Thrust tectonics or contractional tectonics is concerned with the structures formed by, and the

tectonic Tectonics ( via Latin ) are the processes that result in the structure and properties of the Earth's crust and its evolution through time. The field of ''planetary tectonics'' extends the concept to other planets and moons. These processes ...

processes associated with, the shortening and thickening of the crust or

lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the lithospheric mantle, the topmost portion of the upper mantle that behaves elastically on time ...

. It is one of the three main types of tectonic regime, the others being

extensional tectonics Extensional tectonics is concerned with the structures formed by, and the Tectonics, tectonic processes associated with, the stretching of a planetary body's Crust (geology), crust or lithosphere. Deformation styles The types of structure and the ...

and strike-slip tectonics. These match the three types of

plate boundary Plate tectonics (, ) is the scientific theory that the Earth's lithosphere comprises a number of large tectonic plates, which have been slowly moving since 3–4 billion years ago. The model builds on the concept of , an idea developed durin ...

, 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 In geology, continental collision is a phenomenon of plate tectonics that occurs at Convergent boundary, convergent boundaries. Continental collision is a variation on the fundamental process of subduction, whereby the subduction zone is destroy ...

, restraining bends on strike-slip faults and as part of detached fault systems on some

passive margin A passive margin is the transition between Lithosphere#Oceanic lithosphere, oceanic and Lithosphere#Continental lithosphere, continental lithosphere that is not an active plate continental margin, margin. A passive margin forms by sedimentatio ...

Deformation styles

In areas of

thrust Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that ...

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.

Thin-skinned deformation

Thin-skinned deformation refers to shortening that only involves the sedimentary cover. This style is typical of many

fold and thrust belt A fold and thrust belt is a series of mountainous foothills adjacent to an orogenic belt, which forms due to contractional tectonics. Fold and thrust belts commonly form in the forelands adjacent to major orogens as deformation propagates outwards ...

s developed in the foreland of a collisional zone. This is particularly the case where a good basal decollement exists such as

salt In common usage, salt is a mineral composed primarily of sodium chloride (NaCl). When used in food, especially in granulated form, it is more formally called table salt. In the form of a natural crystalline mineral, salt is also known as r ...

or a zone of high pore fluid pressure.

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 Hinterland is a German word meaning the 'land behind' a city, a port, or similar. Its use in English was first documented by the geographer George Chisholm in his ''Handbook of Commercial Geography'' (1888). Originally the term was associated wi ...

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.

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 belt An orogenic belt, orogen, or mobile belt, is a zone of Earth's crust affected by orogeny. An orogenic belt develops when a continental plate crumples and is uplifted to form one or more mountain ranges; this involves a series of geological proc ...

s. The two main types are: the collision of two

continental Continental may refer to: Places * Continental, Arizona, a small community in Pima County, Arizona, US * Continental, Ohio, a small town in Putnam County, US Arts and entertainment * ''Continental'' (album), an album by Saint Etienne * Continen ...

tectonic plates Plate tectonics (, ) is the scientific theory that the Earth's lithosphere comprises a number of large tectonic plates, which have been slowly moving since 3–4 billion years ago. The model builds on the concept of , an idea developed durin ...

(for example the Arabian plate and Eurasian plate, which formed the

Zagros fold and thrust belt The Zagros fold and thrust belt (Zagros FTB) is an approximately 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 ...

) and collisions between a continent and an

island arc Island arcs are long archipelago, chains of active volcanoes with intense earthquake, seismic activity found along convergent boundary, convergent plate tectonics, tectonic plate boundaries. Most island arcs originate on oceanic crust and have re ...

such as that which formed

Taiwan Taiwan, officially the Republic of China (ROC), is a country in East Asia. The main geography of Taiwan, island of Taiwan, also known as ''Formosa'', lies between the East China Sea, East and South China Seas in the northwestern Pacific Ocea ...

Restraining bends on strike-slip faults

When a

strike-slip fault In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

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 In geology, transpression is a type of Strike-slip fault, 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. ...

. Examples include the 'Big Bend' region of the

San Andreas Fault The San Andreas Fault is a continental Fault (geology)#Strike-slip faults, right-lateral strike-slip transform fault that extends roughly through the U.S. state of California. It forms part of the tectonics, tectonic boundary between the Paci ...

, and parts of the

Dead Sea Transform The Dead Sea Transform (DST) fault system, also sometimes referred to as the Dead Sea Rift, is a series of Fault (geology), faults that run for about 1,000 km from the Marash triple junction (a junction with the East Anatolian Fault in south ...

Passive margins

Passive margin A passive margin is the transition between Lithosphere#Oceanic lithosphere, oceanic and Lithosphere#Continental lithosphere, continental lithosphere that is not an active plate continental margin, margin. A passive margin forms by sedimentatio ...

s 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

, 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 The Niger Delta is the delta of the Niger River sitting directly on the Gulf of Guinea on the Atlantic Ocean in Nigeria. It is located within nine coastal southern Nigerian states, which include: all six states from the South South geopolitic ...

(with an overpressured

mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from ''shale'' by its lack of fissility.Blatt, H., and R.J. Tracy, 1996, ''Petrology.'' New York, New York, ...

detachment) and the

Angola Angola, officially the Republic of Angola, is a country on the west-Central Africa, central coast of Southern Africa. It is the second-largest Portuguese-speaking world, Portuguese-speaking (Lusophone) country in both total area and List of c ...

margin (with a salt detachment).Marton, G, & Schoenborn, G. 2008. Salt Tectonics of the Continent-Ocean Transition, Deep-Water Angola. MAPG conference abstract
/ref>

References

External links

Contraction: Chapter 16; A complementary resource to Chapter 16 of the textbook "Strukturgeologi" by Haakon Fossen & Roy Gabrielsen
{{Structural geology Structural geology Tectonics