Thrust fault
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A thrust fault is a particular type of fault, or break in the fabric of the Earth's crust with resulting movement of each side against the other, in which a lower stratigraphic position is pushed up and over another. This is the result of compressional forces.
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[edit] Reverse faults
Another name for high angle thrust fault is reverse fault. The difference between a thrust and a reverse fault is in their influence. A reverse fault occurs primarily across lithological units whereas a thrust usually occurs within or at a low angle to lithological units. It is because of this that it is often difficult to recognise thrusts because their deformation and dislocation can be difficult to detect when they occur within the same rocks without appreciable offset of lithological contacts.
If the angle of the fault plane is low (generally less than 20 degrees from the horizontal) and the displacement of the overlying block is large (often in the kilometer range) the fault is called an overthrust. Erosion can remove part of the overlying block, creating a fenster (or window) when the underlying block is only exposed in a relatively small area. When erosion removes most of the overlying block, leaving only island-like remnants resting on the lower block, the remnants are called klippen (singular klippe).
[edit] 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.
Because of their low dip, thrusts are also difficult to appreciate in mapping, where lithological offsets are generally subtle and stratigraphic repetition difficult to detect especially in peneplanated areas.
[edit] Duplex Thrusts
Duplex thrusts are particular types of thrust faults which occur in orogenic belts above a detachment zone, or are associated with extreme thickening of the stratigraphic section, generally in response to an angle of compression greater than 20 degrees to the sedimentary layering.
Duplexes are, essentially, packets of relatively undisturbied lithology bounded by a floor thrust and a roof thrust, which isolate the duplex packet from rock masses which are faulted around it on these thrusts. The floor thrust and the roof thrust part up-dip of the duplex and merge again down-dip, isolating a lozenge-shaped sliver of rock.
Duplexing is a very efficient mechanism of accommodating shortening of the crust by thickening the section rather than by folding and deformation.[1]
[edit] 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 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 in 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.
[edit] History
Thrust faults were unrecognised until the work of Escher, Heim and Bernard in the Alps working on the Glarus Thrust, and Lapworth, Peach and Horne working on parts of the Moine Thrust Scotland. The realisation that older strata could, via faulting, be found above younger strata, was arrived at more or less independently by geologists in both these areas in about 1884. Geikie in the same year coined the term thrust to describe this special set of faults. "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." Archibald Geikie 1884, Nature.
[edit] External links
- Knockan Crag and the Moine Thrust, Scotland
- Appalachian folding, thrusting and duplexing
- Photos of structures and faults
- Rob Butler's webpage on thrusts
[edit] References
- 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.
