Extensional tectonics is concerned with the structures formed by, and thetectonic processes associated with, the stretching of aplanetary body'scrust orlithosphere.
The types of structure and the geometries formed depend on the amount of stretching involved. Stretching is generally measured using the parameterβ, known as thebeta factor, where
t0 is the initial crustal thickness andt1 is the final crustal thickness. It is also the equivalent of thestrain parameterstretch.[1]
In areas of relatively low crustal stretching, the dominant structures are high to moderate angle normal faults, with associatedhalf grabens andtilted fault blocks.[2]
In areas of high crustal stretching, individualextensional faults may become rotated to too low a dip to remain active and a new set of faults may be generated.[3] Large displacements may juxtapose syntectonic sediments againstmetamorphic rocks of the mid to lower crust and such structures are calleddetachment faults. In some cases the detachments are folded such that the metamorphic rocks are exposed within antiformal closures and these are known asmetamorphic core complexes.[4]
Passive margins above a weak layer develop a specific set of extensional structures. Large listric regional faults dipping towards the ocean develop with rolloveranticlines and related crestal collapsegrabens. On some margins, such as theNiger Delta, large counter-regional faults are observed, dipping back towards the continent, forming large grabenal mini-basins with antithetic regional faults.[5]
Areas of extensional tectonics are typically associated with:
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.[2] In individual rift segments, one polarity (i.e. dip direction) normally dominates, giving ahalf-graben geometry.[6] Other common geometries includemetamorphic core complexes andtilted blocks. Examples of active continental rifts are theBaikal Rift Zone and theEast African Rift.
Divergent plate boundaries are zones of active extension as the crust newly formed at themid-ocean ridge system becomes involved in the opening process.
Zones of thickened crust, such as those formed duringcontinent-continent collision tend to spread laterally; this spreading occurs even when the collisional event is still in progress.[7] After the collision has finished the zone of thickened crust generally undergoesgravitational collapse, often with the formation of very large extensional faults. Large-scaleDevonian extension, for example, followed immediately after the end of theCaledonian orogeny particularly in EastGreenland and westernNorway.[8][9]
When astrike-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 ortranstension is generated. Such bends are known asreleasing bends orextensionalstepovers and often formpull-apart basins orrhombochasms. Examples of active pull-apart basins include theDead Sea, formed at a left-stepping offset of the sinistral senseDead Sea Transform system, and theSea of Marmara, formed at a right-stepping offset on the dextral senseNorth Anatolian Fault system.[10]
Back-arc basins form behind manysubduction zones due to the effects ofoceanic trench roll-back which leads to a zone of extension parallel to theisland arc.
A passive margin built out over a weaker layer, such as an overpressuredmudstone orsalt, tends to spread laterally under its own weight. The inboard part of the sedimentary prism is affected by extensional faulting, balanced by outboard shortening.[11]
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