Thrust tectonics or contractional tectonics is concerned with the structures formed, and the tectonic processes associated with, the shortening and thickening of the crust or lithosphere.
- In areas of thrust tectonics two main styles are recognized; thin-skinned deformation and thick-skinned deformation.
- The distinction is important as attempts to structurally restore the deformation will give very different results depending on the assumed geometry.
- Thick-skinned deformation refers to shortening that involves basement rocks rather than just the overlying cover.
- The two main types are: the collision of two continental tectonic plates (for example the Arabian and Eurasian plates, which formed the Zagros fold and thrust belt) and collisions between a continent and an island arc such as that which formed Taiwan.
- When a strike-slip fault is offset along strike such that the resulting bend in the fault hinders easy movement, e.g. a right stepping bend on a sinistral (left-lateral) fault, this will cause local shortening or transpression.
- Passive margins are characterized by large prisms of sedimentary material deposited since the original break-up of a continent associated with formation of a new spreading center.
- This wedge of material will tend to spread under gravity and, where an effective detachment layer is present such as salt, the extensional faulting that forms at the landward side will be balanced at the front of the wedge by a series of toe-thrusts.
- Examples include the outboard part of the Niger delta (with an over pressured mudstone detachment) and the Angola margin (with a salt detachment).