Thrust tectonics

Last updated
Cross-section diagram of the frontal part of a thin-skinned thrust zone Compressional decollement.svg
Cross-section diagram of the frontal part of a thin-skinned thrust zone

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.

Contents

Deformation styles

In areas of thrust tectonics, two main processes are recognized: thin-skinned deformation and thick-skinned deformation. The distinction is important as attempts to structurally restore the deformation will give very different results depending on the assumed geometry. [1]

Thin-skinned deformation

Thin-skinned deformation refers to shortening that only involves the sedimentary cover. This style is typical of many fold and thrust belts developed in the foreland of a collisional zone. This is particularly the case where a good basal decollement exists such as salt or a zone of high pore fluid pressure. [2]

Thick-skinned deformation

Thick-skinned deformation refers to shortening that involves basement rocks rather than just the overlying cover. This type of geometry is typically found in the hinterland of a collisional zone. This style may also occur in the foreland where no effective decollement surface is present or where pre-existing extensional rift structures may be inverted. [3]

Geological environments associated with thrust tectonics

Collisional zones

The most significant areas of thrust tectonics are associated with destructive plate boundaries leading to the formation of orogenic belts. The two main types are: the collision of two continental tectonic plates (for example the Arabian and Eurasian plates, which formed the Zagros fold and thrust belt) and collisions between a continent and an island arc such as that which formed Taiwan. [4]

Restraining bends on strike-slip faults

When a strike-slip fault is offset along strike such that the resulting bend in the fault hinders easy movement, e.g. a right stepping bend on a sinistral (left-lateral) fault, this will cause local shortening or transpression. Examples include the 'Big Bend' region of the San Andreas fault, [5] and parts of the Dead Sea Transform. [6]

Passive margins

Passive margins are characterised by large prisms of sedimentary material deposited since the original break-up of a continent associated with formation of a new spreading centre. This wedge of material will tend to spread under gravity and, where an effective detachment layer is present such as salt, the extensional faulting that forms at the landward side will be balanced at the front of the wedge by a series of toe-thrusts. Examples include the outboard part of the Niger delta (with an overpressured mudstone detachment) [7] and the Angola margin (with a salt detachment). [8]

Related Research Articles

Orogeny The formation of mountain ranges

Orogeny is the primary mechanism by which mountains are formed on continents. An orogeny is an event that takes place at a convergent plate margin when plate motion compresses the margin. An orogenic belt or orogen develops as the compressed plate crumples and is uplifted to form one or more mountain ranges. This involves a series of geological processes collectively called orogenesis. These include both structural deformation of existing continental crust and the creation of new continental crust through volcanism. Magma rising in the orogen carries less dense material upwards while leaving more dense material behind, resulting in compositional differentiation of Earth's lithosphere. A synorogenic process or event is one that occurs during an orogeny.

Thrust fault Type of reverse fault that has a dip of 45 degrees or less

A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks.

Tectonics Process of evolution of the earths crust

Tectonics are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents known as cratons, and the ways in which the relatively rigid plates that constitute the Earth's outer shell interact with each other. Tectonics also provide a framework for understanding the earthquake and volcanic belts that directly affect much of the global population.

Nappe A large sheetlike body of rock that has been moved a considerable distance above a thrust fault

In geology, a nappe or thrust sheet is a large sheetlike body of rock that has been moved more than 2 km (1.2 mi) or 5 km (3.1 mi) above a thrust fault from its original position. Nappes form in compressional tectonic settings like continental collision zones or on the overriding plate in active subduction zones. Nappes form when a mass of rock is forced over another rock mass, typically on a low angle fault plane. The resulting structure may include large-scale recumbent folds, shearing along the fault plane, imbricate thrust stacks, fensters and klippes.

Sevier orogeny Mountain-building episode in North America

The Sevier orogeny was a mountain-building event that affected western North America from northern Canada to the north to Mexico to the south.

Shear (geology) Response of rock to deformation

In geology, shear is the response of a rock to deformation usually by compressive stress and forms particular textures. Shear can be homogeneous or non-homogeneous, and may be pure shear or simple shear. Study of geological shear is related to the study of structural geology, rock microstructure or rock texture and fault mechanics.

The Lewis Overthrust is a geologic thrust fault structure of the Rocky Mountains found within the bordering national parks of Glacier in Montana, United States and Waterton Lakes in Alberta, Canada. The structure was created due to the collision of tectonic plates about 170 million years ago that drove a several mile thick wedge of rock 50 mi (80 km) eastwards, causing it to overlie softer Cretaceous age rock that is 400 to 500 million years younger.

