Transpression is a geological term used to describe a region of the Earth's crust that experiences strike-slip shear and a component of shortening, resulting in oblique shear. Transpression typically occurs at a regional scale, such as plate boundaries that have an oblique convergence. More locally, transpression can occur along strike-slip fault zones which are not perfectly planar. In reality, many tectonic regimes that were previously defined as simple strike-slip shear zones are actually transpressional. It is generally very unlikely that a deforming body will experience 'pure' shortening or 'pure' strike-slip.

Transpressional shear zones are characterised by the co-existence of different structures, related to both strike-slip shear and shortening. End member structures include pure strike-slip faults and pure thrust (reverse) faults

Thrust fault

A thrust fault is a type of fault, or break in the Earth's crust across which there has been relative movement, in which rocks of lower stratigraphic position are pushed up and over higher strata. They are often recognized because they place older rocks above younger...

. Faults which have components of both (termed 'oblique' slip faults) are abundant. In addition, structures such as folds

Fold (geology)

The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in...

, 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 bends

Restraining bends are transpressional structures that form at any point where the orientation of a strike-slip fault becomes oblique to the regional slip vector. They also form where two segments of a strike-slip fault overlap, and the relay zone between the segments experiences transpression.

Transpressional regions

• Altai Mountains (Western Mongolia and Southern Siberia, Russia)
• Gobi Altai (Central Mongolia)
• San Andreas Fault Zone, Salton Trough (California, USA)