Thrust tectonics

Short description: Concept in structural geology

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.[1]

## Contents

* 1 Deformation styles
* 1.1 Thin-skinned deformation
* 1.2 Thick-skinned deformation
* 1.3 In-sequence thrusting
* 1.4 Out-of-sequence thrusting
* 2 Geological environments associated with thrust tectonics
* 2.1 Collisional zones
* 2.2 Restraining bends on strike-slip faults
* 2.3 Passive margins
* 3 References
* 4 External links

## 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.[2]

### 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.[3]

### 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.[4]

### In-sequence thrusting

In-sequence thrusting is when younger thursts develop progressively towards the foreland, away from the already deformed orogenic belts. The younger faults break underneath previous thrust sheets towards the undeformed basins.[5] Break-back thrusting is another type of in-sequence thrusting. After a thrust develop towards the foreland, a younger thrust develop above and hindward of it. Subsequent younger thrusts develop further hindward into the hinterland.[5]

Forelandward-developing thrust systems have been adopted as the typical deformation style in thrust tectonics. Break-back thrusting was previously classified under out-of-sequence thrusting. The current scientific vernacular has reconsidered it in-sequence thrusting.[6]

### Out-of-sequence thrusting

Out-of-sequence thrusting occurs when thrust deformation develops behind the active deformation front, violating the in-sequence thrusting principle. It involves reactivation of an old thrust that had formed in-sequence.[5] Out-of-sequence thrusting can also occur concurrently with a predominantly foreland-propagating thrust such as in central Nepal where active thrusting has been observed about 100 km (62 mi) hindward of the thrust front.[7]

It can also involve development of a new thrust fault within already deformed regions. All thrust deformation that occur behind the deformation front is considered out-of-sequence thrusting; including the development or reactivation of one or more thrust.[5]

Out-of-sequence thrust propagation may be the result of; a prolonged hiatus and associated erosion between two phases of thrusting in a multi-phase thrust belt, the development of thick-skinned thrusting on a reactivated basement fault after an earlier thin-skinned thrust belt has formed above it, a change in the presence or effectiveness along a basal detachment surface.[8]

## 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 plate and Eurasian plate, which formed the Zagros fold and thrust belt) and collisions between a continent and an island arc such as that which formed Taiwan.[9]

### 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,[10] and parts of the Dead Sea Transform.[11]

### 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)[12] and the Angola margin (with a salt detachment).[13]

## References

1. ↑ Butler, R.; Bond, C. (2020). "Chapter 9 – Thrust systems and contractional tectonics". in Scarselli, N.. Principles of Geologic Analysis. Regional Geology and Tectonics. 1 (2 ed.). Elsevier. pp. 149–167. doi:10.1016/B978-0-444-64134-2.00008-0. ISBN 9780444641359.
2. ↑ 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): 805–827. doi:10.1016/j.marpetgeo.2003.11.020. Bibcode: 2004MarPG..21..805S.
3. ↑ 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". AAPG Search and Discovery Article 70034. http://www.searchanddiscovery.com/documents/2007/07038hatcher/images/hatcher.pdf.
4. ↑ Butler, R. W. H.; Mazzoli, S.; Corrado, S.; De Donatis, M.; Di Bucci, D.; Gambini, R.; Naso, G.; Nicolai, C. et al. (2004). "Applying thick-skinned tectonic models to the Apennine thrust belt of Italy—Limitations and implications". Thrust tectonics and hydrocarbon systems: AAPG Memoir 82. pp. 647–67. http://www.see.leeds.ac.uk/research/igt/faultzone/pdf/betal_aapg.pdf.
5. ↑ 5.0 5.1 5.2 5.3 Morley, C. K. (1988). "Out-of-Sequence Thrusts". Tectonics 7 (3): 539-561. doi:10.1029/TC007i003p00539.
6. ↑ Li, Chengming; Zhang, Changhou; Cope, Tim; D., Yi Lin (2016). "Out-of-sequence thrusting in polycyclic thrust belts: An example from the Mesozoic Yanshan belt, North China Craton". Tectonics 35 (9): 2082-2116. doi:10.1002/2016TC004187.
7. ↑ Wobus, Cameron; Heimsath, Arjun; Whipple, Kelin; Hodges, Kip (2005). "Active out-of-sequence thrust faulting in the central Nepalese Himalaya". Nature 434: 1008–1011. doi:10.1038/nature03499.
8. ↑ Li, C.; Zhang, C.; Cope, T.D.; Lin, Y. (2016). "Out-of-sequence thrusting in polycyclic thrust belts: An example from the Mesozoic Yanshan belt, North China Craton". Tectonics 35 (9): 2082–2116. doi:10.1002/2016TC004187.
9. ↑ 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. https://books.google.com/books?id=8sqMGwkFtxAC.
10. ↑ 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; pp. 119–126
11. ↑ 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
12. ↑ 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
13. ↑ Marton, G, & Schoenborn, G. 2008. Salt Tectonics of the Continent-Ocean Transition, Deep-Water Angola. MAPG conference abstract

## External links

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

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Structural geology

Underlying theory|

* Strain
* Stress
* Stress field
* Mohr–Coulomb theory
* Mohr's circle
* Lamé's stress ellipsoid
* Strain partitioning
* Tension
* Overburden pressure
* Deformation mechanism
* Rock mechanics

Measurement conventions|

* Strike and dip
* Rake
* Inclinometer
* Brunton compass
* Stereographic projection
* Orthographic projection

Large-Scale Tectonics|

* Plate tectonics
* Orogeny
* Structural basin
* Rift
* Extensional tectonics
* Graben
* Half-graben
* Décollement
* Obduction
* List of tectonic plate interactions
* Nappe
* Continental collision
* Convergent boundary
* Fenster
* Klippe
* Fold and thrust belt
* Foreland basin
* Accretionary wedge
* Terrane
* Mountain formation
* Fold mountains
* Fault block
* Horst
* Rift valley
* Mélange
* Thick-skinned deformation
* Thin-skinned deformation
* Thrust tectonics
* Passive margin
* Extensional tectonics
* Divergent boundary
* Back-arc basin
* Intra-arc basin
* Strike-slip tectonics
* Syneclise
* Suture
* Saddle
* Inversion
* Horst and graben
* Horse
* Pull-apart basin
* Autochthon
* Allochthon

Fracturing|

* Joint
* Exfoliation joint
* Vein
* Dike
* Columnar jointing

Faulting|

* Thrust fault
* Detachment fault
* Disturbance
* Fault mechanics
* Fault trace
* Fault scarp
* Transform fault
* Transfer zone
* Cataclastic rock
* Cataclasite

Foliation and Lineation|

* Rock microstructure
* Crenulation
* Cleavage
* Slickenside
* Pressure solution
* Stylolite
* Compaction
* Fissility
* Oblique foliation
* Tectonic phase
* Tectonite

Folding|

* Vergence
* Anticline
* Syncline
* Dome
* Monocline
* Chevron
* Homocline
* Detachment fold

Boudinage|

* Competence

Kinematic Analysis|

* 3D fold evolution
* Section restoration
* Paleostress
* Paleostress inversion

Shear zone|

* Shear
* Pure shear
* Mylonite

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