Structural Geology

Heat from the Earth’s interior is released through the processes of volcanic eruptions and motions of the lithospheric plates. As described in the section on plate tectonics, plate motion is accomplished in three ways: convergent motion resulting in compressional stress; divergent motion resulting in tensional stress; and transform motion resulting in sheering stress. As stress is applied to crustal rock, deformation occurs. Characteristics of the deformation are dependent upon four factors: the type of stress; the rate the stress is applied; the material being stressed; and physical parameters of the environment of deformation (temperature and lithostatic pressure). Described in this section are the principle styles of rock deformation that occur in the Earth’s crust.

Analysis of any Earth system requires an understanding of the components of that system. The study of structural geology is founded in the geometric descriptions of rocks. Sedimentary rocks, those formed at the Earth’s surface, provide the most accessible starting place for a discussion of structural geology.

Dip and Strike

Sedimentary rocks occur in beds. These thin, laterally extensive, layers of rock form parallel to the Earth’s surface. As such, sedimentary beds have the geometry of horizontal tablets. Deformation, however, changes the orientation of these beds. The orientation of a tilted layer of rock can be described by two angular measurements. The deviation of the layer from the horizontal is termed the dip angle. Steeply inclined beds have a large dip angle while nearly horizontal beds have a small dip angle. In addition to the angle of inclination, it is necessary to describe the direction the bed is dipping. Geologists define this orientation by the line formed by the intersection of the tilted bed and an imaginary horizontal surface. This line of intersection is known as the strike line and its orientation is measured in terms of a compass direction. The line of strike is exactly perpendicular to the direction of dip. Taken together, the strike and dip of a tilted bed of rock  describes its spatial orientation.

Folds

There are two principle mechanisms by which rocks deform: plastically to form folds, and brittly to form faults. When exposed to compressional stress, rock can either fold or fault. However, rocks lack the ability to experience significant plastic deformation under conditions of tensional or transform stress. As such, folds are nearly always the products of compressional force. When applied to sedimentary rocks, compression results in the formation of sets of folds that are oriented perpendicular to the stress direction.

When viewed in cross-section, folds can be recognized as either concave up or concave down. The concave up (or U-shaped) folds are termed synclines while the concave down (or A-shaped) folds are known as anticlines. Commonly, these two fold types occur as linked structures. Each fold consists of two sides, or limbs, that are separated by an imaginary axial plane that divides the fold.

Fold geometries are categorized by the angular relationships between the fold limbs, the axial plane, and an imaginary horizontal plane. Symmetrical folds are recognized by the angular symmetry of the limbs on either side of the axial plane. A special case is the upright symmetrical fold, wherein the axial plane is vertical and the dip angles of beds in both limbs are equal. Conversely, the limbs of asymmetrical folds have dip angles that are unequal. Thus, an asymmetrical fold has a steeply dipping limb and a shallowly dipping limb. Importantly, asymmetrical folds are also characterized by a dipping axial plane wherein the axial plane dips in the same direction as the shallowly dipping limb of the fold.

As the dip angle of the axial plane decreases, the steeply dipping limb reaches a vertical orientation. Continued deformation past this point produces an overturned fold. Structures of this type are recognized by the "turned-over" nature of the steeply dipping limb. In this case, both limbs and the axial plane dip in the same direction. If deformation is sufficiently intense, the axial plane of the fold will be pushed over to a horizontal position. In this extreme situation, both limbs of the fold and the axial plane are parallel. These very tightly folded structures are common in intensely deformed mountain ranges such as the Alps and are known as recumbent.

Plunging Folds

So far, folds have been described in terms of their two-dimensional cross-sections. However, many folds are more complex. In order to understand such three-dimensional complexity consider the case of a simple, upright, symmetrical anticline. The axial plane intersects the fold along a line at the top of the structure. This line of intersection is known as the trace of the axial plane because the line can be "traced" or "drawn" on the folded bed. On a simple upright fold, the trace of the axial plane is a horizontal line. However, in many folds this line is inclined to the horizontal. When this occurs the entire fold is tilted in a direction that is perpendicular to the dip direction of the limbs – it is a plunging fold. To more completely visualize this three-dimensional geometry take a sheet of paper and draw a line length-wise down its center. Fold the paper so that a symmetrical upright anticline is formed with the line at the top of the fold. Orient the paper so the trace of the axial plane is pointing towards you. Now, tilt the paper so the fold plunges towards you. Reverse the plunge direction so the fold is tilted away.

Age Relations

Prior to deformation, sedimentary rocks exist as horizontal beds, the oldest of which are on the bottom (first formed) and the youngest of which are on top (last formed). When folded, this simple sequential age relationship produces patterns on the eroded landscape. The presence of these patterns allows for rapid determination of the geometry of folding. Anticlines are characterized by the presence of the oldest rocks in the center of their structure while synclines have their youngest beds in the central position. When folds are plunging the rocks of different ages exhibit curving patters but retain the basic age relationships of synclines and anticlines. In order to fully grasp the relationship between fold geometry and age, draw a few examples in cross-section and map-view.

