Normal faults

Normal faults are common features that occur when the earth's crust is subjected to tensional forces. The sense of movement is primarily vertical and results in an extension of the crust. These faults are generally associated with broad-flexed or uplifted areas and are an integral part of the modern concept of plate tectonics.

Normal Fault Formation

A normal fault is a fracture that separates two crustal blocks, one of which has been displaced downward along the fractured surface. Some workers use the term “gravity fault” to indicate an apparent normal fault if genesis, rather than geometry, is implied. Crustal blocks overlying or underlying a normal or reverse fault are commonly designated as the hanging wall and footwall blocks, respectively. In a normal fault, the hanging wall moves downward relative to the footwall. In a reverse fault, the hanging wall moves upward relative to the footwall, with a dip greater than 45 degrees. These are old descriptive terms that were used in the early English coal-mining districts. Faults that were inclined toward the down-dropped side were common in the area and the term “normal fault” was applied. At places where the movement was in the opposite direction, the breaks in the rock were designated as reverse faults. The displacement of normal faults, which can be intermittent, ranges from less than a meter up to thousands of meters. The inclination or dip of the fault can be from nearly horizontal to vertical but generally ranges from 45 to 60 degrees. In some areas, the angle of the dip decreases with depth and results in a curved surface that is concave upward. This curved surface is termed a listric normal fault and is a common type of fracture in the Gulf coast region of the United States.

Normal faults are the product of a dynamic process that results in conditions of changing stress (force per unit area) along a plane of weakness in the earth's crust. The fault develops from a point along this plane. According to Lamoraal de Sitter, the stress is at a minimum at the starting point along the surface and at a maximum at the edges. Because of the edge conditions, the plane steepens and splits into several divergent smaller faults, or splays. These small segments may join to form a larger normal fault with a scalloped trace. Subparallel normal faults with smaller displacements generally accompany the large faults. At places, the adjacent beds are systematically fractured without significant displacement. These fractures are termed joints and generally have a high density (close spacing) near the fault.

The deforming forces can be related to three mutually perpendicular but unequal axes designated as maximum principal stress (σ₁), intermediate principal stress (σ₂), and minimum principal stress (σ₃). In the case of normal faults, the primary deforming force (σ₁) is vertical or nearly vertical. The least stress (σ₃) is horizontal. The normal faults are actually steeply inclined shear fractures that formed in response to forces promoting the sliding of adjacent blocks past each other. These fractures generally form at an angle of 30 degrees from the maximum principal stress. The orientation of the maximum principal stress is horizontal for thrust faults and for wrench (transform) faults.

Normal Fault Classification

Normal faults are classified according to the type of displacement of fault blocks relative to a known point. Based on the slip or actual movement of formerly adjacent points on opposing fault blocks, three types are commonly designated: strike-slip, or movement along the trend of the fault; dip-slip, or movement directly down the fault surface; and oblique-slip, or diagonal movement down the fault surface. The movement along all these examples is translational. Consequently, no rotation of the blocks in respect to each other has occurred outside a disturbed zone adjacent to the fault. If the actual displacement is not known, the term “separation” is used by most geologists to indicate the apparent movement on a map or cross-section. Heave and throw are the horizontal and vertical components, respectively, of the dip separation as measured along a vertical profile that is at right angles to the trend of the fault.

There are several varieties of normal faults. Detachment (denudation) faults have a low angle of dip (usually less than 30 degrees) and are common features in the western United States. Growth or contemporaneous faults are listric normal faults that are active during sediment accumulation. Layered rocks on the downthrown side of the fault are thicker than equivalent beds on the upthrown side. Smaller subsidiary or antithetic faults commonly form on the downthrown side of the main fault but dip in a direction opposite to the master fault. These are common features along the Gulf coast of the United States.

Some special faults may result from the same stress orientation as normal faults; that is, the maximum principal stress is vertical. These closely related faults, however, are characterized by rotational movement between blocks. For example, hinge faults increase in displacement along the length of the fault; linear features that were parallel before faulting are not all parallel after faulting. A pivotal (scissor) fault is another example of a rotational fault. In this type, the fault blocks pivot about an axis that is at right angles to the fault surface. The movement on the downthrown side is in opposite directions (up and down) along the length of the fault.

Associated Features

Major steeply dipping normal faults occur in the Colorado Plateau (Arizona and New Mexico) where these features are closely associated with regional flexures called monoclines. The western part of the plateau along the Colorado River is divided into large structural blocks by three north-trending faults. One of these faults, the Hurricane fault of Arizona and Utah, dips to the west and has a maximum displacement of 3,048 meters.

