Fault breccia

Fault breccia (IPA: /ˈbrɛtʃiə, ˈbrɛʃ-/, Italian: breach), or tectonic breccia is a breccia (a rock type consisting of angular clasts) that was formed by tectonic forces. Fault breccia has no cohesion, it is normally an unconsolidated rock type, unless cementation took place at a later stage. Sometimes a distinction is made between fault gouge and fault breccia, the first has a smaller grain size. [Twiss, R.J. & Moores, E.M., 2000 (6^th edition): "Structural Geology", W.H. Freeman & co, ISBN 0-7167-2252-6; p. 55]

Zones of fault breccia and fault gouge in rocks can be a hazard for the construction of tunnels and mines, as the non-cohesive zones form weak places in the rock where a tunnel can collapse more easily.

Origin

Fault breccia is a tectonite, it forms by tectonic movement along a localized zone of brittle deformation (a fault zone) in a rock. The grinding and milling occurring when the two sides of the fault zone moving along each other results in a material that is made of loose fragments. Because of this fragmentation fault zones are easily infiltrated by groundwater. Minerals like calcite, epidote, quartz or talc can precipitate from the water depositing so the rock cements. However, when the tectonic movement along the fault zone continues the cement itself can be fragmented leading to a new gouge material containing neoformed clasts.

Deeper in the Earth's crust, where temperatures and pressures are higher, the rocks in the fault zone can still brecciate, but they keep their internal cohesion. The resulting type of rock is called a cataclasite.

ee also

*Breccia
*Cataclasite
*Fault (geology)

References