Fractures in Geology: Types, Mechanisms, Andersonian Theory, and Fracture Zone Aquifers

A fracture is a general term for a break in a rock or other body that may or may not exhibit observable displacement. Fractures include joints, faults, and cracks formed under brittle deformation conditions and represent a type of permanent (nonelastic) strain. Brittle deformation processes generally involve fracture growth or sliding along existing fractures. Frictional sliding occurs on preexisting fracture surfaces, whereas cataclastic flow involves grain-scale fracturing and frictional sliding that produces macroscopic ductile flow over a band of finite width. Tensile cracking involves crack propagation into unfractured material under tensile stress perpendicular to the maximum compressive stress, while shear rupture refers to fracture initiation 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 fracture plane at the tip. Mode II propagation occurs when the fracture grows by incremental shear parallel to the fracture plane at the tip, in the direction of propagation. Mode III is when the fracture grows by incremental shear parallel to the fracture plane at the tip but 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 appear 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 angular relationships between joints. Most joints are continuous for only short distances, but in many regions master joints may run for long distances, controlling geomorphology or forming air photo lineaments. Microfractures or micro-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. Spacing 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, the location of aquifers, and the migration and storage of petroleum and gas.

Joints and fractures, found in all kinds of environments, form by a variety of mechanisms. Contraction of materials induces the formation of desiccation cracks and columnar joints. Mineral changes during diagenesis that lead to volume changes in a 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, while others relate to regional doming, folding, and faulting.

Many fractures and joints exhibit striated or ridged surfaces known as plumose structures, so named because they vaguely resemble feathers. Plumose structures develop in response to local variations in propagation velocity and the stress field. The origin is the point where the fracture started; the mist is the small ridging on the surface; the plume axis is the line 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 1951 work, The Dynamics of Faulting and Dyke Formation. General acceptance of this model led to Andersonian theory, and many fault and fracture sets are described in its terms. According to this theory, the attitude of a fracture plane reveals much 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 Earth’s surface may be the maximum, minimum, or intermediate principal stress because the surface can transmit no shear stress. If the maximum compressive stress is vertical, two fracture sets 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 form, with the maximum compressive stress bisecting the acute angle between them. If the least compressive stress is vertical, two gently dipping fractures form, and their intersection will be parallel to the intermediate principal stress.

Other interpretations of fractures and joints include modifications of Andersonian geometries that account for 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 fold axial surfaces and others crossing them. Features on joint surfaces may be used to interpret their mode of formation. For instance, plumose structures typically indicate Mode I (extensional) 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 and to structures such as folds and faults, and their regional orientation and distribution can lead to a clear understanding of their origin and significance.

Fracture Zone Aquifers. Fractures and joints are in many places important aquifers, forming deep spaces in the Earth where water can be stored without evaporation or contamination for centuries or even thousands of years. Faults and fractures develop at various scales, from continent-crossing faults to fractures visible only microscopically. The internal properties of the rock and the external stresses imposed on it determine the location and orientation of these discontinuities in the rock fabric. Fractures at various scales represent zones of increased porosity and permeability, and by forming networks they are able to store and carry vast amounts of water.

The concept of fracture zone aquifers explains groundwater behavior in large fault-controlled watersheds. Fault zones in this case serve as collectors and transmitters of water from one or more recharge zones, with surface and subsurface flow strongly controlled by regional tectonism. Both the yield and quality of water in these zones are usually higher than those of average wells in any rock type. High-grade water for such a region would be 250 gallons (950 liters) per minute or greater, and total dissolved solids measured in water from such high-yielding wells will be lower than the regional average.

The fracture zone aquifer concept examines variations in groundwater flow as influenced by secondary porosity over an entire watershed. It attempts to integrate basin data to describe the unique effects of secondary porosity on groundwater flow, infiltration, transmissivity, and storage. The concept includes variations in precipitation over the catchment area; for example, orographic effects cause mountainous terrain precipitation to be substantially greater than at lower elevations. Rainfall collects over a large catchment area containing zones of high permeability due to intense bedrock fracturing associated with major fault zones.

The multitude of fractures within these highly permeable zones “funnel” water into other fracture zones that are down gradient from where the water enters the system. These funnels may form a network covering hundreds of square miles (or kilometers). Fault and fracture zones serve as conduits for groundwater and often act as channelways for surface flow. Their intersections form rectilinear drainage patterns sometimes exposed at the surface but also present below ground, converging down the hydrologic gradient (places to which water naturally flows downhill). In some regions, these rectilinear patterns are not always visible on the surface owing to vegetation and sediment cover.

