Thrust fault in the Qilian Shan, China. The older (left, blue and red) thrust over the younger (right, brown).
The Glencoul Thrust at Aird da Loch, Assynt in Scotland. The irregular grey mass of rock is formed of Archaean or PaleoproterozoicLewisian gneisses thrust over well-bedded Cambrianquartzite, along the top of the younger unit.
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Small thrust fault in the cliffs at Lilstock Bay, Somerset, England, displacement of about two metres (6.6 ft).
A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks.
- 1Thrust geometry and nomenclature
Thrust geometry and nomenclature[edit]
Diagram of the evolution of a fault-bend fold or 'ramp anticline' above a thrust ramp, the ramp links decollements at the top of the green and yellow layers
Diagram of the evolution of a fault propagation fold
Development of thrust duplex by progressive failure of ramp footwall
Antiformal stack of thrust imbricates proved by drilling, Brooks Range Foothills, Alaska
Reverse faults[edit]
A thrust fault is a type of reverse fault that has a dip of 45 degrees or less.[1][2]
If the angle of the fault plane is lower (often less than 15 degrees from the horizontal[3]) and the displacement of the overlying block is large (often in the kilometer range) the fault is called an overthrust or overthrust fault.[4] Erosion can remove part of the overlying block, creating a fenster (or window) – when the underlying block is exposed only in a relatively small area. When erosion removes most of the overlying block, leaving island-like remnants resting on the lower block, the remnants are called klippen (singular klippe).
Blind thrust faults[edit]
If the fault plane terminates before it reaches the Earth's surface, it is referred to as a blind thrust fault. Because of the lack of surface evidence, blind thrust faults are difficult to detect until they rupture. The destructive 1994 quake in Northridge, California, was caused by a previously undiscovered blind thrust fault.
Because of their low dip, thrusts are also difficult to appreciate in mapping, where lithological offsets are generally subtle and stratigraphic repetition is difficult to detect, especially in peneplain areas.
Fault-bend folds[edit]
Thrust faults, particularly those involved in thin-skinned style of deformation, have a so-called ramp-flat geometry. Thrusts mostly propagate along zones of weakness within a sedimentary sequence, such as mudstones or salt layers, these parts of the thrust are called decollements. If the effectiveness of the decollement becomes reduced, the thrust will tend to cut up the section to a higher stratigraphic level until it reaches another effective decollement where it can continue as bedding parallel flat. The part of the thrust linking the two flats is known as a ramp and typically forms at an angle of about 15°-30° to the bedding. Continued displacement on a thrust over a ramp produces a characteristic fold geometry known as a ramp anticline or, more generally, as a fault-bend fold.
Fault-propagation folds[edit]
Fault-propagation folds form at the tip of a thrust fault where propagation along the decollement has ceased but displacement on the thrust behind the fault tip is continuing. The continuing displacement is accommodated by formation of an asymmetric anticline-syncline fold pair. As displacement continues the thrust tip starts to propagate along the axis of the syncline. Such structures are also known as tip-line folds. Eventually the propagating thrust tip may reach another effective decollement layer and a composite fold structure will develop with characteristics of both fault-bend and fault-propagation folds.
Thrust duplex[edit]
Duplexes occur where there are two decollement levels close to each other within a sedimentary sequence, such as the top and base of a relatively strong sandstone layer bounded by two relatively weak mudstone layers. When a thrust that has propagated along the lower detachment, known as the floor thrust, cuts up to the upper detachment, known as the roof thrust, it forms a ramp within the stronger layer. With continued displacement on the thrust, higher stresses are developed in the footwall of the ramp due to the bend on the fault. This may cause renewed propagation along the floor thrust until it again cuts up to join the roof thrust. Further displacement then takes place via the newly created ramp. This process may repeat many times, forming a series of fault bounded thrust slices known as imbricates or horses, each with the geometry of a fault-bend fold of small displacement. The final result is typically a lozenge shaped duplex.
Most duplexes have only small displacements on the bounding faults between the horses and these dip away from the foreland. Occasionally the displacement on the individual horses is greater, such that each horse lies more or less vertically above the other, this is known as an antiformal stack or imbricate stack. If the individual displacements are greater still, then the horses have a foreland dip.
Duplexing is a very efficient mechanism of accommodating shortening of the crust by thickening the section rather than by folding and deformation.[5]
Tectonic environment[edit]
An example of thin-skinned deformation (thrusting) in Montana. Note that the white Madison Limestone is repeated, with one example in the foreground and another at a higher level to the upper right corner and top of the picture.
Large overthrust faults occur in areas that have undergone great compressional forces.
These conditions exist in the orogenic belts that result from either two continentaltectonic collisions or from subduction zone accretion.
The resultant compressional forces produce mountain ranges. The Himalayas, the Alps, and the Appalachians are prominent examples of compressional orogenies with numerous overthrust faults.
