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Image by Stan Celestian
CHAPTER TABLE OF CONTENTS Chapter Introductory Picture: Asymmetrical folds of gypsum from the Castile Formation in New Mexico. Page 3 Stress and Strain and the Types of Stress Page 4 The Type of Deformation, Brittle Deformation and Plastic Deformation Described Page 5 Temperature and Time as Factors Controlling Deformation Page 6 The Stress and Strain of a Snickers Bar Page 7 Brittle Deformation, Footwall, Hanging Wall, Normal Faults Page 8 The Basin and Range Province Page 9 Reverse Fault Page 10 Thrust Fault and Strike Slip Fault (Right Lateral and Left Lateral) Page 11 The San Andreas Fault Page 12 Plastic Deformation - Folds, The Parts of a Fold Page 13 Types of Folds, Anticline, Syncline, Symmetrical, Asymmetrical, Overturned and Recumbent Page 14 Isoclinal Fold, Chevron Fold and Monocline Page 15 Plunging Anticlines and Synclines Page 16 Basins and Domes The Black Hills Dome Page 17 The Michigan Basin Page 18 Age Relationships of Sedimentary Strata in Folded Rocks Page 19 Joints at Arches National Park Page 20 Joint Pictures and Summary of the Important Factors Influencing the Deformation of Rocks Page 21 Key Terms
As you have seen, Earth is a dynamic planet. The forces that shape the crustal topography are ultimately traced back to the forces of plate tectonics. The shifting of the plates creates everything from mountain ranges along convergent boundaries, to the deep ocean trenches along subduction zones. The movement of crustal plates creates enormous stress on crustal rocks. These rocks often succumb to these stresses to create many different geologic structures. Stress is force exerted, typically measured in pounds per square inch. When the stress exerted on a rock exceeds it strength, the rock deforms, or strains. This strain (the resulting deformation of stress) is the topic of this chapter. Geologists have identified three different types of stress that create strain in rock, as shown in Figure 14.1a, b, c, and d): Compressional stress - A force acting to confine or squeeze together a rock. Tensional stress - A force acting to stretch or pull a rock apart. Shear stress - Most commonly, it is two forces that act in opposite but offset parallel directions to create a sliding or shearing force.
FIGURE 14.1a An Un-deformed Cube
FIGURE 14.1b Compression Compression results when opposing forces act toward the cube .
FIGURE 14.1c Tension Tensional forces result act out from the center of the cube, in opposite directions.
FIGURE 14.1d Shear Shear results when the opposing forces act toward the cube, but are offset from the center of the cube. Illustrations by Stan Celestian
Strain is the result of stress. In Figure 14.2, a deck of playing cards illustrates this relationship. The cards were pushed in the direction indicated by the left arrow (pointing right). The arrow on the right (pointing left) represents the opposing force, which in this case is the friction between the cards, and between the cards and the table top. The friction between the cards represents the strength of the card deck. The cards were stressed (as represented by the arrows). The result was a strain, or deformation, of the cards. In this simple example, the force holding the cards together was very small, just a little static friction and the force of gravity pushing down on the deck of cards. In nature, of course, the forces holding rocks together is much greater, and tied to the crystal structure of the minerals within the rock, as well as the boundaries between mineral grains, and possible cement holding grains together, as in the case of clastic sedimentary rocks. However, the forces of stress can be enough to strain the rocks that produce mountain ranges.
FIGURE 14.2 A shearing force acting on a deck of cards. Photo by Stan Celestian
The type of deformation is dependent upon: - Rock type - Pressure and direction of pressure - Duration and intensity of pressure - Temperature Some rock types yield to stress very easily, as shown by the beds of 2-million-year-old gypsum and clay beds along the San Andreas Fault near Palmdale, California, illustrated in Figure 14.3. The same forces acting in this area have created brittle failure elsewhere along the San Andreas Fault, as evident by the many earthquakes in the area. However, instead of breaking, these layers of gypsum and clay simply bend under the compressive stress, and did not produce a fault and subsequent earthquake. This type of deformation is called ductile or plastic deformation. Brittle deformation involves rupture of rock into smaller pieces. Brittle deformation is a common deformation style for glass, wood, china, and other rigid materials. In the FIGURE 14.3 Folded beds of gypsum and clay near Earth’s crust, most rocks at the surface will behave in a Palmdale, California. Photo by Stan Celestian brittle manner. The gypsum and clay beds at Palmdale are a notable exception. Conglomerate
Shale Siltstone FIGURE 14.4 Brittle deformation produces a fault plane. Photo courtesy of USGS; modified by Stan Celestian
Figure 14.4 is an example of a surface deposit of conglomerate, shale, and siltstone. These relatively weak rocks experienced brittle deformation in response to the stress applied to them. In this case, the fault that was created was the result of the tensional forces, indicated by the red arrows.
