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Fault and fault structures

Products of fracturing and differential movements along fractures in continental and oceanic crustal rocks. Faults range in length and magnitude of displacement from small structures visible in hand specimens, displaying offsets of a centimeter (1 cm = 0.4 in.) or less, to long, continuous crustal breaks, extending hundreds of kilometers (1 km = 0.6 mi) in length and accommodating displacements of tens or hundreds of kilometers. Faults exist in deformed rocks at the microscopic scale, but these are generally ignored or go unrecognized in most geological studies. Alternatively, where microfaults systematically pervade rock bodies as sets of very closely spaced subparallel, planar fractures, they are recognized and interpreted as a type of cleavage which permitted flow of the rock body. Fractures along which there is no visible displacement are known as joints. These include shear joints, formed by fracturing and imperceptible movement of the walls of the fractures parallel to fracture surfaces, and tension joints, formed by negligible or barely visible displacement of the walls of the fractures perpendicular to the fracture surfaces. Large fractures which have accommodated major dilational openings (a meter or more) perpendicular to the fracture surfaces are known as fissures. Formation of fissures is restricted to near-surface conditions, for example, in areas of crustal stretching of subsidence. When faulting takes place under conditions of high temperature or pressure, zones of penetrative shear flow may develop which are best described as ductile fault zones.

Locating faults

The recognition of faults in continental regions of moderate to excellent rock exposure is generally straightforward. Classically, the process of systematic geological mapping has proved to be a powerful method for locating faults. In essence, faults can be identified and tracked by recognizing in mapped patterns the truncation and offset of one or several bedrock units. Depending on the nature of the exposure and movement on the fault, truncation and offset might produce a simple horizontal shifting of the dominant mapped pattern of bedrock units; alternatively, the faulting might lead to a pattern of repetition or omission of specific rock formations within the geologic column.

Valuable physical signatures which reveal the presence of faults include abnormally straight top­ographic lineaments or fault-line scarps; aligned springs issuing from fractured and favorably displaced bedrock; intensely fractured rocks, perhaps with zones of angular chunks of brecciated, rotated materials; fracture surfaces naturally polished through the movement process and etched with striations or grooves; dragging (folding) of rock layers out of their normal orientation; loosely consolidated, ground-up rock flour or paste, commonly referred to as gouge; radically crushed, cataclastically deformed rocks known as mylonites; and alteration, silicification, or mineralization brought about by circulation of hot fluids through shattered bedrock. Faults are simple to locate in areas of active or very recent mountain building, especially where faulting has broken the ground surface and produced scarps. See also: Mylonite

Fig. 1  Transform faults linking oceanic ridge segments. (After W. J. Morgan, Rises, trenches, great faults, and crustal blocks, J. Geophys. Res., 73:1959–1982, 1968)

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In continental areas of very poor rock exposure, in the subsurface, and in ocean basins, faults are much more difficult and costly to locate. However, major faults are routinely discovered through application of geophysical methods, especially seismic, gravity, and magnetic surveying. Abrupt contrasts in the geophysical signatures of rocks at depth signal the sharp truncation of bedrock by faults and allow the pattern of faults to be mapped. The geophysical exploration of fault and fault structures in the ocean floor has completely changed the way in which geologists view the Earth and earth dynamics. It is known that major fracture zones, unlike any recognized in continental regions, exist in the ocean floor. These fractures, hundreds to thousands of kilometers in length and spaced at tens of kilometers, pervade the Mid-Oceanic Ridge system and are usually oriented perpendicular to the crest of the ridge segment which they occupy. Interpreting these to be enormous faults which accommodate the movement of newborn oceanic crust as it spreads bilaterally from the Mid-Oceanic Ridge system, J. T. Wilson named them transform faults. See also: Geophysical exploration; Mid-Oceanic Ridge; Marine geology

Transform faults

Three fundamental types of transform faults exist: the first connects one ridge segment to another; the second connects a ridge segment (where new oceanic crust forms) to a trench (where oceanic crust is consumed through subduction); the third connects two trenches. Ridge-ridge transform faults, perhaps the easiest to visualize (Fig. 1), link parallel but offset ridge segments, and are characterized by shallow earthquake activity restricted to the part of the transform between the two ocean-ridge segments. The sense of movement along each ridge-ridge transform fault, as deduced from fault-plane solutions, is opposite to that which would explain the offset of the oceanic ridge. In essence, the movement along the transform fault does not produce the offset, but rather records the differential sliding-past (shearing) of the new ocean floor moving in opposite directions from the two ocean-ridge segments which are connected by the transform. Thus the transform faults serve, in this example, as part of the divergent plate boundary which accommodates sea-floor spreading and the movement of the two plates can be described by an imaginary pole of rotation located on the Earth's surface at a position defined by the intersection of great circles drawn perpendicular to points on the array of transform faults. The rate of relative movement along the ridge-ridge transform is a function of the rate of generation of new oceanic crust along the ridge and is generally on the order of several centimeters per year. See also: Mountain systems; Plate tectonics; Transform fault

Transform fault patterns are quite complicated, as is their evolution through time. Boundary zones between three adjacent tectonic plates (or triple junctions) are particularly difficult to evaluate with respect to geometry and kinematics (motions). Nonetheless, plate-tectonic analysis, including detailed assessment of transform faults, has had a revolutionary impact on understanding of the Cenozoic tectonic evolution of the Earth's crust. For example, scientists generally accept Tanya Atwater's hypothesis that the infamous San Andreas fault system of California is a transform fault boundary separating two enormous crustal plates, the North American and the Pacific. Relative movement between these plates is horizontal and right-handed, and in magnitude amounts to hundreds of kilometers.

