = 145 | pages = 46–73 | doi = 10.1016/j.jseaes.2017.03.009 | bibcode = 2017JAESc.145...46Z | doi-access = free }}</ref>

===Post-rift subsidence===
Once rifting ceases, the mantle beneath the rift cools and this is accompanied by a broad area of post-rift subsidence. The amount of subsidence is directly related to the amount of thinning during the rifting phase calculated as the beta factor (initial crustal thickness divided by final crustal thickness), but is also affected by the degree to which the rift basin is filled at each stage, due to the greater density of sediments in contrast to water. The simple 'McKenzie model' of rifting, which considers the rifting stage to be instantaneous, provides a good first order estimate of the amount of crustal thinning from observations of the amount of post-rift subsidence.<ref name="McKenzie">{{cite journal|last=McKenzie|first=D.|year=1978|title=Some remarks on the development of sedimentary basins|journal=Earth and Planetary Science Letters|volume=40|issue=1|pages=25–32|url=http://www.earth.ox.ac.uk/~tony/watts/downloads/McKenzie_1978_Basins.pdf|access-date=25 October 2012|doi=10.1016/0012-821x(78)90071-7|archive-url=https://web.archive.org/web/20140301085415/http://www.earth.ox.ac.uk/~tony/watts/downloads/McKenzie_1978_Basins.pdf|archive-date=1 March 2014|citeseerx=10.1.1.459.4779|bibcode=1978E&PSL..40...25M}}</ref><ref name="Kusznir">{{cite book|last=Kusznir|first=N.J. |author2=Roberts A.M. |author3=Morley C.K.|title=Hydrocarbon habitat in rift basins|editor=Lambiase J.J.|publisher=[[Geological Society]]|location=London|year=1995|series=Special Publications|volume=80|pages=33–56|chapter=Forward and reverse modelling of rift basin formation|isbn=9781897799154|chapter-url=http://sp.lyellcollection.org/content/80/1/33.abstract|access-date=25 October 2012}}</ref> This has generally been replaced by the 'flexural cantilever model', which takes into account the geometry of the rift faults and the [[Lithospheric flexure|flexural]] [[isostasy]] of the upper part of the crust.<ref name="Nøttvedt">{{cite journal|last=Nøttvedt|first=A. |author2=Gabrielsen R.H. |author3=Steel R.J.|year=1995|title=Tectonostratigraphy and sedimentary architecture of rift basins, with reference to the northern North Sea|journal=[[Marine and Petroleum Geology]]|volume=12|issue=8|pages=881–901|doi=10.1016/0264-8172(95)98853-W|bibcode=1995MarPG..12..881N }}</ref>

===Multiphase rifting===
Some rifts show a complex and prolonged history of rifting, with several distinct phases. The [[Geology of the North Sea|North Sea rift]] shows evidence of several separate rift phases from the [[Permian]] through to the Earliest [[Cretaceous]],<ref name="Rodmar">{{cite book|last=Ravnås|first=R. |author2=Nøttvedt A. |author3=Steel R.J.  |author4=Windelstad J.|publisher=[[Geological Society]]|location=London|year=2000|series=Special Publications|volume=167|title=Dynamics of the Norwegian Margin|pages=133–177|chapter=Syn-rift sedimentary architectures in the Northern North Sea|isbn=9781862390560|chapter-url=http://sp.lyellcollection.org/content/167/1/133.abstract|access-date=28 October 2012}}</ref> a period of over 100 million years.

===Rifting to break-up===
Rifting may lead to continental breakup and formation of oceanic basins. Successful rifting leads to seafloor spreading along a mid-oceanic ridge and a set of conjugate margins separated by an oceanic basin.<ref>{{cite journal|author1=Ziegler P.A. |author2=Cloetingh S.|title=Dynamic processes controlling evolution of rifted basins |journal=Earth-Science Reviews |date=January 2004 |volume=64 |issue=1–2 |pages=1–50 |doi=10.1016/S0012-8252(03)00041-2|bibcode=2004ESRv...64....1Z }}</ref> Rifting may be active, and controlled by [[mantle convection]]. It may also be passive, and driven by far-field tectonic forces that stretch the lithosphere. Margin architecture develops due to spatial and temporal relationships between extensional deformation phases. Margin segmentation eventually leads to the formation of rift domains with variations of the [[Mohorovičić discontinuity|Moho]] topography, including proximal domain with fault-rotated crustal blocks, necking zone with thinning of crustal [[Basement (geology)|basement]], distal domain with deep sag basins, ocean-continent transition and oceanic domain.<ref name="Marine and Petroleum Geology">{{cite journal|author1=Péron-Pinvidic G.|author2=Manatschal G.|author3=Osmundsen P.T.|title=Structural comparison of archetypal Atlantic rifted margins: a review of observations and concepts|journal=Marine and Petroleum Geology|date=May 2013|volume=43 |pages=21–47 |doi=10.1016/j.marpetgeo.2013.02.002|bibcode=2013MarPG..43...21P }}</ref>

