Rocks Produced in Shear Zones
The deformations in shear zones are responsible for the development of characteristic fabrics and mineral assemblages reflecting the reigning pressure–temperature (pT) conditions, flow type, movement sense, and deformation history. Shear zones are therefore very important structures for unravelling the history of a specific terrane.
Starting at the Earth's surface, the following rock types are usually encountered in a shear zone:
- uncohesive fault rocks. Examples being fault gouge, fault breccia, and foliated gouge.
- cohesive fault rocks like crush breccias and cataclasites (protocataclasite, cataclasite, and ultracataclasite).
- glassy pseudotachylites.
Both fault gouge and cataclasites are due to abrasive wear on brittle, seismogenic faults.
- foliated mylonites (phyllonites).
- striped gneiss.
Mylonites start to occur with the onset of semibrittle behaviour in the alternating zone characterised by adhesive wear. Pseudotachylites can still be encountered here. By passing into greenschist facies conditions, the pseudotachylites disappear and only different types of mylonites persist. Striped gneisses are high-grade mylonites and occur at the very bottom of ductile shear zones.
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