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fracture A general name for a break in a rock or other body that may or may not have any observable displacement. Fractures include joints, faults, and cracks formed under brittle deformation conditions and are a kind of permanent (nonelastic) strain. Brittle deformation processes generally involve the growth of fractures or sliding along existing fractures. Frictional sliding involves the sliding on preexisting fracture surfaces, whereas cataclastic flow includes grain-scale fracturing and frictional sliding producing macroscopic ductile flow over a band of finite width. Tensile cracking involves the propagation of cracks into unfractured material under tensile stress perpendicular to the maximum compressive stress, whereas shear rupture refers to the initiation of fracture at an angle to the maximum principal stress.
Fractures may propagate in one of three principal modes. Mode I refers to fracture growth by incremental extension perpendicular to the plane of the fracture at the tip. in mode ii propagation is where the fracture grows by incremental shear parallel to the plane of the fracture at the tip, in the direction of fracture propagation. mode iii is when the fracture grows by incremental shear parallel to the plane of the fracture at the tip, perpendicular to the direction of propagation.
Joints are fractures with no observable displacement parallel to the fracture surface. They generally occur in subparallel joint sets, and several sets often occur together in a consistent geometric pattern, forming a joint system. Joints are sometimes classified into extension joints or conjugate sets of shear joints, a subdivision based on the angular relationships between joints. most joints are continuous for only short distances, but in many regions master joints may run for long distances and control geomorphol-ogy or form air photo lineaments. microfractures or joints are visible only under the microscope and affect only a single grain.
many joints are contained within individual beds and have a characteristic joint spacing, measured perpendicular to the joints. This is determined by the relative strength of individual beds or rock types, the thickness of the jointed layer, and structural position, and is very important for determining the porosity and permeability of the unit. in many regions fractures control groundwater flow; location of aquifers, and migration and storage of petroleum and gas.
Joints and fractures, found in all kinds of environments, form by a variety of mechanisms. The contraction of materials induces the formation of desiccation cracks and columnar joints. mineral changes during diagenesis that lead to volume changes in the layer produce bedding plane fissility, characterized by fracturing parallel to bedding. unloading joints form by stress release, such as during uplift, ice sheet withdrawal, or quarrying operations. Exfoliation joints and domes may form by mineral changes, including volumetric changes during weathering, or by diurnal temperature variations. most joints have tectonic origins, typically forming in response to the last phase of tectonic movements in an area. other joints seem to be related to regional doming, folding, and faulting.
many fractures and joints exhibit striated or ridged surfaces known as plumose structures, since they vaguely resemble feathers. Plumose structures develop in response to local variations in propagation velocity and the stress field. The origin is the point at which the fracture originated, the mist is the small ridging on the surface, and the plume axis is the line that starts at the axis and from which individual barbs propagate. The twist hackle refers to the steps at the edge of the fracture plane along which the fracture has split into a set of smaller en echelon fractures.
British geologist E. m. Anderson elegantly explained the geometry and orientation of some fracture sets in 1905 and 1942, and in a now classic work published in 1951, The Dynamics of Faulting and Dyke Formation. General acceptance of this model by the scientific community led to Anderson's model being adopted, and many fault and fracture sets are described in terms of Anderson's theory. According to Andersonian theory the attitude of a fracture plane tells a lot about the orientation of the stress field that operated when the fracture formed. Fractures are assumed to form as shear fractures in a conjugate set, with the maximum compressive stress bisecting an acute (60°) angle between the two fractures. in most situations the surface of the Earth may be the maximum, minimum, or intermediate principal stress, since the surface can transmit no shear stress. if the maximum compressive stress is vertical, two fracture sets will form, each dipping 60° toward each other and intersecting along a horizontal line parallel to the intermediate stress. if the intermediate stress is vertical, two vertical fractures will form, with the maximum compressive stress bisecting the acute angle between the fractures. if the least compressive stress is vertical, two gently dipping fractures will form, and their intersection will be parallel to the intermediate principal stress.
other interpretations of fractures and joints include modifications of Andersonian geometries that include volume changes and deviations of principal stresses from the vertical. many joints show relationships to regional structures such as folds, with some developing parallel to the axial surfaces of folds and others crossing axial surfaces. other features on joint surfaces may be used to interpret their mode of formation. For instance, plumose structures typically indicate mode i or extensional types of formation, whereas the development of fault striations (known as slickensides) indicates mode ii or mode iii propagation. observations of these surface features, the fractures' relationships to bedding, structures such as folds and faults, and their regional orientation and distribution can lead to a clear understanding of their origin and significance.
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