Temperature also influences the strength of rocks experiencing stress. In particular, a high temperature (but still below the rock’s melting point) will make the rock “softer”, and capable of plastic deformation. Even a brittle rock, like granite, can be made to flow, if the stress and temperatures are sufficiently high. Figure 14.5 is an example of a granitic gneiss that shows evidence of plastic flow.
FIGURE 14.5 Plastic deformation in a granitic gneiss. The left image shows flow banding in the gneiss. The right image is a close-up view of the upper left side of the left image, showing tighter folds resulting from compression. Photos by Stan Celestian
Figure 14.6 illustrates another example of a very plastic rock. In this example, a quartz-rich gneiss has been contorted to tight folds, by compressional forces. The gneiss must have been under high pressure, and near to the melting point of the rock, to produce this type of ductile deformation. Rock type, temperature, and pressure all play important roles in the deformation of rocks. Time, is another factor that can allow even small amounts of stress to create deformation, if FIGURE 14.6 Plastic deformation of a quartz-rich gneiss, near Bouse, Arizona. Photo by Stan Celestian the stress is constant, and allowed to act over long periods of time. Figure 14.7 is a diagram of a marble bench that has sagged, due to the downward force of gravity. This type of deformation is the result of millions of micro-fractures in the rock, that take place over hundreds of years. Another time-related deformation is the folding of incompetent rocks, near the surface. Figure 14.8 illustrates GRAVITY Soil C reep this deformation in a very weak schist exposed in a roadcut near Custer, Wyoming. The partiallyweathered schist is GRAVITY bending downslope, as the soil above exerts a FIGURE 14.7 The effect of time and gravity stress on the foliation on a marble bench Illustration by Stan Celestian of the schist. This process normally takes centuries to develop. FIGURE 14.8 Folding of schist due to prolonged influence of gravity induced soil creep Photo by Stan Celestian
THE STRESS AND RESULTING STRAIN OF A SNICKERS® BAR The deformation of a Snickers bar nicely illustrates the type of strain (deformation) created in different materials when the same stress is applied. The Snickers bar consists of tasty nougat, caramel, and peanuts, covered with milk chocolate. At room temperature, the chocolate coating is brittle and deforms by cracking, similar to a rock near the surface of the Earth. The caramel, however, behaves as a plastic, and it bends, rather than break. The sequence of photographs in Figure 14.9 illustrate the strain induced by the stress created by the hands pushing upward in the center (thumbs) and downward on the ends (forefingers). Figure 14.10 shows another Snickers bar that has been frozen, and is being slammed into the table-top. At about 0oF, the caramel behaves as a brittle material. The results of the rapid stress applied to the Snickers bar was that all of the components of the bar behaved as brittle material. This experiment illustrates the role of temperature in the deformation of different materials, including rocks.
Milk Chocolate
Caramel
Peanut
Nougat FIGURE 14.10 The deformation of a frozen snickers bar. Photos by Stan Celestian FIGURE 14.9 Finger-induced stress and resulting strain on a Snickers bar. Photos by Stan Celestian
(Note: All Snickers fragments were properly and safely recycled.)
