Thrust fault

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Thrust fault in the Qilian Shan, China. The older (left, blue and red) thrust over the younger (right, brown). Thrust fault Qilian Shan.jpg
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 Paleoproterozoic Lewisian gneisses thrust over well-bedded Cambrian quartzite, along the top of the younger unit. Glencoul Thrust Fault Zone in Scotland 2014.jpg
The Glencoul Thrust at Aird da Loch, Assynt in Scotland. The irregular grey mass of rock is formed of Archaean or Paleoproterozoic Lewisian gneisses thrust over well-bedded Cambrian quartzite, along the top of the younger unit.
Small thrust fault in the cliffs at Lilstock Bay, Somerset, England; displacement of about two metres (6.6 ft) Small thrust North verging.jpg
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.

Contents

Thrust geometry and nomenclature

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 Faultbendfold.png
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 Fault-propagation fold.gif
Diagram of the evolution of a fault propagation fold
Development of thrust duplex by progressive failure of ramp footwall Duplex1.png
Development of thrust duplex by progressive failure of ramp footwall
Antiformal stack of thrust imbricates proved by drilling, Brooks Range Foothills, Alaska Antiformal stack.jpg
Antiformal stack of thrust imbricates proved by drilling, Brooks Range Foothills, Alaska

Reverse faults

A thrust fault is a type of reverse fault that has a dip of 45 degrees or less. [1] [2]

Strike and dip terms used to denote the orientation of a geologic feature

Strike and dip refer to the orientation or attitude of a geologic feature. The strike line of a bed, fault, or other planar feature, is a line representing the intersection of that feature with a horizontal plane. On a geologic map, this is represented with a short straight line segment oriented parallel to the strike line. Strike can be given as either a quadrant compass bearing of the strike line or in terms of east or west of true north or south, a single three digit number representing the azimuth, where the lower number is usually given, or the azimuth number followed by the degree sign.

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 ).

Window (geology) used in geology to describe areas in which a lower geologic unit crops out in the middle of higher units

A tectonic window is a geologic structure formed by erosion or normal faulting on a thrust system. In such a system the rock mass that has been transported by movement along the thrust is called a nappe. When erosion or normal faulting produces a hole in the nappe where the underlying autochthonous rocks crop out this is called a window. Klippen are also a feature near windows.

Klippe

A klippe is a geological feature of thrust fault terrains. The klippe is the remnant portion of a nappe after erosion has removed connecting portions of the nappe. This process results in an outlier of exotic, often nearly horizontally translated strata overlying autochthonous strata. Examples of klippes include:

Blind thrust faults

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.

1994 Northridge earthquake Earthquake

The 1994 Northridge earthquake was a magnitude of 6.7 (Mw), blind thrust earthquake that occurred on January 17, 1994, at 4:30:55 a.m. PST in the San Fernando Valley region of the County of Los Angeles. Its epicenter was in Reseda, a neighborhood in the north-central Valley. The quake had a duration of approximately 10–20 seconds, and its peak ground acceleration of 1.8g (16.7 m/s2) was the highest ever instrumentally recorded in an urban area in North America. Strong ground motion was felt as far away as Las Vegas, Nevada, about 220 miles (360 km) from the epicenter. The peak ground velocity at the Rinaldi Receiving Station was 183 cm/s, the fastest ever recorded.

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.

Peneplain A low-relief plain formed by protracted erosion

In geomorphology and geology, a peneplain is a low-relief plain formed by protracted erosion. This is the definition in the broadest of terms, albeit with frequency the usage of peneplain is meant to imply the representation of a near-final stage of fluvial erosion during times of extended tectonic stability. Peneplains are sometimes associated with the cycle of erosion theory of William Morris Davis, but Davis and other workers have also used the term in a purely descriptive manner without any theory or particular genesis attached.

Fault-bend folds

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.