Domes and Basins

Anticlines and synclines are typically formed along plate boundaries due to compressional stresses during collision. There exists, however, an additional class of fold structures that commonly form in the center of continents. Rather than being created by horizontal compressional forces, these structures are produced by the vertically directed stresses of uplift and subsidence. The anticline-like structure is called a dome while the syncline-like structure is a basin. As illustrated by synclines and anticlines, basins and domes exhibit age relationships in the beds that form them. However, the patterns that develop consist of concentric bands of similar aged rock. These large-scale fold structures are important reservoirs of oil, natural gas, and mineral resources.

Faults

The folding of rock most commonly occurs under conditions of compressional stress. However, the brittle failure of rocks to produce faults occurs in a wide-range of complex ways. Generally, faults occur under conditions of low lithostatic pressure in the upper regions of the crust. Additionally, faults are associated with all three forms of force: compression, tension, and transform stress. In each case, a specific geometry of faulting is associated with each stress type.

Technically, a fault is a break in rock along which some movement, or displacement, has occurred; breaks in rock that do not have any measurable displacement are known as fractures or joints. Faults are classified by the nature of the displacement. That is, by how one side of the fault moved relative to the other. The reason movement occurs along faults is because such motion is the mechanism by which stress is transformed into strain during brittle deformation. There are two general types of motion on faults: dip-slip and strike-slip. A fault plane can be thought of in the same way as any other plane in space – such as a bed of sedimentary rock – it has a dip angle and a strike angle that are measured in the same way as they are for sedimentary beds. However, in this case, the significance of these angles is that they can be used to describe displacement along the fault. In the case of dip-slip movement, one side of the fault moves up or down along the dip of the fault plane. Conversely, in strike-slip motion, one side of the fault moves laterally past the other along the fault's surface. Dip-slip displacement occurs during the application of compressional and tensional stresses while strike-slip motion occurs by transform stress.

Dip-slip Faults

Description of dip-slip motion begins with an understanding of the two sides of a fault plane. Consider a fault that is dipping at a uniform angle to the right. The right hand side of the fault is not only to the right of the fault plane, it is also everywhere above the fault. The left hand side is likewise everywhere under the fault plane. Importantly, the upper and lower sides of the fault plane are defined independently of the right or left hand sides. The side of the fault that is above the fault plane is always known as the hanging wall of the fault while the underside of the fault is known as the foot wall. These names were derived from early miners who tunneled along the fault surface in search of  mineral deposits.

Within the general class of dip-slip faults, there are two important divisions. In those faults that are formed by tensional stresses there must be a net lengthening of the total area being deformed. That is, tension tries to stretch rocks. When that stress is taken up by brittle deformation the hanging wall (upper side) of the fault moves downward along the fault plane. This form of displacement – hanging wall down – produces a dip-slip fault known as a normal fault.

The second form of dip-slip fault is produced by compressional deformation. In this case, the compressional forces act to shorten the rocks. This is accomplished by motion of the hanging wall upward along the dip of the fault. Thus, hanging wall up motion is driven by compressional force and the resultant structure is termed a reverse fault.

A special case of reverse faults occurs when thick sequences of sedimentary rocks are put under compressional stress. In these cases the reverse faulting occurs in a series of steeply dipping ramps and bedding parallel flats to produce a thrust fault. The significance of this style of faulting is that a great deal of compressional displacement can be accomplished and, as such, thrust faulting provides an important style of deformation in compressional settings.

Strike-slip Faults

The second major class of faults are those that experience strike-slip motion. Transform forces produce a sheering stress in rock. Importantly, however, rocks are very weak in sheering and tend to only deform brittly. When rocks break due to transform stress, the dip angle of the fault plane is less important than the strike angle. The sense of motion along the fault is in the direction of the strike of the fault plane. That is, one side of the fault slips past the other. Importantly, the graphical description of motion along a strike-slip fault is done by map-view illustrations rather than cross-sections.

Strike-slip faults are broken into two different geometries based upon the sense of sheer that occurs along the fault. In this case the sides of the fault plane are not given special names. However, the direction of motion does define the specific type of strike-slip fault. Imagine looking down on a strike-slip fault, the trace of the fault is trending north-south. Thus, there is an eastern side of the fault and a western side of the fault. There are two possible types of strike-slip motion possible. The eastern side of the fault can either move north or south relative to the western side. These two forms of motion are named based upon a simple convention. Imagine standing astride the trace of the fault, looking north, with your right foot on the eastern block and your left on the western block. If the eastern block moves south, past you, that sense of sheer is termed right lateral (imagine your right hand sliding back past you as it follows the moving block). Conversely, if the western block moves south relative to the eastern block, this geometry is termed a left lateral motion. Importantly, the sense of motion is independent of east and west or north and south. A left lateral fault is left lateral no matter what orientation it is observed from. The same is true of right lateral motion.