Movements

This interpretation of the San Andreas Fault clearly demonstrates that some major fault systems in continental regions can be better understood in the context of the “new rules” afforded by study of transform faulting and plate tectonics. However, most fault systems in continental regions cannot be clearly and quantitatively linked to specific plate-tectonic movements or configurations, present or past, and they are analyzed and understood in an entirely different way; the guidelines for analysis are well established. In addition to describing the physical and geometric nature of faults and interpreting time of formation, it has been found to be especially important to determine the orientations of minor fault structures (such as striae and drag folds) which record the sense of relative movement.

Fig. 2  Slip on faults. (a) Block before faulting; (b) normal-slip; (c) reverse -slip; (d) strike-slip; (e) oblique-slip. (After F. Press and R. Siever, Earth, 2d ed., W. H. Freeman, 1978)

Evaluating the movement of faulting can be difficult, for the apparent relative movement (separation) of fault blocks as seen in map or outcrop may bear little or no relation to the actual relative movement (slip). The slip of the fault is the actual relative movement between two points or two markers in the rock that were coincident before faulting (Fig. 2). Strike-slip faults have resulted in horizontal movements between adjacent blocks; dip-slip faults are marked by translations directly up or down the dip of the fault surface; in oblique-slip faults the path of actual relative movement is inclined somewhere between horizontal and dip slip. Strike-slip faults are described as left- or right-handed, depending on the sense of actual relative movement; dip-slip faults are described as normal-slip, thrust-slip, or reverse-slip, depending on the sense of actual relative movement and on the dip and dip direction of the fault surface. Listric normal- slip faults are a type of normal-slip fault in which the inclination of the fault decreases. Movement on such a curved fault surface produces a profound backward rotation of the upper block.

Recognizing even the simplest translational fault movements in nature is often enormously difficult because of complicated and deceptive patterns created by the interference of structure and topography, and by the absence of specific fault structures which define the slip path (Fig. 3). While mapping, the geologist mainly documents apparent relative movement (separation) along a fault, based on what is observed in plan-view or cross-sectional exposures. In the separation sense, left- and right-lateral faults are those displaying apparent horizontal shifts of rock in map view. Such shifts are said to be apparent because the left- or right-lateral offset might actually have been produced by dip-slip, not strike-slip, movements. Normal, reverse, and thrust are separation terms for faults, and again the usage of each is based on apparent offset in cross-sectional view and on the dip and dip direction of the fault.

Fig. 3  Slip versus separation. (a) AB is the dip-slip. (b) After erosion of top of footwall block. The block has undergone a right separation. (After M. P. Billings, Structural Geology, 3d ed., Prentice-Hall, 1972)

Movement and offset on large, regional fault systems must be evaluated on the basis of displaced geologic terrains and abnormal stratigraphic relations. For example, 435 to 500 mi (700 to 800 km) of left-slip fault movement has been postulated in northern Mexico during the time period 150,000,000 to 170,000,000 years ago. The basis of the interpretation is truncation and offset of terrains of Precambrian and Paleozoic rocks in California and Arizona. Low-angle thrust movements in western Utah produced in Cretaceous time a 44-mi (70-km) west-to-east transport of thick Precambrian through Paleozoic miogeoclinal strata onto thin shelf and platform strata of the same age.

Stress conditions

Theory on faulting is based on applied physics and engineering, and focuses on the stress conditions under which rocks break. The theory is almost exclusively concerned with the brittle behavior of crustal rocks; as such, it is most applicable to faulting at upper crustal layers in the Earth. Results of deformational experiments under controlled temperature, pressure, and strain-rate conditions bear importantly on modern understanding of the dynamics of faulting. The results of theoretical and experimental work provide insight into why faults can be conveniently separated into categories of normal-slip, thrust-slip, and strike-slip.

Fig. 4  Mohr circle diagram constructed from three sets of compressional tests of limestone. Angles (61°, 65°, and 71°) are 2θ values. 1 psi = 6.89 kPa.

Fig. 5  Cross section of listric normal faults in the Basin and Range Province. (After R. E. Anderson, Geologic Map of the Black Canyon 15-Minute Quadrangle, Mohave County, Arizona, and Clark County, Nevada, USGS Map GQ-1394, 1978)

Forces which act on a rock body may be resolved by vector analysis into components of force acting in specific directions. These, in turn, can be converted to magnitudes and directions of stresses which tend to deform the body. This is done by dividing the force component by the surface area (of the body) on which it acts. Two types of stresses are distinguished, normal stress (σ), which acts perpendicular to a given surface, and shear stress (τ), which acts parallel to the given surface. Stress analysis, in effect, evaluates the magnitude of shear and normal stresses acting in all directions throughout a body and predicts the orientations of the surfaces along which faulting should occur.

