Extensional tectonics

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Extensional tectonics is concerned with the structures formed by, and the tectonic processes associated with, the stretching of a planetary body's crust or lithosphere.

Contents

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

t0 is the initial crustal thickness and t1 is the final crustal thickness. It is also the equivalent of the strain parameter stretch. [1]

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. [2]

High beta factor

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. [3] 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. [4]

Passive margins

Passive margins above a weak layer develop a specific set of extensional structures. Large listric regional faults dipping towards the ocean develop 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. [5]

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). Fault-Horst-Graben.svg
Horst and graben structure, typical rift related structure (direction of extension shown by red arrows).

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. [2] In individual rift segments, one polarity (i.e. dip direction) normally dominates, giving a half-graben geometry. [6] 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. [7] 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. [8] [9]

Releasing bends along strike-slip faults

When a 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 stepovers and often form 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. [10]

Back-arc basins

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 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. [11]

See also

Related Research Articles

<span class="mw-page-title-main">Rio Grande rift</span> Continental rift zone in the southwest United States

The Rio Grande rift is a north-trending continental rift zone. It separates the Colorado Plateau in the west from the interior of the North American craton on the east. The rift extends from central Colorado in the north to the state of Chihuahua, Mexico, in the south. The rift zone consists of four basins that have an average width of 50 kilometres (31 mi). The rift can be observed on location at Rio Grande National Forest, White Sands National Park, Santa Fe National Forest, and Cibola National Forest, among other locations.

<span class="mw-page-title-main">Basin and Range Province</span> Physiographic region extending through western United States and Mexico

The Basin and Range Province is a vast physiographic region covering much of the inland Western United States and northwestern Mexico. It is defined by unique basin and range topography, characterized by abrupt changes in elevation, alternating between narrow faulted mountain chains and flat arid valleys or basins. The physiography of the province is the result of tectonic extension that began around 17 million years ago in the early Miocene epoch.

<span class="mw-page-title-main">Tectonics</span> Process of evolution of the Earths crust

Tectonics are the processes that result in the structure and properties of the Earth's crust and its evolution through time.

<span class="mw-page-title-main">Rift</span> Geological linear zone where the lithosphere is being pulled apart

In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-graben with normal faulting and rift-flank uplifts mainly on one side. Where rifts remain above sea level they form a rift valley, which may be filled by water forming a rift lake. The axis of the rift area may contain volcanic rocks, and active volcanism is a part of many, but not all, active rift systems.

<span class="mw-page-title-main">Passive margin</span> Transition between oceanic and continental lithosphere that is not an active plate margin

A passive margin is the transition between oceanic and continental lithosphere that is not an active plate margin. A passive margin forms by sedimentation above an ancient rift, now marked by transitional lithosphere. Continental rifting forms new ocean basins. Eventually the continental rift forms a mid-ocean ridge and the locus of extension moves away from the continent-ocean boundary. The transition between the continental and oceanic lithosphere that was originally formed by rifting is known as a passive margin.

<span class="mw-page-title-main">Thrust tectonics</span> Concept in structural geology

Thrust tectonics or contractional tectonics is concerned with the structures formed by, and the tectonic processes associated with, the shortening and thickening of the crust or lithosphere. It is one of the three main types of tectonic regime, the others being extensional tectonics and strike-slip tectonics. These match the three types of plate boundary, convergent (thrust), divergent (extensional) and transform (strike-slip). There are two main types of thrust tectonics, thin-skinned and thick-skinned, depending on whether or not basement rocks are involved in the deformation. The principle geological environments where thrust tectonics is observed are zones of continental collision, restraining bends on strike-slip faults and as part of detached fault systems on some passive margins.

<span class="mw-page-title-main">Extensional fault</span> Fault caused by stretching of the Earths crust

An extensional fault is a fault caused by stretching of the Earth's crust. Stretching reduces the thickness and horizontally extends portions of the crust and/or lithosphere. In most cases such a fault is also a normal fault, but may create a shallower dip usually associated with a thrust fault. Extensional faults are generally planar. If the stress field is oriented with the maximum stress perpendicular to the Earth's surface, extensional faults will create an initial dip of the associated beds of about 60° from the horizontal. The faults will typically extend down to the base of the seismogenic layer. As crustal stretching continues, the faults will rotate, resulting in steeply-dipping fault blocks between them.