The convergence of these groundwater conduits increases the amount of water available for recharge. The increased permeability, water volume, and ratio of water to minerals within these fault/fracture zones help maintain water supply quality. These channels occur in fractured, nonporous media (crystalline rocks) as well as in fractured, porous media (sandstone, limestone). At some point in the groundwater course after convergence, the gradient decreases. The sediment cover over the major fracture zone becomes thicker and acts as a water storage unit with primary porosity. The major fracture zone acts both as a transmitter of water along conduits and as a water storage basin along connected zones with secondary (and/or primary) porosity.

Groundwater within this layer or lens often flows at accelerated rates. The result can be pressurization of groundwater both in the fracture zone and in the surrounding material. Precipitation can almost instantaneously replenish the rapid flow in the conduit. The surrounding materials are replenished more slowly but also release water more slowly, serving as a storage unit to replenish the conduit between precipitation events. Once the zones are saturated, any extra water that flows into them will overflow if an exit is available. In a large-area watershed, this water likely flows along subsurface channelways under pressure until some form of exit is found in the confining environment. Substantial amounts of groundwater may flow along an extension of the main fault zone controlling the watershed and may vent at submarine extensions of the fault zone, forming coastal or offshore freshwater springs.

The fracture zone aquifer concept is particularly applicable to areas underlain by crystalline rocks that have undergone a multiple deformational history including extensional tectonics. This is especially true for areas where recharge is possible from seasonal and/or sporadic rainfall on mountainous regions adjacent to flat desert areas. Fracture zone aquifers are distinguished from horizontal aquifers in that: (a) they drain numerous wadis over extensive areas, many extending for tens of miles (dozens of kilometers); (b) they constitute conduits to mountainous regions where recharge potential from rainfall is high; (c) some may connect several horizontal aquifers, thereby increasing the volume of accumulated water; (d) because the water source is at higher elevations, the artesian pressure at the groundwater level may be high; and (e) they are usually missed by conventional drilling because water is often at depths of up to 1,000 feet (hundreds of meters).

The characteristics of fracture zone aquifers make them an excellent water source in arid and semiarid environments. Fracture zone aquifers are located by seeking major faults, which are usually clearly displayed in images obtained from spacecraft in Earth’s orbit because they are emphasized by drainage. Thus the first step in evaluating groundwater potential of any region is to study structures displayed in satellite images to map faults, fractures, and linear features of uncertain origin (called lineaments). Such a map is then compared with a drainage map showing wadi locations. The combination of many wadis and major fractures indicates a larger potential for groundwater storage. Furthermore, the intersection between major faults would increase both porosity and permeability and, hence, water-collection capacity.

Groundwater resources in arid and semiarid lands are scarce and must be properly used and thoughtfully managed. Most of these resources are “fossil” waters, having accumulated under wet climates during the geological past. The present rates of recharge from occasional rainfall cannot sufficiently replenish the aquifers. Therefore the resources must be used sparingly without exceeding the optimum pumping rates for each water well field.

FURTHER READING: Anderson, E. M. The Dynamics of Faulting and Dyke Formation. London: Oliver and Boyd, 1951.
Bisson, Robert A., and Farouk El-Baz. “Megawatersheds Exploration Model.” Proceedings of the 23rd International Symposium on Remote Sensing of Environment. Ann Arbor, Mich.: Environmental Research Institute of Michigan, 1990.

El-Baz, Farouk. “Utilizing Satellite Images for Groundwater Exploration in Fracture Zone Aquifers.” International Conference on Water Resources Management in Arid Countries. Muscat, Oman: Ministry of Water Resources, 1995.
Gale, J. E. “Assessing the Permeability Characteristics of Fractured Rock.” In Recent Trends in Hydrogeology, edited by T. N. Narasimhan. Geological Society of America Special Paper 189 (1982).
Kusky, Timothy M., and Farouk El-Baz. “Structural and Tectonic Evolution of the Sinai Peninsula, using Landsat Data: Implications for Groundwater Exploration.” Egyptian Journal of Remote Sensing 1 (1999): 69-100.

National Academy of Sciences. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, D.C.: National Academy Press, 1996.
Pollard, David D., and Aydin Atilla. “Progress in Understanding Jointing over the Past Century.” Geological Society of America 100 (1988): 1181-1204.
Ramsay, John G., and Martin I. Huber. The Techniques of Modern Structural Geology, Volume 2: Folds and Fractures. London: Academic Press, 1987.
Wright, E. P., and W. G. Burgess. “The Hydrogeology of Crystalline Basement Aquifers in Africa.” Geological Society of London Special Publication 66 (1992).