Mudstone Fine grained sedimentary rock whose original constituents were clays or muds

Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.063 millimetres (0.0025 in) with individual grains too small to be distinguished without a microscope. With increased pressure over time, the platy clay minerals may become aligned, with the appearance of fissility or parallel layering. This finely bedded material that splits readily into thin layers is called shale, as distinct from mudstone. The lack of fissility or layering in mudstone may be due to either original texture or the disruption of layering by burrowing organisms in the sediment prior to lithification. Mud rocks such as mudstone and shale account for some 65% of all sedimentary rocks. Mudstone looks like hardened clay and, depending upon the circumstances under which it was formed, it may show cracks or fissures, like a sun-baked clay deposit.

Fault-propagation folds

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

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.

Sandstone A clastic sedimentary rock composed mostly of sand-sized particles

Sandstone is a clastic sedimentary rock composed mainly of sand-sized mineral particles or rock fragments.

Horse (geology) Any block of rock completely separated from the surrounding rock either by mineral veins or fault planes.

Horse is the geological technical term used for any block of rock completely separated from the surrounding rock either by mineral veins or fault planes. In mining the term refers to a block of country rock entirely encased within a mineral lode. In structural geology the term was first used to describe the thrust-bounded imbricates found within a thrust duplex. In later literature it has become a general term for any block entirely bounded by faults, whether the overall deformation type is contractional, extensional or strike-slip in nature.

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

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. SunRiver.JPG
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 continental tectonic 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. When thrusts are developed in orogens formed in previously rifted margins, inversion of the buried paleo-rifts can induce the nucleation of thrust ramps. [6]

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. Thrusts have also been detected in cratonic settings, where "far-foreland" deformation has advanced into intracontinental areas. [7]

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.

History

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 Thrust Scotland; Alfred Elis Törnebohm in the Scandinavian Caledonides and R. G. McConnell in the Canadian Rockies. [8] [9] 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. [10] [11]

Related Research Articles

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 movement. Large faults within the Earth's crust result from the action of plate tectonic forces, with the largest forming the boundaries between the plates, such as subduction zones or transform faults. Energy release associated with rapid movement on active faults is the cause of most earthquakes.

Fold (geology) fold in geology

In structural geology, folds occur when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in size from microscopic crinkles to mountain-sized folds. They occur singly as isolated folds and in extensive fold trains of different sizes, on a variety of scales.

Anticline geological term

In structural geology, an anticline is a type of fold that is an arch-like shape and has its oldest beds at its core. A typical anticline is convex up in which the hinge or crest is the location where the curvature is greatest, and the limbs are the sides of the fold that dip away from the hinge. Anticlines can be recognized and differentiated from antiforms by a sequence of rock layers that become progressively older toward the center of the fold. Therefore, if age relationships between various rock strata are unknown, the term antiform should be used.

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 klippe.

Sevier orogeny

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

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.

Décollement

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.

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.

Fracture (geology) Geologic structure

A fracture is any separation in a geologic formation, such as a joint or a fault that divides the rock into two or more pieces. A fracture will sometimes form a deep fissure or crevice in the rock. Fractures are commonly caused by stress exceeding the rock strength, causing the rock to lose cohesion along its weakest plane. Fractures can provide permeability for fluid movement, such as water or hydrocarbons. Highly fractured rocks can make good aquifers or hydrocarbon reservoirs, since they may possess both significant permeability and fracture porosity.

Strike-slip tectonics is concerned with the structures formed by, and the tectonic processes associated with, zones of lateral displacement within the Earth's crust or lithosphere.

Salt tectonics study of salt-controlled structures (like salt domes), mechanisms, and tectonic deformation involving salt or other evaporates

Salt tectonics, or halokinesis, or halotectonics, is concerned with the geometries and processes associated with the presence of significant thicknesses of evaporites containing rock salt within a stratigraphic sequence of rocks. This is due both to the low density of salt, which does not increase with burial, and its low strength.

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

Fold mountains are mountains that form mainly by the effects of folding on layers within the upper part of the Earth's crust. Before either plate tectonic theory developed, or the internal architecture of thrust belts became well understood, the term was used for most mountain belts, such as the Himalayas. The term is still fairly common in physical geography literature but has otherwise generally fallen out of use except as described below. The forces responsible for formation of fold mountains are called orogenic movements. The term orogenic has derived from a Greek word meaning mountain building. These forces act at tangent to the surface of the earth and are primarily a result of plate tectonics.