Brittle Deformation Brittle deformation results from sudden failure of the rock. Much like the breaking of glass, the sudden rupture releases energy. For rocks, this results in a faults and earthquakes. Geologists have grouped the way in which rocks break into categories, based on the relative motion of the rocks on either side of the fault line. The concept of relative motion is shown in Figure 14.11. In the illustration, block B has moved down relative to block A. However, there are a variety of overall motions to be considered. Both blocks may have been moved downward, but block B has moved down more than block A. Conversely, the blocks may both have moved upward, but block A has moved up more than block B. Additionally, either block may have moved independently of the other. A B In describing the motion of blocks on either side of a fault line, a geologists uses the terminology of “relative motion” of the blocks. For example, block B has moved down relative to block A. Fault planes are rarely perfectly vertical. Hanging When blocks slip past each other along a B fault zone, one block is called the footwall, FIGURE 14.11 Relative Motion and the other is called the hanging wall. Illustration by Stan Celestian These terms originated in mines, many years ago. Fault zones are areas where mineral-laden water can precipitate many different types of economically important ore minerals. Many underground mines are thus along fault planes. In an underground mining operation, a drift (passageway along the ore body), is constructed. Miners drill into the Footwall walls of the ore body, in preparation for blasting and removal of ore material. The miner stands on the footwall and the hanging wall hangs overhead. In Figure 14.12 the miner is drilling into the footwall and the FIGURE 14.12 The hanging wall and hanging wall is overhead. The relative motion of the blocks along the fault footwall of an underground mine. Illustration by Stan Celestian zone are show by the arrows. In this example, the hanging wall has moved up relative to the footwall. A
Normal Faults (also called dip-slip or gravity gaults) are created by tensional forces, the extension (pulling apart) of the crust. These types of faults are called gravity faults because the downward pull of gravity is the driving force that moves the blocks one past the other. (Dip slip refers to the fact that the fault movement is parallel to the dip of the fault plane.) In a normal fault, the hanging wall moves down relative to the footwall, as shown by the arrows in Figure 14.13. The tensional forces necessary to produce normal faults can be found at divergent plate boundaries. Fault Scarp
Movement along a fault zone is not smooth, as usually pressure builds and the rock moves along the fault line, however, in some places more than other. Pressure continues to build, and the rocks slip along the fault again. The movement of one block of rock against another generates large amounts of friction, and the fault zone can consist of very broken (brecciated) rocks. The Footwall brecciation creates an increase in porosity and Hanging Brecciated permeability. These zones are often mineralized, Wall especially where one fault zone intersects FIGURE 14.13 Crustal extension and the development of normal another. faults.
Illustration by Stan Celestian
X
X
Y
Top View
Y B
A
C
FIGURE 14.14 Fault gouge breccia, intersecting fault planes, and mineralization Photo A is an example of a brecciated zone found along a fault plane in Nevada. A layer of chert was brecciated to produce what is known as a fault gouge breccia. The red is mercury ore, cinnabar, that has filled the pore spaces in the breccia. Photo and Graphics by Stan Celestian
Figure 14.14 illustrates a situation where two fault planes (X and Y) intersect. Each has its own brecciated zone, but where they intersect, a much more extensive brecciated zone is created. The United Verde Mine at Jerome, Arizona, was one such occurrence. About 10 million tons of ore were recovered from the mine. This yielded about 675 million pounds of copper, 20 million ounces of silver, and about 700 million ounces of gold. Many other mines in the area around Jerome also produced the same materials, but none could match the overall production of the United Verde Mine, primarily because of its unique geology. The Basin and Range Province A geologic province is a large area characterized by a similar geology. The Basin and Range Province of the western United States is dominated by normal faults, created by crustal extension. Figure 14.15 shows the location of the Basin and Range Province as well as portions of surrounding provinces. Figure 14.16 illustrates the crustal extension that created the Basin and Range Province. Block A illustrates the area before extension, and Block B illustrates the normal faults created by the extension. Notice also that the segments of the fault that are higher are called horsts, and the downthrown blocks are called grabens. The grabens are the basins, and the horsts are the ranges in the Basin and Range Province.
