Fault breccia ( or ; Italian for "breach"), or tectonic breccia, is a
breccia
Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix.
The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of ...
(a
rock type consisting of angular
clasts) that was formed by
tectonic
Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents ...
forces.
Fault breccia is a
tectonite formed by localized zone of
brittle deformation (a
fault zone
In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectoni ...
) in a rock. Brecciation in fault zones influences
fault zone hydrogeology in its interaction with
groundwater
Groundwater is the water present beneath Earth's surface in rock and Pore space in soil, soil pore spaces and in the fractures of stratum, rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit ...
and
petroleum
Petroleum, also known as crude oil, or simply oil, is a naturally occurring yellowish-black liquid mixture of mainly hydrocarbons, and is found in geological formations. The name ''petroleum'' covers both naturally occurring unprocessed crud ...
deposits.
Origin
Fault breccias are tectonites formed primarily by tectonic movement along a localized zone of
brittle deformation (a
fault zone
In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectoni ...
) in a rock formation or province.
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
Groundwater is the water present beneath Earth's surface in rock and Pore space in soil, soil pore spaces and in the fractures of stratum, rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit ...
.
Secondary minerals such as
calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scratc ...
,
epidote
Epidote is a calcium aluminium iron sorosilicate mineral.
Description
Well developed crystals of epidote, Ca2Al2(Fe3+;Al)(SiO4)(Si2O7)O(OH), crystallizing in the monoclinic system, are of frequent occurrence: they are commonly prismatic in ...
,
quartz
Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical f ...
or
talc
Talc, or talcum, is a clay mineral, composed of hydrated magnesium silicate with the chemical formula Mg3Si4O10(OH)2. Talc in powdered form, often combined with corn starch, is used as baby powder. This mineral is used as a thickening agent a ...
can
precipitate
In an aqueous solution, precipitation is the process of transforming a dissolved substance into an insoluble solid from a super-saturated solution. The solid formed is called the precipitate. In case of an inorganic chemical reaction leading ...
from the circulating groundwater filling the voids and cementing the rock. 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
Cataclasite is a cohesive granular fault rock. Comminution, also known as cataclasis, is an important process in forming cataclasites. They fall into the category of cataclastic rocks which are formed through faulting or fracturing in the upper ...
.
Properties
Fault breccia has no
cohesion; it is normally an
unconsolidated
Soil consolidation refers to the mechanical process by which soil changes volume gradually in response to a change in pressure. This happens because soil is a two-phase material, comprising soil grains and pore fluid, usually groundwater. Whe ...
rock type, unless
cementation took place at a later stage. Sometimes a distinction is made between
fault gouge
Fault gouge is a type of fault rock best defined by its grain size. It is found as incohesive fault rock (rock which can be broken into its component granules at the present outcrop, only aided with fingers/pen-knife), with less than 30% clasts ...
and fault breccia, the first has a smaller
grain size
Grain size (or particle size) is the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may also be applied to other granular materials. This is different from the crystallite size, which ...
.
[Twiss, R.J. & Moores, E.M., 2000 (6th edition): ''Structural Geology'', W.H. Freeman & co, ; p. 55]
Zones of fault breccia and fault gouge in rocks can be a hazard for the construction of
tunnel
A tunnel is an underground passageway, dug through surrounding soil, earth or rock, and enclosed except for the entrance and exit, commonly at each end. A pipeline is not a tunnel, though some recent tunnels have used immersed tube cons ...
s and
mines, as the non-cohesive zones form weak places in the rock where a tunnel can collapse more easily.
See also
*
Breccia
Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix.
The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of ...
*
Cataclasite
Cataclasite is a cohesive granular fault rock. Comminution, also known as cataclasis, is an important process in forming cataclasites. They fall into the category of cataclastic rocks which are formed through faulting or fracturing in the upper ...
*
Mylonite
*
Fault (geology)
In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...
References
{{reflist, 30em
Rocks
Tectonics
Breccias