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.

Geology of the Pyrenees

The Pyrenees are a 430-kilometre-long, roughly east–west striking, intracontinental mountain chain that divide France, Spain, and Andorra. The belt has an extended, polycyclic geological evolution dating back to the Precambrian. The chain's present configuration is due to the collision between the microcontinent Iberia and the southwestern promontory of the European Plate. The two continents were approaching each other since the onset of the Upper Cretaceous (Albian/Cenomanian) about 100 million years ago and were consequently colliding during the Paleogene (Eocene/Oligocene) 55 to 25 million years ago. After its uplift, the chain experienced intense erosion and isostatic readjustments. A cross-section through the chain shows an asymmetric flower-like structure with steeper dips on the French side. The Pyrenees are not solely the result of compressional forces, but also show an important sinistral shearing.

Section restoration

In structural geology section restoration or palinspastic restoration is a technique used to progressively undeform a geological section in an attempt to validate the interpretation used to build the section. It is also used to provide insights into the geometry of earlier stages of the geological development of an area. A section that can be successfully undeformed to a geologically reasonable geometry, without change in area, is known as a balanced section.

Detachment fold

A detachment fold, in geology, occurs as layer parallel thrusting along a decollement develops without upward propagation of a fault; the accommodation of the strain produced by continued displacement along the underlying thrust results in the folding of the overlying rock units. As a visual aid, picture a rug on the floor. By placing your foot on one end and pushing toward the other, the rug slides across the floor (decollement) and folds upward. Figure 1, is a generalized representation of the geometry assumed by a detachment fault.

Zagros fold and thrust belt zone of deformed crustal rocks, formed in the foreland of the collision between the Arabian Plate and the Eurasian Plate

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 and about 7% of all reserves globally.

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.

Growth fault

Growth faults are syndepositional or syn-sedimentary extensional faults that initiate and evolve at the margins of continental plates. They extend parallel to passive margins that have high sediment supply. Their fault plane dips mostly toward the basin and has long-term continuous displacement. Figure one shows a growth fault with a concave upward fault plane that has high updip angle and flattened at its base into zone of detachment or décollement. This angle is continuously changing from nearly vertical in the updip area to nearly horizontal in the downdip area.

3D fold evolution

In geology, 3D fold evolution is the study of the full three dimensional structure of a fold as it changes in time. A fold is a common three-dimensional geological structure that is associated with strain deformation under stress. Fold evolution in three dimensions can be broadly divided into two stages, namely fold growth and fold linkage. The evolution depends on fold kinematics, causes of folding, as well as alignment and interaction of the each structure with respect to each other. There are several ways to reconstruct the evolution progress of folds, notably by using depositional evidence, geomorphological evidence and balanced restoration. Understanding the evolution of folds is important because it helps petroleum geologists to gain a better understanding on the distribution of structural traps of hydrocarbon.

References

  1. "dip slip". Earthquake Glossary. USGS . Retrieved 5 December 2017.
  2. "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.
  3. Crosby, G. W. (1967). "High Angle Dips at Erosional Edge of Overthrust Faults". Bulletin of Canadian Petroleum Geology. 15 (3): 219–229.
  4. 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)
  5. "diagrams of the Brooks Range Thrust" (PDF).
  6. Martins-Ferreira, Marco Antonio Caçador (April 2019). "Effects of initial rift inversion over fold-and-thrust development in a cratonic far-foreland setting". Tectonophysics. 757: 88–107. doi:10.1016/j.tecto.2019.03.009.
  7. Martins-Ferreira, Marco Antonio Caçador (April 2019). "Effects of initial rift inversion over fold-and-thrust development in a cratonic far-foreland setting". Tectonophysics. 757: 88–107. doi:10.1016/j.tecto.2019.03.009.
  8. 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.
  9. McConnell, R. G. (1887) Report on the geological structure of a portion of the Rocky Mountains: Geol. Surv. Canada Summ. Rept., 2, p. 41.
  10. "Thrust Tectonics". www.see.leeds.ac.uk.
  11. 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.