![Transpression model](https://upload.wikimedia.org/wikipedia/commons/2/26/Transpression_model.png)
In
geology
Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Ea ...
, transpression is a type of
strike-slip 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 ...
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,
stylolites,
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
![Aksai bend](https://upload.wikimedia.org/wikipedia/commons/3/39/Aksai_bend.png)
A fault bend, or
fault stepover, 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
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 right or a
dextral 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 survey
Geophysical survey is the systematic collection of geophysical data for spatial studies. Detection and analysis of the geophysical signals forms the core of Geophysical signal processing. The magnetic and gravitational fields emanating from the E ...
s, 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
Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never f ...
.
Transpressional regions
*
Altai Mountains (Western Mongolia and Southern Siberia, Russia)
* Western
Aleutian Subduction Zone
* Gobi Altai (Mongolia)
* "Big Bend" of the
San Andreas Fault Zone, (California, USA)
*
Alpine Fault
The Alpine Fault is a geological fault that runs almost the entire length of New Zealand's South Island (c. 480 km) and forms the boundary between the Pacific Plate and the Indo-Australian Plate. The Southern Alps have been uplifted on the f ...
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]
See also
*
Strike-slip tectonics
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 plates, ...
*
Structural geology
*
Transtension Transtension is the state in which a rock mass or area of the Earth's crust experiences both ''extensive'' and ''transtensive'' shear. As such, transtensional regions are characterised by both extensional structures ( normal faults, grabens) and ...
References
Structural geology