Faults, like joints, are the result of overstressing of the rock but are distinct from joints in that they exhibit measurable shear displacement. Faults are predominantly associated with mountain-building episodes and tectonic movements. They can occur as single features or more commonly as fault systems. Faults are often highly important to rock-engineering projects because they are often associated with large zones of weakened rock. They also act as barriers or passageways for fluids, cause general disruption to the geology, and are associated with sudden releases of energy (earthquakes). An example of a disruptive fault (inactive) is illustrated in figure below.

Figure (a) Roadside cutting above Morenos, Portugal. (b) Interpretation. Fractured, faulted and folded
Carboniferous schist (with alternating sandstone and shaley horizons). One major fault is quite obvious but there has also been slippage along schistosity with a well-defined fracture cleavage confined to one shaley horizon. The degree of jointing is remarkably low. To the right of the walking stick, the rock below the main fault has lost a recognisable structure compared with the folded zone to its left. Clearly, the history here is complicated – not a simple single faulting episode.

Evidence of a fault:
– Stretch marks

– Mountain mirror (smooth paraclase)

– Fault breccias

Types of faults

Fault system

Tectonic Graben or trench – relatively lowered central part

Horst – relatively elevated central part

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