Deformation and magmatism interact during rift evolution. Magma-rich and magma-poor rifted margins may be formed.<ref name="Marine and Petroleum Geology"/> Magma-rich margins include major volcanic features. Globally, volcanic margins represent the majority of passive continental margins.<ref>{{cite book|author1=Reston T.J.|author2=Manatschal G.|title=Building blocks of later collision|editor=Brown D. & Ryan P.D.|publisher=Frontiers in Earth Sciences|year=2011|chapter=Arc-Continent Collision}}</ref> Magma-starved rifted margins are affected by large-scale faulting and crustal hyperextension.<ref>{{cite journal|author1=Péron-Pinvidic G.|author2=Manatschal G.|title=The final rifting evolution at deep magma-poor passive margins from Iberia-Newfoundland: a new point of view|journal=International Journal of Earth Sciences|year=2009|volume=98|issue=7 |page=1581 |doi=10.1007/s00531-008-0337-9 |bibcode=2009IJEaS..98.1581P |s2cid=129442856 }}</ref> As a consequence, upper mantle peridotites and gabbros are commonly exposed and serpentinized along extensional detachments at the seafloor.

==Magmatism==
[[File:20190621 CPH-KEF 7314 (48453477346).jpg|thumb|upright=1.35|Volcano-tectonic landforms connected to rifting on [[Geology of Reykjanes Peninsula|Reykjanes Peninsula]], [[Iceland]]: [[Fault (geology)|fault]]s, [[Fissure vent|fissure]]s, elongated [[volcano]]es of [[Subglacial volcano|subglacial origin]], postglacial [[lava field]]s]]
Many rifts are the sites of at least minor [[Magmatism|magmatic activity]], particularly in the early stages of rifting.<ref name="White">{{cite journal|last=White|first=R.S.|author2=McKenzie D. |year=1989|title=Magmatism at Rift Zones: The Generation of Volcanic Margins and Flood Basalts|journal=Journal of Geophysical Research|volume=94|issue=B6|pages=7685–7729|url=http://www.mantleplumes.org/WebDocuments/White1989.pdf|access-date=27 October 2012|doi=10.1029/jb094ib06p07685|bibcode=1989JGR....94.7685W}}</ref> [[Alkali basalt]]s and [[bimodal volcanism]] are common products of rift-related magmatism.<ref name="Farmer">{{cite book|last=Farmer|first=G.L.|title=Treatise on Geochemistry: The crust|editor=Rudnick R.L.|publisher=Gulf Professional Publishing|year=2005|pages=97|chapter=Continental Basaltic Rocks|isbn=9780080448473|chapter-url=https://books.google.com/books?id=QT52UZxDFwgC&q=%22alkali+basalt%22+rift-related+volcanism&pg=PA98|access-date=28 October 2012}}</ref><ref name="Cas">{{cite book|last=Cas|first=R.A.F.|title=Volcanoes and the Environment|editor=Marti J. & Ernst G.G.|publisher=Cambridge University Press|year=2005|pages=145|chapter=Volcanoes and the geological cycle|isbn=9781139445108|chapter-url=https://books.google.com/books?id=4LswmjBnlJMC&q=bimodal+volcanism+rifting&pg=PA145|access-date=28 October 2012}}</ref>

Recent studies indicate that post-collisional granites in collisional orogens are the product of rifting magmatism at converged plate margins.{{citation needed|date=October 2018}}

==Economic importance==
The sedimentary rocks associated with continental rifts host important deposits of both minerals and [[hydrocarbon]]s.<ref name="USGS">{{cite web|url=http://marine.usgs.gov/fact-sheets/baikal/|title=Lake Baikal - A Touchstone for Global Change and Rift Studies|last=United States Geological Survey|year=1993|access-date=28 October 2012|archive-url=https://web.archive.org/web/20120629180319/http://marine.usgs.gov/fact-sheets/baikal/|archive-date=29 June 2012}}</ref>

===Mineral deposits===
[[Sedimentary exhalative deposits|SedEx]] mineral deposits are found mainly in continental rift settings. They form within post-rift sequences when hydrothermal fluids associated with magmatic activity are expelled at the seabed.<ref name="Groves">{{cite journal|last=Groves|first=D.I.|author2=Bierlein F.P. |year=2007|title=Geodynamic settings of mineral deposit systems|journal=Journal of the Geological Society|volume=164|pages=19–30|url=http://jgs.lyellcollection.org/content/164/1/19.abstract|access-date=27 October 2012|issue=1|doi=10.1144/0016-76492006-065|bibcode=2007JGSoc.164...19G|s2cid=129680970|url-access=subscription}}</ref>

===Oil and gas===
Continental rifts are the sites of significant oil and gas accumulations, such as the [[Geology of the North Sea|Viking Graben]] and the [[Gulf of Suez Rift]]. Thirty percent of [[giant oil and gas fields]] are found within such a setting.<ref name="Mann">{{cite web|url=http://www.worldoil.com/September-2001-Tectonic-setting-of-the-worlds-giant-oil-fields.html|title=Tectonic setting of the world's giant oil fields|last=Mann|first=P. |author2=Gahagan L.  |author3=Gordon M.B.|year=2001|work=WorldOil Magazine|access-date=27 October 2012}}</ref> In 1999 it was estimated that there were 200 [[1000000000 (number)|billion]] [[Barrel (unit)#Oil barrel|barrels]] of recoverable oil reserves hosted in rifts. [[Source rock]]s are often developed within the sediments filling the active rift ([[Tectonostratigraphy#Effects of active tectonics on lithostratigraphy|syn-rift]]), forming either in a [[wikt:lacustrine|lacustrine]] environment or in a restricted marine environment, although not all rifts contain such sequences. [[Petroleum reservoir|Reservoir rocks]] may be developed in pre-rift, syn-rift and post-rift sequences. 

