Fault Structures

Faults are commonly associated with minor structures indicating shear:

• Grooves, Striations and Slickensides

  Fault surfaces often show grooves, straitions (scratches) and asymmetric fracture patterns called slickensides. All these features provide lineations on the fault surface indicating the direction of shear. Only slickensides provide evidence of the sense of movement. The asymmetric surface roughness features are steepest downhill or down the shear direction.

• Breccia and Gouge

  Faults are shear planes and commonly contain the debris from the frictional contact of the two surfaces. In strong rocks, material is fragmented to create a zone of crushed rock or fault breccia. In weaker rocks, the material in the fault plane can be reduced to a very fine clay-size infill known as fault gouge. Gouge is very significant in engineering terms since the shear strength of the discontinuity is that of the weak gouge rather than the wall rock.