In geology, transpression is a type of

strike-slip In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

deformation that deviates from simple shear because of a simultaneous component of shortening perpendicular to the fault plane. This movement ends up resulting in oblique shear. It is generally very unlikely that a deforming body will experience "pure" shortening or "pure" strike-slip. The relative amounts of shortening and strike-slip can be expressed in the convergence angle alpha which ranges from zero (ideal strike-slip) to 90 degrees (ideal convergence). During shortening, unless material is lost, transpression produces vertical thickening in the crust. Transpression that occurs on a regional scale along

plate boundaries Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...

is characterized by oblique convergence. More locally, transpression occurs within restraining bends in strike-slip fault zones.

Transpressional structures

Transpressional shear zones are characterized by an association of structures that suggest zone-normal shortening and zone-parallel shearing. Commonly developed features include transposition foliations, lineations,

stylolite Stylolites (Greek: ''stylos'', pillar; ''lithos'', stone) are serrated surfaces within a rock mass at which mineral material has been removed by pressure dissolution, in a deformation process that decreases the total volume of rock. Minerals whi ...

s, folds, and reverse faults. Pure shear-dominated transpression usually gives steep lineations, while simple shear-dominated transpression favors horizontal lineations. It is also common for non-vertical transpressional zones to have a significant component of shearing parallel to the dipline of the zone boundary. In these zones, the lineations are between horizontal and vertical. The complete geometry presented by all structural elements in the zone is used to constrain the actual boundary displacements. .

Restraining bend

A fault bend, or

fault stepover Strike-slip tectonics or wrench tectonics is the type of tectonics that is dominated by lateral (horizontal) movements within the Earth's crust (and lithosphere). Where a zone of strike-slip tectonics forms the boundary between two tectonic plate ...

, forms when individual segments of the fault overlap and link together. The type of structures which form along the strike-slip fault depend on the sense of slip relative to the sense of stepping. When a sinistral fault steps to the right or a

dextral Sinistral and dextral, in some scientific fields, are the two types of chirality ("handedness") or relative direction. The terms are derived from the Latin words for "left" (''sinister'') and "right" (''dexter''). Other disciplines use different ...

fault steps to the left, a restraining bend is formed. Geologists may also refer to a restraining bend as a left bend. These are areas of positive relief (topographic uplift), crustal shortening, and exhumation of crystalline basement. As seen in deeply eroded outcrop exposures or from subsurface geophysical surveys, restraining bends commonly define positive flower structures. In plan view we see them form contractional strike-slip duplexes, subparallel reverse or oblique-slip contractional faults that are bounded by two strike-slip segments. Restraining bends are widespread on the Earth's surface, from sub-outcrop-scale examples to large scale mountain ranges. They have been theorized to occur on extraterrestrial bodies, like Jupiter's icy moon

Europa Europa may refer to: Places * Europe * Europa (Roman province), a province within the Diocese of Thrace * Europa (Seville Metro), Seville, Spain; a station on the Seville Metro * Europa City, Paris, France; a planned development * Europa Cliff ...

and on Venus.

Transpressional regions

Altai Mountains The Altai Mountains (), also spelled Altay Mountains, are a mountain range in Central Asia, Central and East Asia, where Russia, China, Mongolia and Kazakhstan converge, and where the rivers Irtysh and Ob River, Ob have their headwaters. The m ...

(Western Mongolia and Southern Siberia, Russia) * Western Aleutian Subduction Zone * Gobi Altai (Mongolia) * "Big Bend" of the

San Andreas Fault Zone The San Andreas Fault is a continental transform fault that extends roughly through California. It forms the tectonic boundary between the Pacific Plate and the North American Plate, and its motion is right-lateral strike-slip (horizontal). ...

, (California, USA) * Alpine Fault in New Zealand * Galeh–Doz pluton (Sanandaj–Sirjan Zone, Iran){{Cite journal, title = Dextral transpression in Late Cretaceous continental collision, Sanandaj–Sirjan Zone, western Iran, journal = Journal of Structural Geology, date = 2000-08-01, pages = 1125–1139, volume = 22, issue = 8, doi = 10.1016/S0191-8141(00)00023-7, first = Mohammad, last = Mohajjel, first2 = Christopher L, last2 = Fergusson

References

Structural geology

Transpressional structures

Restraining bend

Transpressional regions

See also

References