Evaluating the distribution of stresses in a body that is acted upon by forces discloses that there are three unique directions within the stress field, called principal stress directions. These stress directions are mutually perpendicular, and the value of shear stress equals zero only along these three directions. Furthermore, one of the principal stress directions is characterized by the maximum value of normal stress (σ1) within the system, and another (σ3) is characterized by the minimum normal stress. Maximum shear stress values (τmax) occur along lines oriented 45° to the principal stress directions.

Two-dimensional mathematical analysis demonstrates that the magnitude of normal stress (σ) on any plane (that is, on any potential surface of faulting) in the body is given by Eq. (1), where σ =

 (1) 

greatest principal normal stress, σ3 = least principal normal stress and θ = angle between the greatest principal stress axis and the direction of shearing stress (in the plane for which σ is being evaluated). The magnitude of shear stress (τ) on that same plane is given by Eq. (2). The distribution of

 (2) 

paired values of normal stress and shear stress as a function of θ is such that shear stress (τ) is zero at values of 0°, 90°, 180°, and 270°, but attains maximum values at 45°, 135°, 225°, and 310°. Normal stress reaches a maximum at 90° and 270°, but is minimal at 0° and 180°. These variations in normal and shear stress values may be portrayed on a Mohr circle diagram, a graphical representation of the above equations (Fig. 4). Points on the periphery of the circle have coordinates (σ,τ) which correspond in value to normal stress and shear stress on a plane which makes an angle of θ with σ1, the greatest principal stress direction.

Given such a stress distribution, and assuming that the differential stress conditions (σ1 − σ3) exceed the strength of the rock body, the orientation of faulting can be determined. The Mohr-Coulomb law of failure [Eq. (3)]

 (3) 

predicts that faulting should occur at a critical shear stress level (τc) where τ0 = cohesive strength of the rock, σ = normal stress on the fault plane, and φ = angle of internal friction. The coefficient of internal friction, tan φ, equals σ/τ at failure. For most rocks, the coefficient of internal friction has a value between 0.4 and 0.7; thus the angle of internal friction, φ, commonly varies from 20 to 35°. The value of θ for such internal friction typically ranges from 27 to 35°, and is often 30°. In practice, the failure points on Mohr circle diagrams, as generated through experimental deformation, do not conform to the ideal straight-line failure envelope predicted on the basis of the Mohr-Coulomb law. Rather, the failure envelopes are smoothly curved in a way that describes an increase in 2θ with an increase in confining pressure (σ3). Predictions as to when and at what angle faulting should occur are further complicated by variables of temperature, strain rate, pore-fluid pressure, and presence of fractures. In fact, fundamental questions have been raised regarding the extent to which theory and short-term experimental work can be applied to some natural geological systems.

What arises from theory and experiments is that fractures form in an orientation such that they contain the axis of intermediate stress (σ2) and make an angle of θ (commonly around 30°) with σ1 and 90 − θ with σ3. Since the Earth's surface has no shear stress, principal stress directions near the Earth's surface tend to be vertical and horizontal, and depending on the relative configuration of the principal stresses, will give rise to thrust-slip, normal-slip, or strike-slip faults. The direction of movement on these faults is such that the wedge receiving the greatest compressive stress moves inward, whereas the wedge receiving the least compressive stress moves outward.

Examples

There are many excellent natural examples of normal-slip, thrust-slip, and strike-slip faults. The Basin and Range Province of the western United States displays a unique physiographic basin/range style because of pervasive large-scale normal-slip faulting (Fig. 5), which resulted from regional crustal extension within the last 15,000,000 years. Normal-slip faults accommodate extension.

Thrust-slip faults are an integral part of the tectonic framework of the southern Appalachian Mountains, the Sevier orogenic belt of western Utah and western Wyoming/eastern Idaho, and the Canadian Rockies. In mountain belts throughout the world, thrusting has played a major role in accommodating crustal shortening during mountain building. Major mechanical questions have arisen as to how enormous masses of crustal rocks can be thrust tens or hundreds of kilometers, since the apparent force required should have crushed the rock mass before it moved. The paradox of regional overthrusting has led to theories of faulting that emphasize the importance of factors such as high pore-fluid pressure, gravitational sliding, viscous creep of ductile materials, and underthrusting.

The best-documented strike-slip faulting is concentrated near margins of lithospheric plates, but such faulting has occurred in foreland tectonic regions as well, including the Rocky Mountains of the American West and mainland China. A major theme that has emerged from the study of high-angle faults, especially strike-slip faults, is that ancient faults in basement rocks are commonly reactivated in post-Precambrian time, producing zones of concentrated, superposed strain. This interpretation has been used to explain the classic monoclinal uplifts of the Colorado Plateau. See also: Graben; Horst; Structural geology

George H. Davis