<span class="mw-page-title-main">Detachment fault</span> Geological term associated with large displacements

A detachment fault is a gently dipping normal fault 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. They are thought to have formed as either initially low-angle structures or by the rotation of initially high-angle normal faults modified also by the isostatic effects of tectonic denudation. They may also be called denudation faults. Examples of detachment faulting include:

Non-volcanic passive margins (NVPM) constitute one end member of the transitional crustal types that lie beneath passive continental margins; the other end member being volcanic passive margins (VPM). Transitional crust welds continental crust to oceanic crust along the lines of continental break-up. Both VPM and NVPM form during rifting, when a continent rifts to form a new ocean basin. NVPM are different from VPM because of a lack of volcanism. Instead of intrusive magmatic structures, the transitional crust is composed of stretched continental crust and exhumed upper mantle. NVPM are typically submerged and buried beneath thick sediments, so they must be studied using geophysical techniques or drilling. NVPM have diagnostic seismic, gravity, and magnetic characteristics that can be used to distinguish them from VPM and for demarcating the transition between continental and oceanic crust.

<span class="mw-page-title-main">Geology of Russia</span>

The geology of Russia, the world's largest country, which extends over much of northern Eurasia, consists of several stable cratons and sedimentary platforms bounded by orogenic (mountain) belts.

Tectonic subsidence is the sinking of the Earth's crust on a large scale, relative to crustal-scale features or the geoid. The movement of crustal plates and accommodation spaces produced by faulting brought about subsidence on a large scale in a variety of environments, including passive margins, aulacogens, fore-arc basins, foreland basins, intercontinental basins and pull-apart basins. Three mechanisms are common in the tectonic environments in which subsidence occurs: extension, cooling and loading.

<span class="mw-page-title-main">Half-graben</span> Geological structure bounded by a fault along one side of its boundaries

A half-graben is a geological structure bounded by a fault along one side of its boundaries, unlike a full graben where a depressed block of land is bordered by parallel faults.

<span class="mw-page-title-main">Tilted block faulting</span>

Tilted block faulting, also called rotational block faulting, is a mode of structural evolution in extensional tectonic events, a result of tectonic plates stretching apart. When the upper lithospheric crust experiences extensional pressures, the brittle crust fractures, creating detachment faults. These normal faults express themselves on a regional scale; upper crust fractures into tilted fault blocks, and ductile lower crust ascends. This results in uplift, cooling, and exhumation of ductilely deformed deeper crust. The large unit of tilted blocks and associated crust can form an integral part of metamorphic core complexes, which are found on both continental and oceanic crust.

<span class="mw-page-title-main">Gulf of Corinth basin</span> Extensional marine sedimentary basin

The Gulf of Corinth basin, or Corinth rift, is an active extensional marine sedimentary basin thought to have started deforming during the late Miocene – Pleistocene epoch. The dimensions of the Gulf of Corinth are approximately 105 km long and 30 km wide with a basement depth of 3 km at its center. This half-graben basin is formed by a N100°E-oriented rift which separates the Peloponnese peninsula from the continental mainland of Greece. Currently the Corinth rift is opening at rate of 10–15 mm/yr, with respect to the Eurasia Plate. The basin is bounded by the Peloponnese highlands to the south and the westward-moving Anatolian Fault to the north. Major and minor fault planes make up the north and south margins, and its north-south extension is due to activity along an E-W to NW-SE oriented coastal southern margin. The basin's active and inactive faults create associated syn-rift sediment fill. These aspects provide a unique opportunity for scientists to study the tectonic and stratigraphic development of a rift, while further understanding how a basin is actually made.

<span class="mw-page-title-main">Northern North Sea basin</span>

The North Sea is part of the Atlantic Ocean in northern Europe. It is located between Norway and Denmark in the east, Scotland and England in the west, Germany, the Netherlands, Belgium and France in the south.

