Shear zone

In the upper crust, where rock is brittle, the shear zone takes the form of a fracture called a fault.

In the lower crust and mantle, the extreme conditions of pressure and temperature make the rock ductile.

Between these end-members, there are intermediate types of brittle–ductile (semibrittle) and ductile–brittle shear zones that can combine these geometric features in different proportions.

Mylonites start to occur with the onset of semibrittle behaviour in the alternating zone characterised by adhesive wear.

By passing into greenschist facies conditions, the pseudotachylites disappear and only different types of mylonites persist.

En echelon tension gash arrays (or extensional veins), characteristic of ductile-brittle shear zones, and sheath folds can also be valuable macroscopic shear-sense indicators.

Crustal-scale shear zones (megashears) can become 10 km wide and consequently show very large displacements from tens to hundreds of kilometers.

The softening can be brought about by the following phenomena: Furthermore, for a material to become more ductile (quasi-plastic) and undergo continuous deformation (flow) without fracturing, the following deformation mechanisms (on a grain scale) have to be taken into account: Due to their deep penetration, shear zones are found in all metamorphic facies.

Ductile shear zones start at greenschist facies conditions and are therefore restricted to metamorphic terranes.

Most often they are not isolated in their occurrence, but commonly form fractal-scaled, linked up, anastomosing networks which reflect in their arrangement the underlying dominant sense of movement of the terrane at that time.

Shear zones can host economically viable mineralizations, examples being important gold deposits in Precambrian terranes.