Foreland basin Structural basin that develops adjacent and parallel to a mountain belt

A foreland basin is a structural basin that develops adjacent and parallel to a mountain belt. Foreland basins form because the immense mass created by crustal thickening associated with the evolution of a mountain belt causes the lithosphere to bend, by a process known as lithospheric flexure. The width and depth of the foreland basin is determined by the flexural rigidity of the underlying lithosphere, and the characteristics of the mountain belt. The foreland basin receives sediment that is eroded off the adjacent mountain belt, filling with thick sedimentary successions that thin away from the mountain belt. Foreland basins represent an endmember basin type, the other being rift basins. Space for sediments is provided by loading and downflexure to form foreland basins, in contrast to rift basins, where accommodation space is generated by lithospheric extension.

Décollement Geological feature

Décollement is a gliding plane between two rock masses, also known as a basal detachment fault. Décollements are a deformational structure, resulting in independent styles of deformation in the rocks above and below the fault. They are associated with both compressional settings and extensional settings.

Transpression Type of strike-slip deformation

In geology, transpression is a type of strike-slip deformation that deviates from simple shear because of a simultaneous component of shortening perpendicular to the fault plane. This movement ends up resulting in oblique shear. It is generally very unlikely that a deforming body will experience "pure" shortening or "pure" strike-slip. The relative amounts of shortening and strike-slip can be expressed in the convergence angle alpha which ranges from zero to 90 degrees. During shortening, unless material is lost, transpression produces vertical thickening in the crust. Transpression that occurs on a regional scale along plate boundaries is characterized by oblique convergence. More locally, transpression occurs within restraining bends in strike-slip fault zones.

Strike-slip tectonics is concerned with the structures formed by, and the tectonic processes associated with other zones of lateral displacement within the Earth's crust or lithosphere, such as transform boundaries. It is one of the three main types of plate tectonic regime, the others being extensional tectonics and thrust tectonics. Areas of strike-slip tectonics are associated with particular deformation styles including Riedel shears, flower structures and strike-slip duplexes. This type of tectonics is characteristic of several geological environments, including oceanic and continental transform faults, zones of oblique collision and the deforming foreland of a zone of continental collision.

Inversion (geology) Relative uplift of a sedimentary basin or similar structure as a result of crustal shortening

In structural geology inversion or basin inversion relates to the relative uplift of a sedimentary basin or similar structure as a result of crustal shortening. This normally excludes uplift developed in the footwalls of later extensional faults, or uplift caused by mantle plumes. "Inversion" can also refer to individual faults, where an extensional fault is reactivated in the opposite direction to its original movement.

Accretionary wedge The sediments accreted onto the non-subducting tectonic plate at a convergent plate boundary

An accretionary wedge or accretionary prism forms from sediments accreted onto the non-subducting tectonic plate at a convergent plate boundary. Most of the material in the accretionary wedge consists of marine sediments scraped off from the downgoing slab of oceanic crust, but in some cases the wedge includes the erosional products of volcanic island arcs formed on the overriding plate.

Fold mountains Mountains formed by compressive crumpling of the layers of rock

Fold mountains are formed by the effects of folding on layers within the upper part of the Earth's crust. Before the development of the theory of plate tectonics and before the internal architecture of thrust belts became well understood, the term was used to describe most mountain belts, e.g. the Himalayas. The term is still fairly common in the literature of physical geography but has otherwise fallen out of use.

Thin-skinned deformation

Thin-skinned deformation is a style of deformation in plate tectonics at a convergent boundary which occurs with shallow thrust faults that only involves cover rocks, and not deeper basement rocks.

Zagros fold and thrust belt Geologic zone

The Zagros fold and thrust belt is an approximately 1,800-kilometre (1,100 mi) long zone of deformed crustal rocks, formed in the foreland of the collision between the Arabian Plate and the Eurasian Plate. It is host to one of the world's largest petroleum provinces, containing about 49% of the established hydrocarbon reserves in fold and thrust belts (FTBs) and about 7% of all reserves globally.

Tectonics of the Tian Shan

The Tian Shan is a mountain range in central Asia that extends through western China, Kazakhstan, and Kyrgyzstan. The Tian Shan is 2,800 kilometres (1,700 mi) long, and up to 7,400 metres (24,300 ft) high. Throughout the Tian Shan there are several intermontane basins separated by high ranges. Plate tectonic theory makes the assumption that deformation is concentrated along plate boundaries. However, active deformation is observed in the Tian Shan, far from plate boundaries. This apparent contradiction of plate tectonic theory makes the Tian Shan a key place to study the dynamics of intracontinental deformation.

