Strike-slip tectonics

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

Deformation styles

Development of riedel shears in a zone of dextral shear
Flower structures developed along minor restraining and releasing bends on a dextral (right-lateral) strike-slip fault

Riedel shear structures

In the early stages of strike-slip fault formation, displacement within basement rocks produces characteristic fault structures within the overlying cover. This will also be the case where an active strike-slip zone lies within an area of continuing sedimentation. At low levels of strain the overall simple shear causes a set of small faults to form. The dominant set, known as R shears, form at about 15° to the underlying fault with the same shear sense. The R shears are then linked by a second set, the R' shear that form at about 75° to the main fault trace.[1] These two fault orientations can be understood as conjugate fault sets at 30° to the short axis of the instantaneous strain ellipse associated with the simple shear strain field caused by the displacements applied at the base of the cover sequence. With further displacement the Riedel fault segments will tend to become fully linked, often with the development of a further set of shears known as 'P shears', which are roughly symmetrical to the R shears with respect to the overall shear direction, until a throughgoing fault is formed.[2] The somewhat oblique segments will link downwards into the fault at the base of the cover sequence with a helicoidal geometry.[3]

Flower structures

In detail many strike-slip faults at surface consist of en echelon and/or braided segments in many cases probably inherited from previously formed riedel shears. In cross-section the displacements are dominantly reverse or normal in type depending on whether the overall fault geometry is transpressional (i.e. with a small component of shortening) or transtensional (with a small component of extension). As the faults tend to join downwards onto a single strand in basement, the geometry has led to these being termed flower structure. Fault zones with dominantly reverse faulting are known as positive flowers, those with dominantly normal offsets are known as negative flowers. The identification of such structures, particularly where positive and negative flowers are developed on different segments of the same fault, are regarded as reliable indicators of strike-slip.[4]

Geological environments associated with strike-slip tectonics

San Andreas Transform Fault on the Carrizo Plain

Areas of strike-slip tectonics are associated with:

Oceanic transform boundaries

Mid-ocean ridges are broken into segments offset from each other by transform faults. The active part of the transform links the two ridge segments. Some of these transforms can be very large, such as the Romanche fracture zone, whose active portion extends for about 300 km.

Continental transform boundaries

Transform faults within continental plates include some of the best known examples of strike-slip structures, such as the San Andreas Fault, the Dead Sea Transform, the North Anatolian Fault and the Alpine Fault.

Lateral ramps in areas of extensional or contractional tectonics

Major lateral offsets between large extensional or thrust faults are normally connected by diffuse or discrete zones of strike-slip deformation allowing transfer of the overall displacement between the structures.

Zones of oblique collision

In most zones of continent-continent collision the relative movement of the plates is oblique to the plate boundary itself. The deformation along the boundary is normally partitioned into dip-slip contractional structures in the foreland with a single large strike-slip structure in the hinterland accommodating all the strike-slip component along the boundary. Examples include the Main Recent Fault along the boundary between the Arabian and Eurasian plates behind the Zagros fold and thrust belt,[5] the Liquiñe-Ofqui Fault that runs through Chile and the Great Sumatran fault that runs parallel to the subduction zone along the Sunda Trench.

The deforming foreland of a zone of continent-continent collision

The process sometimes known as escape tectonics, first elucidated by Paul Tapponnier, occurs during a collisional event where one of the plates deforms internally along a system of strike-slip faults. The best known active example is the system of strike-slip structures observed in the Eurasian plate as it responds to collision with the Indian plate, such as the Kunlun fault and Altyn Tagh fault.[6]

See also

References

  1. ↑ Katz, Y.; Weinberger R. & Aydin A. (2004). "Geometry and kinematic evolution of Riedel shear structures, Capitol Reef National Park, Utah". Journal of Structural Geology 26: 491–501. Bibcode:2004JSG....26..491K. doi:10.1016/j.jsg.2003.08.003. Retrieved 6 May 2011. 
  2. ↑ Tchalenko, J.S. (1970). "Similarities between Shear Zones of Different Magnitudes". Geological Society of America Bulletin 81 (6): 1625–1640. Bibcode:1970GSAB...81.1625T. doi:10.1130/0016-7606(1970)81[1625:SBSZOD]2.0.CO;2. Retrieved 6 May 2011. 
  3. ↑ Ueta, K.; Tani, K. 2001. Ground Surface Deformation in Unconsolidated Sediments Caused by Bedrock Fault Movements: Dip-Slip and Strike-Slip Fault Model Test and Field Survey. American Geophysical Union, Fall Meeting 2001, abstract #S52D-0682
  4. ↑ Harding, T.P. 1990. Bulletin American Association of Petroleum Geologists. 74
  5. ↑ Talebian, M. Jackson, J. 2004. A reappraisal of earthquake focal mechanisms and active shortening in the Zagros mountains of Iran. Geophysical Journal International, 156, pages 506-526
  6. ↑ Tapponnier, P. & Molnar, P. 1979. Active faulting and Cenozoic tectonics of the Tien Shan, Mongolia and Baykal regions. Journal Geophysical Research, 84, B7, 3425 – 3459.

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