A
GRABEN
HORST
B Figure 14.15 The location of the Basin and Range Province. Map Courtesy of the USGS
Figure 14.16 Normal faulting in the Basin and Range Province Illustration by Stan Celestian
FIGURE 14.17 A map view (left) and a geologic map view (right) of a portion of the southwestern United States, showing the Basin and Range Province. The Geologic Map of the United States can be viewed by clicking on this hyperlink: http://pubs.usgs.gov/dds/dds11/kb.html Maps Courtesy of the USGS
Figure 14.17 illustrates the Basin and Range Province on a shaded relief map (left) and a geologic map (on the right). A geologic map is one in which the various colors represent different rock types. Nevada is especially endowed with Basin and Range geology. Reverse Fault (also called a dip-slip fault)
Hanging Wall Footwall
Before After
A reverse fault is the result of compression, and consequent brittle failure of the rock. In a reverse fault, the hanging wall moves up relative to the footwall. Figure 14.18 is an illustration of a high angle, reverse fault. (The high angle refers to the angle at which the fault plane tilts from the horizontal position. In this case, it is about 60o.) The illustration also shows the confining pressures, as indicated by the orange arrows; and the amount of shortening, by the before and after bars below the block diagram. The compressional forces needed to produce reverse faults are found along convergent plate boundaries.
Figure 14.19 illustrates a reverse fault in Iceland. In the photo, a geologist is measuring the amount of vertical displacement along the fault. In this situation, the rock below the glacial ice faulted. The movement was abrupt and caused the ice, which is normally plastic, to break in a brittle manner. This illustrates the concept that a sudden burst of force can cause even soft material to act in a brittle manner. FIGURE 14.18 A reverse fault The bars below the block diagram show the amount of crustal shortening caused by the compressional forces (orange arrows). Illustration by Stan Celestian
FIGURE 14.19 A reverse fault in glacial ice Photo courtesy of the USGS
Nappe Proterozoic Rocks
Chief Mt. (Klippe)
Thrust Fault (red line)
FIGURE 14.20 The Lewis Overthrust and the erosional remnant of Chief Mountain.
Cretaceous Rocks Illustration by Stan Celestian
Another example of a compressional fault is the low-angle reverse fault, also called a thrust fault. In northern Montana, Chief Mountain, is a remnant of a huge thrust fault. About 170 million years ago the North American Plate collided with a crustal block. That collision pushed a huge mass of old (~2 billion years old) Proterozoic rocks eastward for about 50 miles, over much younger (~100 million years old) Cretaceous-age sedimentary rocks. This massive low-angle thrust fault, called the Lewis Overthrust, is shown in Figure 14.20. The angle of the thrust fault plane averages about 10o. Figure 14.21 is a photograph of Chief Mountain, which is a classic example of a “ klippe” (German for cliff). Klippes are erosional remnants of “nappes” (French for tablecloth). The geological concept is that a wedge of continental crust was thrust up and over other crustal rocks. In the case of Chief Mountain, it was part of the nappe, the older Proterozoic rocks. In Figure 14.20, the thrust plane (fault plane) is represented by the red line just above the yellow Cretaceous rocks. The nappe is the main body of the overthrust rock. As it was thrust over the younger Cretaceous rocks, it was folded and contorted by the compression, much like a tablecloth being pushed over a tabletop. Millions of years of erosion has reduced the forward edge of the nappe to fragments, the largest of which are called klippes, like Chief Mountain.
Thrust Plane
FIGURE 14.21 The Chief Mountain Klippe
Photo by Stan Celestian
Strike-Slip Faults A strike-slip fault is one that moves in the direction of the fault plane’s surface trace. In Figure 14.22, the trace of the fault is shown as a turquoise line. Note that no vertical movement on the fault is shown. All of the movement is parallel to the fault trace (the strike of the fault). The relative motion of the faults is shown by the black arrows. There is a distinction between the two faults. The blocks on the left represent a right-lateral strike-slip fault, and the blocks on the right represent a left-lateral strike-slip fault.
Figure 14.x A Strike Slip (Transform) Fault.
Left Lateral
Right Lateral FIGURE 14.22 A strikes-slip (transform) fault Strike-slip faults are produced by shear forces as indicated by the blue arrows. These forces act in opposing directions but are offset. Illustration by Stan Celestian
The names of the fault are derived from their relative motion, with respect to a person straddling the fault. The fault is named a right-lateral if the right side moves toward the person. Conversely, the fault is named a left-lateral if the left side moves toward the person. From your perspective, as you view the image (opposite of the person straddling the fault) the names still apply. For example, in the right-lateral strike-slip fault, the right side is moving toward you.
