Structural Geology

Stress and Strain

All materials respond to stress. But each material responds in a unique way that may be different under altered conditions. This course will review the basics of how rocks respond to tectonic forces acting over millions of years.

Stress is a force applied to an object for a given area. Many complex styles of forces can be applied (ex. torque). Let's restrict it to three types: compressional, tensional and shear stresses. Compressional stress occurs when two objects are "squashed" together. This commonly results in the objects being shortened in the direction the force is applied. When two objects are pulled apart, the stress is referred to as tensional stress. Objects tend to be drawn out and lengthened by tensional forces. Shear stress causes objects to be "ripped" apart. This occurs when objects are slid past one another in a "side swiping" motion - much like the blades of a pair of shears (scissors).

Strain is change in an object, such as shape or size, produced when stress is applied. Three common types of strain are associated with rocks: elastic, plastic and brittle responses.

Suppose a stress is applied and when it is removed the object bounces back to its original shape. This non-permanent form of strain is known as elastic strain. It can be observed in rubber balls ("compressed" against the floor and bouncing back) or rubber bands ("stretched" to surround an object and snapping back when released).

Plastic strain occurs when the shape of an object is changed by the stress and the object remains deformed even after the stress is removed. For example: clay may be "compressed" and remains squashed, a piece of plastic can be "stretched" and distorts, or glass being heated can be "blown" to different shapes. All result in permanent change in the object's shape.

When an object is pulled, stretched, or stressed beyond "it's limit" and breaks, the strain is referred to as brittle behavior. Any object can be brittle in nature. Many factors, especially temperature and pressure, control how material responds to stress. The glass needs to be heated to deform without breaking, a rubber ball, if frozen, will shatter when "bounced" and rubber bands, when pulled too hard or too fast, will snap under the extreme pressure.

Rocks can also deform if stress is applied. They fold, stretch, break and distort into different shapes as tectonic forces push them around. This course will look at folds, faults and earthquakes: general responses to tectonic stress. The style in which rock deforms is not fully dependent on the type of stress applied. The depth at which a rock is stressed, it's temperature, the mineral composition, neighboring rocks and time (lots of it!) are important factors that can control the type of deformation a rock undergoes.

Terminology

When discussing tectonics and rock outcrops, geologist communicate and exchange information with a uniform set of terms and symbols. Rock layers, when deposited, are typically flat and horizontal; they have no structure. Deformation usually results in the rock being tilted away from horizontal. The tilted rock layers can be described by s a "line" along the Earth's surface. This "line" is referred to as the strike of the rock bed. Strikes are recorded by the direction that this "line" is oriented away from north. (This is commonly given in degrees.) The rock bed is also be tilted into the surface. The angle that the rock tilts from a horizontal orientation into the ground is referred to as the dip. (This is measured at a 90° orientation from the strike. Both the direction and amount of dip is important.)

Geologic Responses: Structures

Folds

Folds are bends in rock that are evidence of plastic strain. They can range in scale from small features in a rock outcrop to large crustal warping that may affect hundreds of miles. Most folds are a result of compressional forces acting during mountain building events (or orogenies), though other types of stresses can result in folded rock. Because folds can be complex in nature, geologist have a set of terms to describe the orientation and configuration of a fold. A simple fold will have an axis, or imaginary line which runs down the middle of the fold. The orientation of the axis in space is reported as a strike. The axis may be parallel to the surface (non-plunging) or it may be tilted into the Earth's interior (plunging). The sides of the fold, or limbs, are tilted into the ground. The complexity of the fold is determined by using the dip of the limbs and how 'tightly' they are folded together (this course will not use complex fold terminology.)

Simple folds can be described in five geometric configurations (see your text for illustrations). The folds are defined by using the age of the rock formations exposed by erosion and the dip of the limbs. Monoclines are folds were one limb remains horizontal and one limb is tilted into the Earth. Monoclines are typically developed in areas that undergo tensional forces. The tilted limb is 'draped' over a down-dropped region of rock.

Anticlines and synclines are straight, or linear, folds which commonly occur together (similar to the wrinkling of a rug pushed against a wall). When viewed from an airplane, they appear as long ridges (non plunging) or 'V's (plunging). Anticlines are uplifted folds: their limbs dip away from the axis and older rock is exposed in the interior of the fold. Synclines are the opposite configuration: they are down-dropped in nature with limbs dipping into the axis. Younger rocks are exposed in the interior of synclines. Both are created by compressional forces.

Basins and domes are larger features created during crustal warping. They are usually circular or elliptical in character. Basins are down-dropped regions with younger rocks in the central region. Rocks surrounding the structural basin all dip into the center. Domes have the opposite configuration: the rocks dip away from the center and older rocks are exposed in the middle of the structure.

Joints and Faults

When rocks are stressed beyond their 'limit', they fracture and break. Two general types of fractures may occur: joints and faults. Joints occur after a stress has been removed; there is no movement along the crack created in the rock. Cracks can appear when rocks are 'relaxing' after being stressed for millions of years by tectonics or overlying weight. The rocks 'pop and crack" when the 'pressure' is removed. (Exfoliation in the granites of the Sierra Nevadas is a good example.)

Faults occur during the application of stress; movement shifts the rock along the fractured surface. Basic terms for faults are based on old mining terms. When a 'miner' went looking for ore veins, he was often searching for the hydrothermal alteration that occurs along fractures in rock (i.e., faults). The crack intersected the surface along a strike (think 'gold strike'!) and dipped into the Earth's surface. He would dig into the fracture (the fault plane), mining out the good ore. The pit he developed had two walls: the one he walked on (the footwall) and the one that overhung his head (the hanging wall). For convention, the geologist always assumes that the footwall remains stationary and the hanging wall is the piece of rock that moves. The type of fault movement which occurs will be based on the stress applied. (See your text for illustrations.) Remember, when discussing fault movement two things should be kept in mind: a) the movement is best seen in the cross-sectional view, and b) use marker beds to determine the movement (erosion removes fault scraps!)

Normal or gravity faults are formed when tensional stress breaks rock. As the rock is ripped apart, the hanging wall slowly slides down the fault plane. Large areas of crustal extension can create regions of down-dropped blocks (grabens) between high ridges (horsts). The Basin and Range region of the southwestern United States consists of large north-south trending horsts with sediment filled grabens between. One valley, Death Valley, dropped well below current sea level.

When the hanging wall is shoved up the fault plane, as during compressional stress, the fault is known as a reverse fault. A special type of reverse fault is referred to as a thrust fault. This fault has a very long fault plane surface situated at a shallow angle of sliding. (Think of thrusts as cards that are being collected by a dealer at a table: overlapping and gathering the cards into a stacked deck as 'compression' occurs.) The best evidence for a thrust is a repeating section of rock in a limited distance; the Rocky Mountains, for example.

Lateral faults, also known as strike-slip faults, usually show no up or down movement along the fault plane. Shearing motion produces a side-to-side movement along the length of the fault (or its strike). The faults may be referred to as left lateral or right lateral depending on the shift along the fault. (Stand on the fault and face across the fault line. The movement will be either left or right oriented as given in the name.) The San Andreas complex in Southern California is an example of a right lateral fault system.

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