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{{short description|Geological process of stretching planet crust}}
'''Extensional tectonics''' is concerned with the structures formed by, and the [[Tectonics|tectonic]] processes associated with, the stretching of a [[planetary body]]'s [[Crust (geology)|crust]] or [[lithosphere]].

==Deformation styles==
The types of structure and the geometries formed depend on the amount of stretching involved. Stretching is generally measured using the parameter ''β'', known as the ''beta factor'', where

:<math> \beta = \frac{t_1}{t_0} \,,</math>

''t''0 is the initial crustal thickness and ''t''1 is the final crustal thickness. It is also the equivalent of the [[Strain (materials science)|strain]] parameter ''stretch''.<ref>{{cite book |last=Park |first=R. G. |title=Foundations of Structural Geology |url=https://books.google.com/books?id=ycASqdxSG3YC&pg=PA64 |edition=3rd |year=1997 |publisher=Psychology Press |isbn=978-0-7487-5802-9 |page=64}}</ref>

===Low beta factor===
In areas of relatively low crustal stretching, the dominant structures are high to moderate angle normal faults, with associated [[half grabens]] and [[tilted block faulting|tilted fault blocks]].<ref name ="kearey">{{cite book|last1=Kearey|first1=P.|last2=Klepeis|first2=K.A.|last3=Vine|first3=F.J.|title=Global Tectonics|chapter-url=https://books.google.com/books?id=HYqZntfg25UC&pg=PA153|year=2009|publisher=WileyBlackwell|isbn=978-1-4443-0322-3|page=153|chapter=Continental rifts and rifted margins}}</ref>

===High beta factor===
In areas of high crustal stretching, individual [[extensional fault]]s may become rotated to too low a dip to remain active and a new set of faults may be generated.<ref>{{cite journal |last=Proffett |first=John M. |title=Cenozoic geology of the Yerington district, Nevada, and implications for the nature and origin of Basin and Range faulting |journal=Geological Society of America Bulletin |date=1977 |volume=88 |issue=2 |pages=247 |doi=10.1130/0016-7606(1977)88<247:CGOTYD>2.0.CO;2|bibcode=1977GSAB...88..247P }}</ref> Large displacements may juxtapose syntectonic sediments against [[metamorphic rock]]s of the mid to lower crust and such structures are called [[detachment fault]]s. In some cases the detachments are folded such that the metamorphic rocks are exposed within antiformal closures and these are known as [[metamorphic core complex]]es.<ref>{{Cite journal |doi=10.1016/0191-8141(89)90036-9 |last1=Lister |first1=G.S. |last2=Davis |first2=G.A. |year=1989 |title=The origin of metamorphic core complexes and detachment faults formed during Tertiary continental extension in the northern Colorado River region, U.S.A. |url=http://ic.ucsc.edu/~casey/eart150/Death%20Valley%20Reading/Lister_Davis,%201991_MCCs.pdf|journal=J. Struct. Geol. |volume=11 |issue=1–2 |pages=65–94 |bibcode=1989JSG....11...65L }}</ref>

===Passive margins===
[[Passive margin]]s above a weak layer develop a specific set of extensional structures. Large listric regional faults dipping towards the ocean develop with rollover [[anticline]]s and related crestal collapse [[graben]]s. On some margins, such as the [[Niger Delta]], large counter-regional faults are observed, dipping back towards the continent, forming large grabenal mini-basins with antithetic regional faults.<ref>[http://pubs.usgs.gov/of/1999/ofr-99-0050/OF99-50H/OF99-50H.pdf Tuttle, M.L.W., Charpentier, R.R. & Brownfield, M.E. 2002. The Niger Delta Petroleum System: Niger Delta Province, Nigeria, Cameroon, and Equatorial Guinea, Africa. USGS Open-File Report 99-50-H.]</ref>

==Geological environments associated with extensional tectonics==

Areas of extensional tectonics are typically associated with:
[[File:Fault-Horst-Graben.svg|thumbnail|[[Horst and graben]] structure, typical rift related structure (direction of extension shown by red arrows).]]

===Continental rifts===
{{Main article|Rift}}
Rifts are linear zones of localized crustal extension. They range in width from somewhat less than 100 km up to several hundred km, consisting of one or more normal faults and related fault blocks.<ref name ="kearey" /> In individual rift segments, one polarity (i.e. dip direction) normally dominates, giving a [[half-graben]] geometry.<ref>{{cite journal |last1=White |first1=R. S. |last2=Hardman |first2=R. F. P. |last3=Watts |first3=A. B. |last4=Whitmarsh |first4=R. B. |last5=Ebinger |first5=C. J. |last6=Jackson |first6=J. A. |last7=Foster |first7=A. N. |last8=Hayward |first8=N. J. |title=Extensional basin geometry and the elastic lithosphere |journal=Philosophical Transactions of the Royal Society of London. Series A: Mathematical, Physical and Engineering Sciences |date=15 April 1999 |volume=357 |issue=1753 |pages=741–765 |doi=10.1098/rsta.1999.0351 |jstor=55068|bibcode=1999RSPTA.357..741E |s2cid=91719117 }}</ref> Other common geometries include [[metamorphic core complex]]es and [[tilted block faulting|tilted blocks]]. Examples of active continental rifts are the [[Baikal Rift Zone]] and the [[East African Rift]].

===Divergent plate boundaries===
{{Main article|Divergent plate boundary}}
Divergent plate boundaries are zones of active extension as the crust newly formed at the [[mid-ocean ridge]] system becomes involved in the opening process.