ho Elk
Perhaps the most studied strike-slip fault in the world is the San Andreas Fault in California. Figure 14.23 shows offset of a small stream (Wallace Creek) by movement along the San Andreas Fault. There are many examples of this type of stream Wallace offsets, as well as rock outcrops along the Creek fault zone. The San Andreas Fault stretches about 780 miles across California. The San Andreas Fault is really a fault system. It represents a number of faults (including the Hayward and Calaveras Faults) that are more or less parallel to the San Andreas, and result from the same type of stress and strain, as the North American Plate slides past the Pacific Plate. An important aspect of the strike-slip fault is that there is very little vertical motion. One fault block slides FIGURE 14.23 Wallace Creek offset along the San Andreas Fault. Image Courtesy of the USGS, The National Map laterally (horizontally) past the other. The San Andreas Fault System is tens of miles wide in some locations and has been determined to be over 11 miles deep in some places. Movement along the fault system does not always produce destructive earthquakes. In some areas, like around El Centro, the fault movement is considered to creep. Thousands of small earthquakes slowly move one plate past the other over many years. rn
. Rd
FIGURE 14.24 Large fault systems in California Illustration Courtesy of the USGS
FIGURE 14.25 Branches of the San Andreas Fault System Illustration Courtesy of the USGS
Figure 14.24 illustrates the major faults in California. Note the areas where creep takes place (gray areas). Figure 14.25 shows the complex nature of the fault system near San Francisco. PLASTIC DEFORMATION - FOLDS Folds are rocks that have been deformed by compressional forces in a plastic, rather than in a brittle manner. Fold nomenclature is shown in Figure 14.26, which is an example of a type of fold called an anticline. al Axi e n Pla
Hinge Line
Lim
The hinge line is the surface expression of the axial plane. The axial plane divides the fold symmetrically. In simple folds, like this example, the axial plane is a flat, planar surface. In more complex folds, it can be curved.
b Lim b
FIGURE 14.26 Parts of a fold
The hinge line represents the fold axis. It is the line from which the fold tilts in opposite directions. In this example of an anticline, the hinge line is on the top of the crest.
Graphic by Stan Celestian
Limbs are simply the flanks of the fold. They are the rock layers that diverge from the hinge line.
Folds represent crustal shortening, as a result of compressional forces.
Anticline
Anticline
TYPES OF FOLDS
C B
D
The most basic types of folds are anticlines and A synclines. Figure 14.27 illustrates these two types of folds. In an anticline, the limbs tilt away from Syncline the crest; and in a syncline, the limbs tilt toward the trough. (A method to remember the difference between the two is that the word anticline starts FIGURE 14.27 Anticlines and a syncline Layer A is the oldest, and D is the youngest. Graphic by Stan Celestian with an “A”, which is the shape of the fold.) Although folds are the result of compressional forces, those forces are normally not uniform in their direction or consistency. As a result, folds can take different shapes. Figure 14.28 displays the most common varieties of folds. The symmetrical fold is one in which the axial plane (blue line) is vertical and the layers tilt in opposite directions at SYMMETRICAL FOLD approximately the same angle.
A
An asymmetrical fold is one in which the axial plane is tilted from the vertical position. This results in the layers (limbs) tilting more on one side than the other. An overturned fold is similar to the asymmetrical fold, but the layers of rock have been forced over, to the point that they are actually tilted in the same direction. In this example, the beds are tilted toward the lower left of the diagram.
ASYMMETRICAL FOLD
B OVERTURNED FOLD
C RECUMBENT FOLD
D FIGURE 14.28 Common types of folds Graphic by Stan Celestian
A recumbent fold is one in which the axial plane is horizontal or nearly horizontal. In these folds (A, B, C and D), the compressional forces (shown by the orange arrows) have changed their relative positions. In A (symmetrical fold) the compressional forces are equal and in opposite directions. However, in block B, the compressional forces are not precisely opposite. The result is a slight shearing component to the compressional force. This is the cause of the asymmetry in the fold. The fold has been tilted over by the shearing force. The separation of the orange arrows (direction of compressional force) becomes greater in block C, creating an overturned fold. Block D represents a fold where the axial plane is almost horizontal, and the orange arrow separation is the greatest. Notice also the change in the shape of the axial plane from a straight line to a curved line in C and D. The axial plane is also being folded by the shear component of the compressional forces.
