Fault gouge

[2] The grinding and milling from the two sides of the fault moving along each other results in grain size reduction and fragmentation.

[4] To further elucidate, cataclasis involves the granulation of grains due to both brittle fracture and rigid body rotation—where rigid body rotation is when mineral grains exhibit rotation in agreement with the fault plane shear sense.

[1] The fault strength of a gouge is dependent on its composition, its water content, its thickness, temperature and it can easily be affected by any changes in effective normal stress and slip rate.

[7] The composition and concentration of clay minerals will affect the fault behavior in the brittle crust.

Gouges dominated by clay minerals (montmorillonite, illite, and chlorite) are consistently weaker.

The presence of water will reduce the frictional resistance between the grains of phyllosilicate minerals[8] Also, the permeability before shearing is usually higher than after the deformation.

[7] Because chlorite crystals form at higher pressure and temperature, it is most likely to remain as larger aggregates in shear zones compared to the smaller size of montmorillonite or illite grains which explains why the permeability is less affected.

[7] Fault gouges rich in chlorite and quartz keep their high permeability to a significant depth.

A larger thickness of fault gouge is associated with higher degrees of pore fluid pressure.

At the San Andreas Fault Observatory at Depth (SAFOD) they are overarchingly composed of serpentinite porphyroclasts and sedimentary rock amongst a Magnesium-rich clay matrix.

[10] Muddy Mountain Thrust: This fault is located in the southeast of Nevada, USA and represents tens of kilometers of transport at near-surface or surface conditions.

[11] The fault gouge contains less than 30% fragments of hanging wall dolomite and footwall sandstone clasts within a yellow-stained aggregate matrix with granular to foliated texture.