===Gravitational spreading of zones of thickened crust===
{{Main|Orogenic collapse}}
Zones of thickened crust, such as those formed during [[continental collision|continent-continent collision]] tend to spread laterally; this spreading occurs even when the collisional event is still in progress.<ref>{{Cite journal|last1=Ji|first1=Zhou|last2=Tonglin|first2=Han|last3=Armijo|first3=R.|last4=Mercier|first4=J. L.|last5=Tapponnier|first5=P.|date=December 1981|title=Field evidence for active normal faulting in Tibet|journal=Nature|language=en|volume=294|issue=5840|pages=410–414|doi=10.1038/294410a0|issn=1476-4687|bibcode=1981Natur.294..410T|s2cid=4326485 }}</ref> After the collision has finished the zone of thickened crust generally undergoes [[post-orogenic collapse|gravitational collapse]], often with the formation of very large extensional faults. Large-scale [[Devonian]] extension, for example, followed immediately after the end of the [[Caledonian orogeny]] particularly in East [[Greenland]] and western [[Norway]].<ref>{{Cite journal|last1=Dunlap|first1=J. W.|last2=Fossen|first2=H.|year=1998|title=Early Paleozoic orogenic collapse, tectonic stability, and late Paleozoic continental rifting revealed through thermochronology of K-feldspars, southern Norway|url=https://folk.uib.no/nglhe/Papers/Tectonics%201998%20Dunlap%20&%20Fossen.pdf|journal=Tectonics|volume=17|issue=4|pages=604–620|doi=10.1029/98TC01603|bibcode=1998Tecto..17..604D|doi-access=free}}</ref><ref>{{cite journal |last1=Hartz |first1=E. H. |last2=Andresen |first2=A. |last3=Hodges |first3=K. V. |last4=Martin |first4=M. W. |title=U–Pb and 40Ar/39Ar constraints on the Fjord Region Detachment Zone: A long-lived extensional fault in the central East Greenland Caledonides |journal=Journal of the Geological Society |date=July 2000 |volume=157 |issue=4 |pages=795–809 |doi=10.1144/jgs.157.4.795 |bibcode=2000JGSoc.157..795H |s2cid=130773289 |url=http://folk.uio.no/ebbe/Ebbe_Hartz/Publications_files/Hartz%20et%20al.,%202000,%20J.%20Geol.%20Soc.%20London,%20Dating%20the%20Fjord%20Region%20Detachment,%20Greenland.pdf |archive-url=https://web.archive.org/web/20120302193811/http://folk.uio.no/ebbe/Ebbe_Hartz/Publications_files/Hartz%20et%20al.%2C%202000%2C%20J.%20Geol.%20Soc.%20London%2C%20Dating%20the%20Fjord%20Region%20Detachment%2C%20Greenland.pdf|archive-date=2012-03-02|url-status=dead}}</ref>

===Releasing bends along strike-slip faults===
When a [[Fault (geology)#Strike-slip faults|strike-slip fault]] is offset along strike such as to create a gap e.g. a left-stepping bend on a sinistral fault, a zone of extension or [[transtension]] is generated. Such bends are known as ''releasing bends'' or ''extensional [[Fault stepover|stepovers]]'' and often form [[pull-apart basin]]s or ''rhombochasms''. Examples of active pull-apart basins include the [[Dead Sea]], formed at a left-stepping offset of the sinistral sense [[Dead Sea Transform]] system, and the [[Sea of Marmara]], formed at a right-stepping offset on the dextral sense [[North Anatolian Fault]] system.<ref>{{citation|url=http://bmeyer2.free.fr/pdf/2002-TerraNova.pdf|last1=Armijo|first1=R.|last2=Meyer|first2=B.|last3=Navarro|first3=S.|last4=King|first4=G.|last5=Barka|first5=A.|author-link5=Aykut Barka|year=2002|title=Asymmetric slip partitioning in the Sea of Marmara pull-apart: a clue to propagation processes of the North Anatolian Fault?|journal=Terra Nova|publisher=[[Wiley-Blackwell]]|volume=14|issue=2|pages=80–86|doi=10.1046/j.1365-3121.2002.00397.x|bibcode=2002TeNov..14...80A|citeseerx=10.1.1.546.4111|s2cid=49553634 }}</ref>

===Back-arc basins===
{{Main article|Back-arc basin}}
Back-arc basins form behind many [[subduction]] zones due to the effects of [[oceanic trench]] roll-back which leads to a zone of extension parallel to the [[island arc]].

===Passive margins===
{{Main article|Passive margin}}
A passive margin built out over a weaker layer, such as an overpressured [[mudstone]] or [[Rock salt|salt]], tends to spread laterally under its own weight. The inboard part of the sedimentary prism is affected by extensional faulting, balanced by outboard shortening.<ref name="Brun_&_Fort_2011">{{Cite journal |last1=Brun |first1=J,-P. |last2=Fort |first2=X. |title=Salt tectonics at passive margins: Geology versus models |journal=Marine and Petroleum Geology |date=2011 |volume=28 |issue=6 |pages=1123–1145 |doi=10.1016/j.marpetgeo.2011.03.004|bibcode=2011MarPG..28.1123B }}</ref>

==See also==
*[[Anorogenic magmatism]]
*[[Strike-slip tectonics]]
*[[Thrust tectonics]]

==References==
{{Reflist}}

==External links==
* [https://web.archive.org/web/20110716072657/http://folk.uib.no/nglhe/StructModulesTextbook/Extension.swf Extension: Chapter 17; A complementary resource to Chapter 17 of the textbook "Strukturgeologi" by Haakon Fossen & Roy Gabrielsen]

{{Structural geology}}

[[Category:Structural geology]]
[[Category:Tectonics]]

Extensional tectonics - Revision history

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