Thrust faults occur in the foreland basin which occur marginal to orogenic belts. Here, compression does not result in appreciable mountain building, which is mostly accommodated by folding and stacking of thrusts. Instead thrust faults generally cause a thickening of the stratigraphic section.
Foreland basin thrusts also usually observe the ramp-flat geometry, with thrusts propagating within units at a very low angle 'flats' (at 1–5 degrees) and then moving up-section in steeper ramps (at 5–20 degrees) where they offset stratigraphic units. Identifying ramps where they occur within units is usually problematic.
Thrusts and duplexes are also found in accretionary wedges in the ocean trench margin of subduction zones, where oceanic sediments are scraped off the subducted plate and accumulate. Here, the accretionary wedge must thicken by up to 200% and this is achieved by stacking thrust fault upon thrust fault in a melange of disrupted rock, often with chaotic folding. Here, ramp flat geometries are not usually observed because the compressional force is at a steep angle to the sedimentary layering.
![Arcgis Arcgis](/uploads/1/2/4/2/124272309/241069996.png)
History[edit]
Thrust faults were unrecognised until the work of Arnold Escher von der Linth, Albert Heim and Marcel Alexandre Bertrand in the Alps working on the Glarus Thrust; Charles Lapworth, Ben Peach and John Horne working on parts of the Moine ThrustScotland; Alfred Elis Törnebohm in the Scandinavian Caledonides and R. G. McConnell in the Canadian Rockies.[6][7] The realisation that older strata could, via faulting, be found above younger strata, was arrived at more or less independently by geologists in all these areas during the 1880s. Geikie in 1884 coined the term thrust-plane to describe this special set of faults. He wrote:
By a system of reversed faults, a group of strata is made to cover a great breadth of ground and actually to overlie higher members of the same series. The most extraordinary dislocations, however, are those to which for distinction we have given the name of Thrust-planes. They are strictly reversed faults, but with so low a hade that the rocks on their upthrown side have been, as it were, pushed horizontally forward.[8][9]
References[edit]
- ^'dip slip'. Earthquake Glossary. USGS. Retrieved 5 December 2017.
- ^'How are reverse faults different than thrust faults? In what way are they similar?'. UCSB Science Line. University of California, Santa Barbara. 13 February 2012. Retrieved 5 December 2017.
- ^Crosby, G.W. (1967). 'High Angle Dips at Erosional Edge of Overthrust Faults'. Bulletin of Canadian Petroleum Geology. 15 (3): 219–229.
- ^Neuendorf, K.K.E.; Mehl, Jr., J.P.; Jackson, J.A. (editors) (2005). Glossary of Geology (5th edition). Alexandria, Virginia: American Geological Institute. p. 462.CS1 maint: Extra text: authors list (link)
- ^'diagrams of the Brooks Range Thrust'(PDF).
- ^Peach, B.N., Horne, J., Gunn, W., Clough, C.T. & Hinxman, L.W. 1907. The Geological Structure of the North-west Highlands of Scotland (Memoirs of the Geological Survey, Scotland). His Majesty's Stationery Office, Glasgow.
- ^McConnell, R. G. (1887) Report on the geological structure of a portion of the Rocky Mountains: Geol. Surv. Canada Summ. Rept., 2, p. 41.
- ^'Thrust Tectonics'. www.see.leeds.ac.uk.
- ^Archibald Geikie (November 13, 1884). 'The Crystalline Rocks of the Scottish Highlands'. Nature. 31 (785): 29–31. Bibcode:1884Natur..31...29G. doi:10.1038/031029d0.
External links[edit]
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Learn about this topic in these articles:
fault type
- In faultReverse dip-slip faults result from horizontal compressional forces caused by a shortening, or contraction, of the Earth’s crust. The hanging wall moves up and over the footwall. Thrust faults are reverse faults that dip less than 45°. Thrust faults with a very low angle of…Read More
- In metamorphic rock: Structural features…the Main Central Thrust, a thrust fault that runs for hundreds of kilometres from east to west and was responsible for the transportation of rocks belonging to the Eurasian Plate southward over those of the Indian Plate. Along much of the length of this fault, the metamorphic rocks in the…Read More
Precambrian stratigraphy
- In Precambrian time: Structure and formation of greenstone-granite belts…are upside-down and deformed by thrusts and major horizontal folds (nappes). They have been subsequently refolded by upright anticlines (convex folds of rock) and synclines. The result of this thrusting is the repetition of the same stratigraphic successions on top of one another, creating a massive deposit of material up…Read More
superposition principle
- In dating: Determination of sequence…belts is the presence of thrust faults. Such faults, which are common in compression zones along continental edges, may follow bedding planes and then cross the strata at a steep angle, placing older units on top of younger ones. In certain places, the fault planes are only a few centimetres…Read More