Extensional tectonics
Encyclopedia
Extensional tectonics is concerned with the structures formed, and the tectonic
processes associated with, the stretching of the crust
or lithosphere
.
, known as the beta factor where 
, 
is the initial crustal thickness and 
is the final crustal thickness. It is also the equivalent of the strain
parameter stretch.
s 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 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.
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.
and the East African Rift
.
system becomes involved in the opening process.
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
.
, formed at a left-stepping offset of the sinstral sense Dead Sea Transform
system, and the Sea of Marmara
, formed at a right-stepping offset on the dextral sense North Anatolian Fault
system.
zones due to the effects of oceanic trench
roll-back which leads to a zone of extension parallel to the island arc
.
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.
Tectonics
Tectonics is a field of study within geology concerned generally with the structures within the lithosphere of the Earth and particularly with the forces and movements that have operated in a region to create these structures.Tectonics is concerned with the orogenies and tectonic development of...
processes associated with, the stretching of the crust
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...
or lithosphere
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's 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, known as the beta factor where , is the initial crustal thickness and is the final crustal thickness. It is also the equivalent of the strainStrain (materials science)
In continuum mechanics, the infinitesimal strain theory, sometimes called small deformation theory, small displacement theory, or small displacement-gradient theory, deals with infinitesimal deformations of a continuum body...
parameter stretch.
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 fault blocks.High beta factor
In areas of high crustal stretching, individual extensional faultExtensional fault
An extensional fault is a fault that vertically thins and horizontally extends portions of the Earth's crust and/or lithosphere. In most cases such a fault is also a normal fault, but may be rotated to have a shallower geometry normally associated with a thrust fault...
s 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 rock
Metamorphic rock
Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
s of the mid to lower crust and such structures are called detachment fault
Detachment fault
Detachment faulting is associated with large-scale extensional tectonics. Detachment faults often have very large displacements and juxtapose unmetamorphosed hanging walls against medium to high-grade metamorphic footwalls that are called metamorphic core complexes...
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
Metamorphic core complex
Metamorphic core complexes are exposures of deep crust exhumed in association with largely amagmatic extension. They form, and are exhumed, through relatively fast transport of middle and lower continental crust to the Earth's surface...
es.
Passive margins
Passive marginPassive margin
A passive margin is the transition between oceanic and continental crust which is not an active plate margin. It is constructed by sedimentation above an ancient rift, now marked by transitional crust. Continental rifting creates new ocean basins. Eventually the continental rift forms a mid-oceanic...
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
Anticline
In structural geology, an anticline is a fold that is convex up and has its oldest beds at its core. The term is not to be confused with antiform, which is a purely descriptive term for any fold that is convex up. Therefore if age relationships In structural geology, an anticline is a fold that is...
s and related crestal collapse graben
Graben
In geology, a graben is a depressed block of land bordered by parallel faults. Graben is German for ditch. Graben is used for both the singular and plural....
s. On some margins, such as the Niger Delta
Niger Delta
The Niger Delta, the delta of the Niger River in Nigeria, is a densely populated region sometimes called the Oil Rivers because it was once a major producer of palm oil...
, 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:Continental rifts
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. Examples of active continental rifts are the Baikal Rift ZoneBaikal Rift Zone
The Baikal Rift Zone is a divergent plate boundary centered beneath Lake Baikal in southeastern Russia. To its west is the Eurasian Plate and to its east is the Amur Plate which is moving away from the rift toward Japan at about 4 mm per year....
and the East African Rift
East African Rift
The East African Rift is an active continental rift zone in eastern Africa that appears to be a developing divergent tectonic plate boundary. It is part of the larger Great Rift Valley. The rift is a narrow zone in which the African Plate is in the process of splitting into two new tectonic plates...
.
Divergent plate boundaries
Divergent plate boundaries are zones of active extension as the crust newly formed at the mid-ocean ridgeMid-ocean ridge
A mid-ocean ridge is a general term for an underwater mountain system that consists of various mountain ranges , typically having a valley known as a rift running along its spine, formed by plate tectonics. This type of oceanic ridge is characteristic of what is known as an oceanic spreading...
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 collisionContinental collision
Continental collision is a phenomenon of the plate tectonics of Earth that occurs at convergent boundaries. Continental collision is a variation on the fundamental process of subduction, whereby the subduction zone is destroyed, mountains produced, and two continents sutured together...
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
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
extension, for example, followed immediately after the end of the Caledonian orogeny
Caledonian orogeny
The Caledonian orogeny is a mountain building era recorded in the northern parts of the British Isles, the Scandinavian Mountains, Svalbard, eastern Greenland and parts of north-central Europe. The Caledonian orogeny encompasses events that occurred from the Ordovician to Early Devonian, roughly...
particularly in East Greenland
Greenland
Greenland is an autonomous country within the Kingdom of Denmark, located between the Arctic and Atlantic Oceans, east of the Canadian Arctic Archipelago. Though physiographically a part of the continent of North America, Greenland has been politically and culturally associated with Europe for...
and western Norway
Norway
Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...
.
Releasing bends along strike-slip faults
When a 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 is generated. Such bends are known as releasing bends or extensional stepovers and often form pull-apart basins or rhombochasms. Examples of active pull-apart basins include the Dead SeaDead Sea
The Dead Sea , also called the Salt Sea, is a salt lake bordering Jordan to the east and Israel and the West Bank to the west. Its surface and shores are below sea level, the lowest elevation on the Earth's surface. The Dead Sea is deep, the deepest hypersaline lake in the world...
, formed at a left-stepping offset of the sinstral sense Dead Sea Transform
Dead Sea Transform
The Dead Sea Transform fault system, also sometimes referred to as the Dead Sea Rift, is a geologic fault which runs from the Maras Triple Junction to the northern end of the Red Sea Rift...
system, and the Sea of Marmara
Sea of Marmara
The Sea of Marmara , also known as the Sea of Marmora or the Marmara Sea, and in the context of classical antiquity as the Propontis , is the inland sea that connects the Black Sea to the Aegean Sea, thus separating Turkey's Asian and European parts. The Bosphorus strait connects it to the Black...
, formed at a right-stepping offset on the dextral sense North Anatolian Fault
North Anatolian Fault
The North Anatolian Fault is a major active right lateral-moving strike-slip fault in northern Anatolia which runs along the transform boundary between the Eurasian Plate and the Anatolian Plate. The fault extends westward from a junction with the East Anatolian Fault at the Karliova Triple...
system.
Back-arc basins
Back-arc basins form behind many subductionSubduction
In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. These 3D regions of mantle downwellings are known as "Subduction Zones"...
zones due to the effects of oceanic trench
Oceanic trench
The oceanic trenches are hemispheric-scale long but narrow topographic depressions of the sea floor. They are also the deepest parts of the ocean floor....
roll-back which leads to a zone of extension parallel to the island arc
Island arc
An island arc is a type of archipelago composed of a chain of volcanoes which alignment is arc-shaped, and which are situated parallel and close to a boundary between two converging tectonic plates....
.
Passive margins
A passive margin built out over a weaker layer , such as an overpressured mudstoneMudstone
Mudstone is a fine grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.0625 mm with individual grains too small to be distinguished without a microscope. With increased pressure over time the platey clay minerals may become aligned, with the...
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.