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


Thrust geometry and nomenclature


Reverse faults

A thrust fault is a type of reverse fault that has a
dip
dip
of 45 degrees or less. If the angle of the fault plane is lower (often less than 15 degrees from the horizontal) and the displacement of the overlying block is large (often in the kilometer range) the fault is called an ''overthrust'' or ''overthrust fault''. Erosion can remove part of the overlying block, creating a ''fenster'' (or ''
window A window is an Hole, opening in a wall, door, roof, or vehicle that allows the exchange of light and may also allow the passage of sound and sometimes air. Modern windows are usually glazing (window), glazed or covered in some other transparenc ...
'') – 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

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 earthquake in Northridge, Los Angeles, California
1994 earthquake in Northridge, Los Angeles, California
, was caused by a previously undiscovered blind thrust fault. Because of their low
dip
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
peneplain
areas.


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
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 (geology), fissility (parallel layering).Blatt, H., and R.J. Tracy, 1 ...

mudstone
s or halite 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

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 Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. ...

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 The horse (''Equus ferus caballus'') is a Domestication, domesticated, odd-toed ungulate, one-toed, ungulate, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two Extant taxon, extant subspecies of wild horse, ''Equus fer ...
, 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.


Tectonic environment

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
tectonic
collisions or from subduction zone accretion. The resultant compressional forces produce
mountain A mountain is an elevated portion of the Earth's crust, generally with steep sides that show significant exposed bedrock. Although definitions vary, a mountain may differ from a plateau in having a limited Summit (topography), summit area, and ...

mountain
ranges. The
Himalayas The Himalayas, or Himalaya (; ; ), is a mountain range in Asia, separating the plains of the Indian subcontinent from the Tibetan Plateau. The range has some of the planet's highest peaks, including the very highest, Mount Everest. Over list ...

Himalayas
, the
Alps The Alps () ; german: Alpen ; it, Alpi ; rm, Alps ; sl, Alpe . are the highest and most extensive mountain range system that lies entirely in Europe, stretching approximately across seven Alpine countries (from west to east): France, Swi ...

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
rift
rift
ed margins, inversion of the buried paleo-rifts can induce the nucleation of thrust ramps. 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. Thrusts and duplexes are also found in s 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 , and Marcel Alexandre Bertrand in the Alps working on the Glarus Thrust; , Ben Peach and John Horne working on parts of the Moine Thrust
Scotland Scotland (, ) is a Countries of the United Kingdom, country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a Anglo-Scottish border, border with England to the southeast ...

Scotland
; Alfred Elis Törnebohm in the Scandinavian Caledonides and R. G. McConnell in the Canadian Rockies. 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. 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.


References


External links


Appalachian folding, thrusting and duplexing

Rob Butler's webpage on thrusts
{{Authority control Plate tectonics Structural geology Seismology