At some places, normal faults bound narrow blocks that have been displaced up or down. An uplifted or elevated block is called a horst; the depressed block is termed a graben. Topographically, these structural features may be represented by a series of mountain ranges and intervening valleys, respectively. The Basin and Range Province of the western United States is a good example of this horst-and-graben type complex. In this region, both low-and high-angle normal faults have shaped an area that extends from southern Oregon southward to northern Mexico; the area has been broadly uplifted and the crust stretched in an east-west direction by the normal faulting. Some estimates of the total extension across the region are more than 100 percent. The displacement along these large faults ranges up to 5,486 meters. In Europe, the Rhine Graben is a classic example of a well-developed rift system. This narrow structural trough trends northward for nearly 300 kilometers through West Germany and controls the path of the upper Rhine River.

Clues to Identification

Normal faults are recognized in the field or on vertical aerial photographs and satellite images by identifying features characteristic of this type of fault. On the earth's surface, these faults occur as geological lines that are revealed by a sharp, curvilinear line in the bedrock that is usually accentuated by vegetation, a sharp contact in adjacent rock types in section or map view, a marked change in structural style, an abrupt change in topography, or an anomalous drainage pattern. Normal faults are usually recognized in vertical drill holes by an omission of rock layers; comparison of rock samples (drill cuttings) or mechanical logs from adjacent borings will generally reveal the part of the rock column that is missing. Caution, however, must be exercised to make sure that the strata have not been eroded, have not thinned, or have not changed character laterally.

There are many distinctive geometric, mineralogic, or physiographic features that are associated with large normal faults. Some of these features, however, are also characteristic of other types of faults and should thus merely be considered as “clues” in recognizing normal faults. Many normal faults are expressed at the surface by low cliffs or scarps that reflect the minimum displacement along the fault; however, these straight slopes are usually modified by erosion to form faultline scarps. The scarps may be notched by streams crossing the upthrown block at a high angle to the fault trend; continued erosion by these side streams may result in triangular-shaped bedrock facets on the footwall with fan-shaped stream deposits on the downthrown hanging wall block. Movement along the fault usually disturbs rocks adjacent to the break and results in beds along the fault being bent up or down; in other words, the rocks bend before rupture takes place. This phenomenon is called fault drag. Normal drag occurs when rocks on the upthrown side are bent down into the fault and rocks on the downthrown side are bent upward. Reverse drag occurs when beds on both sides of the fracture are bent down.

Because movement along the fault produces an irregular surface between the fault blocks in some places, subsurface fluids are provided an avenue to the surface. Both hot-and cold-water springs are common occurrences along large normal faults. Solutions moving along the fault may also deposit minerals such as calcite or quartz between the blocks. These fillings are usually stained yellow or reddish brown by iron oxide.

Movement along the fault is usually recorded by fine lines or by narrow grooves on the fault surface called slickensides. These features, however, may indicate only the latest movement along the fault. Impressions of the slicken lines are sometimes preserved on the outer surfaces of the mineral fillings. A series of larger scale (several centimeters or more of relief), parallel grooves and ridges may produce an undulating fault surface. The movement of large blocks along the fault usually produces low, steplike irregularities on the surface that are steeply inclined in the direction of movement. These features can be used to identify the direction of movement when a fault surface is poorly exposed.

As the fault blocks slide past each other, angular rock fragments are dislodged and may accumulate to form a tectonic breccia; a microscopic breccia, or mylonite, may also result from movement. In some cases, the dislodged rock may be ground to a pliable, claylike substance called gouge. At places, a large fragment of bedrock, called a horse, is caught along a normal fault.

Characteristic Map Patterns

Normal faults generally occur in definite region patterns that are easily represented on geologic maps. Zones of overlapping, or en echelon, normal faults are common in the Gulf Coast region of the United States. In Texas, individual faults within the Balcones and Mexia-Luling fault systems are not continuous along strike but overlap with adjacent faults that have a similar trend. These fault zones generally follow the path of the buried Ouachita fold belt and mark the boundary between the geologically stable area of Texas and the less stable Gulf coast region. Parallel or subparallel faults in an area also form a distinctive map pattern; if most of the faults are downthrown in the same direction, these structural features are designated as step faults. Radial fault patterns are common over or around central uplifts or domed areas of the crust. These faults are generally associated with local stretching of the crust that results from the emplacement of salt masses (plugs) or igneous intrusions.

