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Transform fault(red)

A transform fault or transform boundary, also known as conservative plate boundary, is a fault which runs along the boundary of a tectonic plate. The relative motion of such plates is horizontal in either sinistral or dextral direction. Typically, some vertical motion may also exist, but the principal vectors in a transform fault are oriented horizontally. Not all faults are transform faults, and not all plate boundaries are transform faults. Most transform faults are found on the ocean floor, where they often offset active spreading ridges to form a zigzag plate boundary. However, the best-known transform faults are found on land.

Transform faults are one of the three types of plate boundaries in plate tectonics. Transform faults occur where plates slide past each other, and crust is neither destroyed nor created. Divergent faults occur where magma seeps up through the earth's crust, and new crust is formed as the plates are pushed away from each other. Convergent faults occur where two plates collide and one plate is forced under the other plate (in a process known as subduction or obduction), and as a consequence, the plate being forced under is melted and destroyed. When two continental plates converge, they may push up against each other (in a process known as continental collision) forming mountain ranges, however, subduction may still occur. Plate tectonics was proposed by J. Tuzo Wilson in 1965 and he particularly recognized the concept in the case of the transverse strike-slip faults along which mid-oceanic ridges are off-set.

Contents

Background

John Tuzo Wilson recognized that because of friction, the plates cannot simply glide past each other. Rather, stress builds up in both plates and when it reaches a level that exceeds the strain threshold of rocks on either side of the fault the accumulated potential energy is released as strain. Strain is both accumulative and/or instantaneous depending on the rheology of the rock; the ductile lower crust and mantle accumulates deformation gradually via shearing whereas the brittle upper crust reacts by fracture, or instantaneous stress release to cause motion along the fault. The ductile surface of the fault can also release instantaneously when the strain rate is too great. The energy released by instantaneous strain release is the cause of earthquakes, a common phenomenon along transform boundaries.

Mechanics

The left- or right-lateral motion of one plate against another along transform faults can cause highly visible seismic lithosphereic crust effects. Because of friction, the plates cannot simply glide past each other. Rather, stress builds up in both plates and when it reaches a level that exceeds the strain threshold of rocks on either side of the fault the accumulated potential energy is released as strain. Strain is both cumulative and instantaneous depending on the rheology of the rock; the ductile lower crust and mantle accumulates deformation gradually via shearing whereas the brittle upper crust reacts by fracture, or instantaneous stress release to cause motion along the fault. The ductile surface of the fault can also release instantaneously when the strain rate is too great. The energy released by instantaneous strain release is the cause of earthquakes, a common phenomenon along transform boundaries.

Examples

The San Andreas fault in California is a major transform fault which runs between the Mendocino Triple Junction in the north and the northern end of the East Pacific Rise somewhere beneath the Imperial Valley in the south.

The Southern Alps rise dramatically beside the Alpine Fault on New Zealand's West Coast. About 500 kilometres (300 mi) long; northwest at top.

Other examples include:

See also

References

  • International Tectonic Dictionary - AAPG Memoir 7, 1967
  • The Encyclopedia of Structural Geology and Plate Tectonics - Ed. by Carl K. Seyfert, 1987
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