Brittle deformation The permanent change that occurs in a solid material due to the growth of fractures and/ or due to sliding on fractures. Brittle deformation only occurs when stresses exceed a critical value, and thus only after a rock has already undergone some elastic and/or plastic behavior.

Brittle fault zone A band of finite width in which slip is distributed among many smaller discrete brittle faults, and/or in which the fault surface is bordered by pervasively fractured rock.
Brittle fault A single surface on which movement occurs specifically by brittle deformation mechanisms.

Cataclasis A deformation process that involves distributed fracturing, crushing, and frictional sliding of grains or of rock fragments.

Crack Verb: to break or snap apart. Noun: a fracture whose displacement does not involve shear displacement (i.e., a joint or microjoint).

Fault Broad sense: a surface or zone across which there has been measurable sliding parallel to the surface. Narrow sense: a brittle fault. The narrow definition emphasizes the distinctions between faults, fault zones, and shear zones.

Fracture zone A band in which there are many parallel or subparallel fractures. If the fractures are wavy, they may anastomose with one another. Note: The term has a somewhat different meaning in the context of ocean-floor tectonics.

Fracture A general term for a surface in a material across which there has been loss of continuity and, therefore, strength. Fractures range in size from grain-scale to continent-scale.
Healed microcrack A microcrack that has cemented back together. Under a microscope, it is defined by a plane containing many fluid inclusions. (Fluid inclusions are tiny bubbles of gas or fluid embedded in a solid).

Joint A natural fracture which forms by tensile loading, i.e., the walls of the fracture move apart veryc slightly as the joint develops. Note: A minority of geologists argue that joints can form due to shear loading.

Microfracture A very small fracture of any type. Microfractures range in size from the dimensions of a single grain to the dimensions of a thin section.

Microjoint A microscopic joint; microjoints range in size from the dimensions of a single grain to the dimensions of a hand-specimen. Synonymous with microcrack.

Shear fracture A macroscopic fracture that grows in association with a component of shear parallel to the fracture. Shear fracturing involves coalescence of microcracks.

Shear joint A surface that originated as a joint but later became a surface of sliding. Note: A minority of geologists consider a shear joint to be a joint that initially formed in response to shear loading.

Shear rupture A shear fracture.

Shear zone A region of finite width in which ductile shear strain is significantly greater than in the surrounding rock. Movement in shear zones is a consequence of ductile deformation mechanisms (cataclasis, crystal plasticity, diffusion).

Vein A fracture filled with minerals precipitated from a water solution.

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