Thick-skinned deformation is a geological term which refers to crustal shortening that involves basement rocks and deep-seated faults as opposed to only the upper units of cover rocks above the basement which is known as thin-skinned deformation. While thin-skinned deformation is common in many different localities, thick-skinned deformation requires much more strain to occur and is a rarer type of deformation.

El Tigre Fault

The El Tigre Fault is a 120 km long, roughly north-south trending, major strike-slip fault located in the Western Precordillera in Argentina. The Precordillera lies just to the east of the Andes mountain range in South America. The northern boundary of the fault is the Jáchal River and its southern boundary is the San Juan River. The fault is divided into three sections based on fault trace geometry, Northern extending between 41–46 km in length, Central extending between 48–53 km in length, and Southern extending 26 km in length. The fault displays a right-lateral (horizontal) motion and has formed in response to stresses from the Nazca Plate subducting under the South American Plate. It is a major fault with crustal significance. The Andes Mountain belt trends with respect to the Nazca Plate/South American Plate convergence zone, and deformation is divided between the Precordilleran thrust faults and the El Tigre strike-slip motion. The El Tigre Fault is currently seismically active.

Geology of Myanmar

The geology of Myanmar is shaped by dramatic, ongoing tectonic processes controlled by shifting tectonic components as the Indian plate slides northwards and towards Southeast Asia. Myanmar spans across parts of three tectonic plates separated by north-trending faults. To the west, a highly oblique subduction zone separates the offshore Indian plate from the Burma microplate, which underlies most of the country. In the center-east of Myanmar, a right lateral strike slip fault extends from south to north across more than 1,000 km (620 mi). These tectonic zones are responsible for large earthquakes in the region. The India-Eurasia plate collision which initiated in the Eocene provides the last geological pieces of Myanmar, and thus Myanmar preserves a more extensive Cenozoic geological record as compared to records of the Mesozoic and Paleozoic eras. Myanmar is physiographically divided into three regions: the Indo-Burman Range, Myanmar Central Belt and the Shan Plateau; these all display an arcuate shape bulging westwards. The varying regional tectonic settings of Myanmar not only give rise to disparate regional features, but they also foster the formation of petroleum basins and a diverse mix of mineral resources.

References

  1. Shiner, P (2004). "Thin-skinned versus thick-skinned structural models for Apulian carbonate reservoirs: constraints from the Val d'Agri Fields, S Apennines, Italy". Marine and Petroleum Geology . 21 (7): 805. doi:10.1016/j.marpetgeo.2003.11.020.
  2. Hatcher, R. D. (2007). "Confirmation of Thin-skinned Thrust Faulting in Foreland Fold-Thrust Belts and Its Impact on Hydrocarbon Exploration: Bally, Gordy, and Stewart, Bulletin of Canadian Petroleum Geology, 1966" (PDF). AAPG Search and Discovery Article. 70034.
  3. Butler, R. W. H.; Mazzoli, S.; Corrado, S.; De Donatis, M.; Di Bucci, D.; Gambini, R.; Naso, G.; Nicolai, C.; Scrocca, D.; Shiner, P.; Zucconi, V. (2004). McClay, K. R. (ed.). "Applying thick-skinned tectonic models to the Apennine thrust belt of Italy—Limitations and implications" (PDF). Thrust tectonics and hydrocarbon systems: AAPG Memoir 82. pp. 647–67.
  4. Timothy Briggs Byrne; Char-Shine Liu (2002). Geology and Geophysics of an Arc-continent Collision, Taiwan. Special Paper 358. Geological Society of America. ISBN   978-0-8137-2358-7.
  5. Rust, D. 1998. Contractional and extensional structures in the transpressive ‘Big Bend’ of the San Andreas fault, southern California. Geological Society, London, Special Publications; 135; p. 119-126
  6. Gomez, F., Nemer, T., Tabet, C., Meghraoui, M. & Barazangi, M. 2007. Strain partitioning of active transpression within the Lebanese restraining bend of the Dead Sea Fault (Lebanon and SW Syria). Geological Society, London, Special Publications; 290; 285–303
  7. Bilotti, F. & Shaw, J.H. 2005. Deep-water Niger Delta fold and thrust belt modeled as a critical taper wedge: The influence of elevated basal fluid pressure on structural styles. AAPG Bulletin; 89; 11; 1475–1491
  8. Marton, G, & Schoenborn, G. 2008. Salt Tectonics of the Continent-Ocean Transition, Deep-Water Angola. MAPG conference abstract