<span class="mw-page-title-main">Kutai Basin</span>

The Kutai sedimentary basin extends from the central highlands of Borneo, across the eastern coast of the island and into the Makassar Strait. With an area of 60,000 km2, and depths up to 15 km, the Kutai is the largest and deepest Tertiary age basin in Indonesia. Plate tectonic evolution in the Indonesian region of SE Asia has produced a diverse array of basins in the Cenozoic. The Kutai is an extensional basin in a general foreland setting. Its geologic evolution begins in the mid Eocene and involves phases of extension and rifting, thermal sag, and isostatic subsidence. Rapid, high volume, sedimentation related to uplift and inversion began in the Early Miocene. The different stages of Kutai basin evolution can be roughly correlated to regional and local tectonic events. It is also likely that regional climate, namely the onset of the equatorial ever wet monsoon in early Miocene, has affected the geologic evolution of Borneo and the Kutai basin through the present day. Basin fill is ongoing in the lower Kutai basin, as the modern Mahakam River delta progrades east across the continental shelf of Borneo.

<span class="mw-page-title-main">Nordfjord-Sogn Detachment</span> Zone of deformed rocks in Norway

The Nordfjord—Sogn Detachment (NSD) is a major extensional shear zone in Norway up to 6 km in thickness, which extends about 120 km along strike from Nordfjord to Sognefjord, bringing Devonian continental coarse clastic sedimentary rocks into close contact with eclogite facies metamorphic rocks of the Western Gneiss Region. It formed towards the end of the Caledonian Orogeny and was mainly active during the Devonian. It has an estimated displacement of at least 70 km and possibly as much as 110 km. It was reactivated during the Mesozoic and may have influenced the development of fault structures in the North Sea rift basin.

<span class="mw-page-title-main">Huangling Anticline</span> Group of rock units in the Yangtze Block, South China

The Huangling Anticline or Complex represents a group of rock units that appear in the middle of the Yangtze Block in South China, distributed across Yixingshan, Zigui, Huangling, and Yichang counties. The group of rock involves nonconformity that sedimentary rocks overlie the metamorphic basement. It is a 73-km long, asymmetrical dome-shaped anticline with axial plane orientating in the north-south direction. It has a steeper west flank and a gentler east flank. Basically, there are three tectonic units from the anticline core to the rim, including Archean to Paleoproterozoic metamorphic basement, Neoproterozoic to Jurassic sedimentary rocks, and Cretaceous fluvial deposit sedimentary cover. The northern part of the core is mainly tonalite-trondhjemite-gneiss (TTG) and Cretaceous sedimentary rock called the Archean Kongling Complex. The middle of the core is mainly the Neoproterozoic granitoid. The southern part of the core is the Neoproterozoic potassium granite. Two basins are situated on the western and eastern flanks of the core, respectively, including the Zigui basin and Dangyang basin. Both basins are synforms while Zigui basin has a larger extent of folding. Yuanan Graben and Jingmen Graben are found within the Dangyang Basin area. The Huangling Anticline is an important area that helps unravel the tectonic history of the South China Craton because it has well-exposed layers of rock units from Archean basement rock to Cretaceous sedimentary rock cover due to the erosion of the anticline.

<span class="mw-page-title-main">Hornelen Basin</span> Sedimentary basin in Vestland, Norway

The Hornelen Basin is a sedimentary basin in Vestland, Norway, containing an estimated 25 km stratigraphic thickness of coarse clastic sedimentary rocks of Devonian age. It forms part of a group of basins of similar age along the west coast of Norway between Sognefjord and Nordfjord, related to movement on the Nordfjord-Sogn Detachment. It formed as a result of extensional tectonics as part of the post-orogenic collapse of crust that was thickened during the Caledonian Orogeny towards the end of the Silurian period. It is named for the mountain Hornelen on the northern margin of the basin.

<span class="mw-page-title-main">Ordos Block</span>

The Ordos Block is a crustal block, that forms part of the larger North China Block (NCB). It is surrounded by active fault systems and has been a distinct block since at least the Mesozoic. It is bordered to the west by the Alxa Block, the westernmost part of the NCB, to the south by the Qinling orogenic belt, to the north by the Yanshan-Yinshan orogenic belt, part of the Central Asian Orogenic Belt and to the east by the Taihangshan mountain range, which forms part of the Trans-North China Orogen. The block is currently stable and large earthquakes are restricted to the bordering fault zones. It has been suggested that the block is currently undergoing anti-clockwise rotation with respect to the Eurasian Plate, as a result of the ongoing eastward spreading of the Tibetan Plateau, although this view has been challenged.

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