Shear Zone - Occurrence and Examples of Shear Zones

Occurrence and Examples of Shear Zones

Due to their deep penetration, shear zones are found in all metamorphic facies. Brittle shear zones are more or less ubiquitous in the upper crust. Ductile shear zones start at greenschist facies conditions and are therefore bound to metamorphic terranes.

Shear zones can occur in the following geotectonic settings:

• transcurrent setting – steep to vertical:
  • strike-slip zones.
  • transform faults.

• compressive setting — low-angle
  • recumbent fold nappes (at the base of).
  • subduction zones.
  • thrust sheets (at the base of).

• extensional setting — low-angle
  • metamorphic core complex detachments.

Shear zones are dependent neither on rock type nor on geological age. 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.

Some good examples of shear zones of the strike-slip type are the South Armorican Shear Zone and the North Armorican Shear Zone in Brittany, the North Anatolian Fault Zone in Turkey, and the Dead Sea Fault in Israel. Shear zones of the transform type are the San Andreas Fault in California, and the Alpine Fault in New Zealand. A shear zone of the thrust type is the Moine Thrust in northwestern Scotland. An example for the subduction zone setting is the Japan Median Tectonic Line. Detachment fault related shear zones can be found in southeastern California, e.g. the Whipple Mountain Detachment Fault. An example of a huge anastomosing shear-zone is the Borborema Shear Zone in Brazil.