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Study of the structures formed by, and the processes associated with, the stretching of a planetary body's crust
In areas of high crustal stretching, individual extensional faults may become rotated to too low a dip to remain active and a new set of faults may be generated. Large displacements may juxtapose syntectonic sediments against metamorphic rocks of the mid to lower crust and such structures are called detachment faults. In some cases the detachments are folded such that the metamorphic rocks are exposed within antiformal closures and these are known as metamorphic core complexes.
Passive margins 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 anticlines and related crestal collapse grabens. 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.
Geological environments associated with extensional tectonics
Areas of extensional tectonics are typically associated with:
Horst and graben structure, typical rift related structure (direction of extension shown by red arrows).
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. In individual rift segments, one polarity (i.e. dip direction) normally dominates giving a half-graben geometry. Other common geometries include metamorphic core complexes and tilted blocks. Examples of active continental rifts are the Baikal Rift Zone and the East African Rift.
Divergent plate boundaries
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 continent-continent collision tend to spread laterally; this spreading occurs even when the collisional event is still in progress. 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.
A passive margin built out over a weaker layer, such as an overpressured mudstone or 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.
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^White, R. S.; Hardman, R. F. P.; Watts, A. B.; Whitmarsh, R. B.; Ebinger, C. J.; Jackson, J. A.; Foster, A. N.; Hayward, N. J. (15 April 1999). "Extensional basin geometry and the elastic lithosphere". Philosophical Transactions of the Royal Society of London. Series A: Mathematical, Physical and Engineering Sciences. 357 (1753): 741-765. doi:10.1098/rsta.1999.0351. JSTOR55068.