Some normal faults are also closely related to the development of plunging (inclined) folds and form characteristic map patterns. According to de Sitter, steeply dipping normal cross faults may form nearly at right angles to the trend of concentric (formed of parallel layers) folds. These faults are parallel to the principal deforming force and occur during the folding process. The maximum displacement occurs along the highest part (crest) of the fold; these faults usually die out along the flanks. Longitudinal crest faults may occur parallel to the trend of the folds. These normal faults are perpendicular to the principal deforming force and probably form as the compressional forces diminish.

Role in Shaping Earth's Surface

Faults have played a significant role in shaping the earth's surface throughout geologic time. The occurrence of normal faults is closely tied to modern plate tectonics, a unifying concept for the geological sciences. The faults are generally associated with modern and ancient divergent lithospheric plate boundaries, both on continents and in the ocean basins. The regions adjacent to modern plate margins, which are characterized by high heat flow and shallow-focus earthquake activity, are places where new oceanic-type crust is generated. In modern ocean basins, inferred normal faults bound narrow down-dropped blocks (grabens) along the axis of the mid-ocean ridge system, the longest continuous geologic feature on earth. Topographically, these structural troughs form deep valleys along the ridge crest. Individual troughs range up to 30 kilometers wide and are filled or partly filled with sediment. The Mid-Atlantic Ridge, a mountain range along the midline of the Atlantic basin, extends northward from Antarctica to Iceland. In Iceland, measurements across the boundary between the North American and Eurasian plates indicate that the crustal blocks are currently moving apart at the rate of a few centimeters per year.

In the Middle East, along the Red Sea, steeply dipping normal faults are associated with a large dome or uplift over a plumelike hot spot in the earth's mantle. Near the Afar region of Ethiopia, the uplift has been subdivided by three radial fault systems that intersect at angles of about 120 degrees. These systems are characterized by large, high-angle normal faults that initially formed a series of down-dropped blocks, or grabens. The three-pronged structural feature represents a “triple junction” that separates the African, Arabian, and Indian-Australian lithospheric plates. The East African rift system, which consists of both east and west zones, forms the second prong; it trends northward for nearly 5,000 kilometers and is marked by a series of elongated lakes. The maximum displacement on the bordering faults is nearly 2,500 meters in some places. The east-trending third arm of this large feature is a rift that is partly occupied by the Gulf of Aden.

Earth scientists have also been able to identify historical divergent lithospheric plate boundaries from regional geologic and geophysical (application of physics to geological problems) studies. In the modern Appalachian mountain chain in the eastern part of the United States, a series of elongated structural troughs (grabens and half-grabens) occur along the axis of the range. These structural features, which extend from Nova Scotia in Canada southwestward to North Carolina, contain thick deposits of Triassic (period of geologic time ranging from about 200 to 245 million years ago) sedimentary rocks with associated igneous rocks. Internally, steeply dipping normal faults divide the troughs into narrow tilted blocks that range up to 10 kilometers wide. Some of the border faults were active during Triassic deposition and have a cumulative displacement of nearly 4,000 meters. The formation of these troughs probably marked the separation of North America and Europe about 200 million years ago.

Economic Importance

Normal faults are also economically important. These faults serve as traps for hydrocarbons in many places. Migrating oil and gas moving updip from a place of origin, usually a sedimentary basin, are trapped against the fault, which acts as an impermeable barrier or seal. If the fault is not completely sealed, however, it may serve as an avenue for fluids to move to a higher level. Most commercial hydrocarbon deposits occur on the upthrown side of the fault where “rollover” of the rock layers has provided a suitable site for the accumulation of hydrocarbons. The faults are also the locus of metallic mineral deposits. Mineralization may occur in the openings along the fault or in the adjacent fractured rock. Drag ore, related to fault drag, occurs in some places. Also, rich ore bodies are moved downward along younger normal faults. A classic example is at the United Verde extension mine near Jerome, Arizona. There, a rich copper deposit was displaced more than 500 meters vertically.

Principal Terms

dip: the angle of inclination of a fault, measured from a horizontal surface; dip direction is perpendicular to strike direction

fault: a break in the earth's crust that is characterized by movement parallel to the surface of the fracture

fault drag: the bending of rocks adjacent to a fault

footwall: the crustal block underlying the fault

graben: a long, narrow depressed crustal block bounded by normal faults that may form a rift valley

hanging wall: the crustal block that overlies the fault

horst: a long, narrow elevated crustal block bounded by normal faults that may result in a fault-block mountain

slickensides: fine lines or grooves along a faulted body that usually indicate the direction of latest movement

stress: the forces acting on a solid rock body within a specified surface area

throw: the vertical displacement of a rock sequence or key horizon measured across a fault

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