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Fault gouge is a tectonite (a rock formed by tectonic forces) with a very small grain size. Fault gouge has no cohesion and it is normally an unconsolidated rock type, unless cementation took place at a later stage. A fault gouge forms in the same way as fault breccia, the latter also having larger clasts. In comparison to fault breccia, which is another incohesive fault rock, fault gouge has less visible fragments (less than 30% visible fragments regarding fault gouges, and more than 30% regarding fault breccia).
Gouge-filled faults can be weak planes in rock masses. If compressive stresses are enough these can cause compressive yielding or eventually rock fracture. 
Fault gouge forms by tectonic movement along a localized zone of brittle deformation (a fault zone) in a rock. The grinding and milling that results when the two sides of the fault zone move along each other results in a material that is made of loose fragments. First a fault breccia will form, but if the grinding continues the rock becomes fault gouge.
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- Bertuzzi, R., 2015: Tunnel Support Loading Mechanism, UNSW, Sydney, Australia; p. 1
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