Australia: The Land Where Time Began

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Strike-Slip Faults

Strike-slip faults form, according to Anderson's theory, when the intermediate principal stress, σ2, is vertical as a result of gravitational loading, meaning that in a horizontal surface of the remaining principal axis 1 direction is subjected to a compression that is greater than the vertical load and the other is subjected to extension, or to a compressive stress that is less intense than the vertical load. This results in a direction of horizontal regional shortening, which is parallel to the direction of σ1, normal to the maximum lengthening direction, which is parallel to the direction of σ3. Strike-slip faults form in a number of geologic settings, such as transform plate boundaries, that are characterised by horizontal shearing, and block movement along faults that are close to vertical, these being the most prominent. These transform faults are perpendicular to the spreading centers at midocean ridges, which separate lithospheric reaches that are expanding at different rates. The term that is strictly used for all faults affecting the entire lithosphere, which mark plate boundaries in both continental and oceanic settings, is transform fault.

Transcurrent faults are other large-scale faults in continental settings not associated with plate boundaries. Strike-slip faults are present in other geotectonic environments, such as compressional settings and extensional provinces, as well as transform plate boundaries, and in mountain belts where they can perform an important function accommodating the overall deformation, even when they are local or minor features. An example of this can be observed in extensional areas or compressive settings, where strike-slip faults, termed transfer faults, that are oriented parallel to the direction of displacement, where they adjust the movement of half-grabens that show different polarities or separate areas that are subjected to different rates of extension. Minor strike-slip faults termed tear faults that are associated with folds, thrusts or normal faults, are similar to transform faults, but are of minor extension. Bends (frontal vertical ramps) and stepovers may form, though most strike-slip faults have vertical surfaces that are roughly planar, which form straight traces on the surface. In both dextral and sinistral faults bends and stepovers can be produced either to the right or the left. The authors1 suggest these features are important as they impose special conditions of stress along the faults, such as a dextral fault with a right bend or stepover is subjected to extension in the bend of the offset area as a result of the separation of blocks that occurs during movement along the fault.

Transtensional areas are areas that are subjected to extension along a strike-slip fault, in such features the bends are extensional or releasing. Pull-apart basins are basins that have developed in transtensional areas. A dextral fault with a left bend or stepover is another example that illustrates a very different behaviour, with the blocks being compressed against each other in the 'bended' or offset area to form a transpressive area, the bends or stepovers being called contractional or restraining. Strike-slip duplexes or flower structures, that are particular structures  defined by horsts between strike-slip vertical faults, are caused by transpressional and transtensional settings. Horsts with a gravitational or normal component, called normal or negative flower structures, develop in transtensional areas, while horsts with a negative component are developed in transpressional contexts, the duplexes being formed are termed reverse or positive flower structures.


Sources & Further reading

  1. Leeder, Mike, Perez-Arlucea, Marta, 2006, Physical Processes in Earth and Environmental Sciences, Blackwell Publishing Ltd.


Author: M. H. Monroe
Last updated 15/05/2013

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