Transpression

File:Transpression model.png

Simple model for transpression: strike-slip zone with an additional and simultaneous shortening across the zone. Also induces vertical uplift.

In geology, 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 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 contractional structures and features; such as stylolites, cleavages, reverse faults, and lineations. Pure shear-dominated transpression gives vertical lineations, while a singular simple shear component favors horizontal lineations. Additionally, structures such as folds, tension fractures, and Riedel shears all form in the shear zone, but at different angles to those observed in simple strike-slip fault zones.

Restraining bend

File:Restraining Bend.png

A contractional duplex that has developed at the bend/stepover along a strike-slip fault.

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 fault steps to the right or a dextral fault steps to the left, a restraining bend is formed. 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 duplexs, 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 and on Venus.

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 in New Zealand
• Galeh–Doz pluton (Sanandaj–Sirjan Zone, Iran)

See also

• Strike-slip tectonics
• Structural geology
• Transtension