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'''Extensional tectonics''' is concerned with the structures formed, and the [[Tectonics|tectonic]] processes associated with, the stretching of the [[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 <math> \beta </math>, known as the ''beta factor'' where <math> \beta = \frac{t_{0}}{t_{1}} </math>, <math> t_{0} </math> is the initial crustal thickness and <math> t_{1} </math> is the final crustal thickness. It is also the equivalent of the [[Strain (materials science)|strain]] parameter ''stretch''.<ref>[http://books.google.co.uk/books?id=ycASqdxSG3YC&pg=PA64&dq=stretch+Foundations+of+structural+geology&lr=&ei=HjYVSrq3EZrEzASuzrH7Ag Park, R.G. 1997. Foundations of Structural Geology. 3rd edition, Routledge, 216pp.]</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">[http://books.google.co.uk/books?id=HYqZntfg25UC&pg=PA153&dq=rift+dimensions&lr=&ei=iloVSvvcO4TOlQTDuvDUCQ Kearey, P., Klepeis, K.A. & Vine, F.J. 2008. Global Tectonics, Chapter 7, Continental rifts and rifted margins, WileyBlackwell, 496pp.]</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>[http://bulletin.geoscienceworld.org/cgi/content/abstract/88/2/247 Proffett, J.M. 1977. Cenozoic geology of the Yerington district, Nevada, and implications for the nature of Basin and Range faulting. Bull. geol. Soc. Am. 88, 247-66.]</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.{{Citation needed|date=May 2009}}

===Passive margins===
[[Passive margin]]s above a weak layer develop a specific set of extensional structures. Large listric regional (i.e. dipping towards the ocean) faults are developed 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|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>[http://www.jstor.org/pss/55068 Ebinger, C.J., Jackson, J.A., Foster, A.N. & Hayward, N.J. 1999. Extensional basin geometry and the elastic lithosphere. Philosophical Transactions of the Royal Society, London, A, 357, 741-765.]</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|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===
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>[http://www.nature.com/nature/journal/v294/n5840/abs/294410a0.html Tapponier, P. Mercier, J.L., Armijo, R., Tonglin, H, & Ji, Z. 1981. Field evidence for active normal faulting in Tibet. Nature, 294, 410-414.]</ref> After the collision has finished the zone of thickened crust generally undergoes 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>[http://folk.uib.no/nglhe/Papers/Tectonics%201998%20Dunlap%20&%20Fossen.pdf Dunlap, J.W. & Fossen, H. 1998: Early Paleozoic orogenic collapse, tectonic stability, and late Paleozoic continental rifting revealed through thermochronology of K-feldspars, southern Norway. Tectonics 17, 604-620.]</ref><ref>[http://folk.uio.no/ebbe/Ebbe_Hartz/Publications_files/Hartz%20et%20al.,%202000,%20J.%20Geol.%20Soc.%20London,%20Dating%20the%20Fjord%20Region%20Detachment,%20Greenland.pdf Hartz, E.H, Andresen, A., Hodges K.V. & Martin, M.W., 2000, The Fjord Region Detachment Zone: A long-lived extensional fault in the East Greenland Caledonides, J. Geol. Soc. London, 158, 795-810.]</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 i.e. 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 stepovers'' and often form [[pull apart basin|pull-apart basins]] 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>[http://bmeyer2.free.fr/pdf/2002-TerraNova.pdf Armijo, R., Meyer, B., Navarro, S., King, G. & Barka, A. 2002. Asymmetric slip partitioning in the Sea of Marmara pull-apart: a clue to propagation processes of the North Anatolian Fault? Terra Nova, 14, 80–86.]</ref>

===Back-arc basins===
{{Main|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===
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.

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

==References==
{{Reflist}}

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

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

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