Effective regional seals may be present within the post-rift sequence if [[mudstone]]s or [[evaporite]]s are deposited. Just over half of estimated oil reserves are found associated with rifts containing marine syn-rift and post-rift sequences, just under a quarter in rifts with a non-marine syn-rift and post-rift, and an eighth in non-marine syn-rift with a marine post-rift.<ref name="Lambiase">{{cite journal|last=Lambiase|first=J.J.|author2=Morley C.K. |year=1999|title=Hydrocarbons in rift basins: the role of stratigraphy|journal=[[Philosophical Transactions of the Royal Society A]]|volume=357|issue=1753|pages=877–900|doi=10.1098/rsta.1999.0356 |bibcode=1999RSPTA.357..877L|s2cid=129564482|citeseerx=10.1.1.892.6422}}</ref>

==Examples==
* The [[Asunción Rift]] in Eastern Paraguay
* The [[Canadian Arctic Rift System]] in northern [[North America]]
* The [[East African Rift]]
* The [[West and Central African Rift System]]
* The [[Red Sea Rift]]
* The [[Gulf of California]]
* The [[Baikal Rift Zone]], the bottom of [[Lake Baikal]] is the deepest continental rift on the earth.
* The [[Gulf of Suez Rift]]
* Throughout the [[Basin and Range Province]] in North America
* The [[Rio Grande Rift]] in the southwestern US
* The rift zone that contains the [[Gulf of Corinth]] in [[Greece]]
* The [[Reelfoot Rift]], an ancient buried failed rift underlying the [[New Madrid Seismic Zone]] in the [[Mississippi embayment]]
* The [[Rhine Rift]], in southwestern Germany, known as the [[Upper Rhine valley]], part of the [[European Cenozoic Rift System]]
* The [[Taupō Volcanic Zone]] in the [[North Island]] of [[New Zealand]]
* The [[Oslo Graben]] in [[Norway]]
* The [[Ottawa-Bonnechere Graben]] in Ontario and Quebec
* The [[Northern Cordilleran Volcanic Province]] in [[British Columbia]], [[Yukon]] and [[Alaska]]
* The [[West Antarctic Rift System]] in [[Antarctica]]
* The [[Midcontinent Rift System]], a late [[Precambrian]] rift in central North America
* The [[Central Lowlands|Midland Valley]] in Scotland
* The [[Fundy Basin]], a [[Triassic]] rift basin in southeastern Canada
* The Cambay, Kachchh, and Narmada rifts<ref>Chouhan, A.K. Structural fabric over the seismically active Kachchh rift basin, India: insight from world gravity model 2012. Environ Earth Sci 79, 316 (2020). https://doi.org/10.1007/s12665-020-09068-2</ref> in northwestern [[Deccan Traps|Deccan volcanic province]] of India<ref>Chouhan, A.K., Choudhury, P. & Pal, S.K. New evidence for a thin crust and magmatic underplating beneath the Cambay rift basin, Western India through modelling of EIGEN-6C4 gravity data. J Earth Syst Sci 129, 64 (2020). https://doi.org/10.1007/s12040-019-1335-y</ref>

== See also ==
{{Portal|Geology}}
* [[Rift zone]]
* [[Wilson Cycle]]

== References ==
{{Reflist|33em}}

== Further reading ==
{{Commons category|Rifts}}
* {{cite journal |last1=Bally |first1=A.W. |last2=Snelson |first2=S. |year=1980 |title=Realms of subsidence |journal=Canadian Society for Petroleum Geology Memoir |volume=6 |pages=9–94}}
* {{cite journal |last1=Kingston |first1=D.R. |last2=Dishroon |first2=C.P. |last3=Williams |first3=P.A. |date=December 1983 |title=Global Basin Classification System |url=http://www.monografias.com/trabajos-pdf4/global-basin-classification-system/global-basin-classification-system.pdf |journal=[[American Association of Petroleum Geologists|AAPG Bulletin]] |volume=67 |issue=12 |pages=2175–2193 |access-date=2017-06-23}}
* {{cite journal |last=Klemme |first=H.D |date=1980 |title=Petroleum Basins - Classifications and Characteristics |doi=10.1111/j.1747-5457.1980.tb00982.x |journal=[[Journal of Petroleum Geology]] |volume=3 |issue=2 |pages=187–207|bibcode=1980JPetG...3..187K }}

{{Structural geology}}
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[[Category:Rifts and grabens|+]]
[[Category:Structural geology]]
[[Category:Plate tectonics]]
[[Category:Rift basins| ]]