RECUMBENT FOLD
THRUST FAULT
FIGURE 14.29 A recumbent fold ceating a thrust fault
Illustration by Stan Celestian
In many cases, the shearing compressional forces may overcome the strength of the rock. The plastic strain may yield to brittle deformation, and result in a low-angele reverse fault (thrust fault). Figure 14.29 illustrates how the pressures that produced a recumbent fold can also create a thrust fault. ISOCLINAL FOLD
Isoclinal folds, Figure 14.30, are produced when a competent bed (stronger) is surrounded by weaker rock units. The compressional force folds all of the rocks, however, the competent bed retains its original thickness, while the incompetent beds are displaced. The isoclinals (iso - same, cline - tilt) limbs of the fold are the same angle on both sides of the axial plane. FIGURE 14.30 Isoclinal Folds GraphicFOLD by Stan Celestian CHEVRON
Chevron folds are very angular. Instead of having a smooth, rounded transition from one limb to another, across the axial plane, chevron folds are marked by abrupt angular transitions. These types of folds are found when competent rocks are found interbedded with incompetent rocks. Figure 14.31 is an example of a chevron fold, in which case the pink beds are a competent sandstone interbedded with shale (blue beds). Another type of fold is the FIGURE 14.31 Chevron Folds Graphic by Stan Celestian monocline. As the name implies the monocline (mono - one, cline - tilt) has only one side of a fold. Monoclines commonly form when a competent rock, deep below the surface, is faulted. The displacement of the upthrown block deforms incompetent rocks above, to create a fold. FIGURE 14.32 Monocline
Graphic by Stan Celestian
Figure 14.33 illustrates the geologic circumstances that can create a monocline. The basement rock in this illustration is a very competent granite, that has been faulted. The fault extends into the sedimentary rocks above, however, the sedimentary rocks behave more plastically and deform by folding, rather than faulting. The result of the fault in the granite is a monocline at the surface. Figure 14.34 is the Raplee Monocline, in southeastern Utah.
Incompetent Rocks (Sedimentary)
Fault
Competent Rock (Granite)
FIGURE 14.33 Monocline over fault Graphic by Stan Celestian
FIGURE 14.34 The Raplee Monocline near Mexican Hat, Utah Photo by Stan Celestian
The fault that provided the force for the creation of the monocline lies far below the surface. The faulting and subsequent monocline occurred during the Laramide Orogeny, during the latest part of the Mesozoic Era and early Cenozoic. Based on the tilt angle of the beds and the geology of other similar structures around the Colorado Plateau, it is believed that the monocline formed above a high-angle reverse fault, due to compressional forces. In Figure 14.34, note that the rocks on top of the ridge are fairly horizontal. They tilt toward the west (left side of the picture) and then become more level, as indicated by the rocks in the foreground. Anticlines and synclines represent strain produced by compressional forces. These folds can also be tilted from the horizontal — that is, their hinge lines can be tilted into the Earth’s surface. Such folds are called plunging anticlines or plunging synclines. Figure 14.36 illustrates folded rocks in which the hinge lines of an anticline and two synclines are tilted from the horizontal. The red lines represent the plunging hinge lines of the folds. Figure 14.35 represents a plunging anticline in which the blue plane represents the axial plane. On the block diagram in Figure 14.36, note the surface pattern of the folds. They trace out a zigzag pattern that is very indicative of plunging anticlines and synclines.
FIGURE 14.35 Plunging anticline Graphic by Stan Celestian
FIGURE 14.36 Plunging anticlines and synclines Graphic by Stan Celestian
Figure 14.37 is a NASA radar image that shows the complex fold pattern produced when folds plunge. This same type of zigzag pattern is evident on Figure 14.38, which is The National Map shaded relief map of an area near Bloomsburg, in Central Pennsylvania.
FIGURE 14.37 A radar image of plunging anticlines and synclines of the Appalachian Mountains. Image Courtesy of NASA
FIGURE 14.38 Shaded relief map of Central Pennsylvania, showing the topography of plunging anticlines and synclines. Image Courtesy of the USGS, The National Map
Domes and Basins
FIGURE 14.39 A structural dome
Graphic by Stan Celestian
A dome is a broad uplift that creates a circular or elongated symmetrical fold. Figure 14.39 illustrates an idealized diagram of a dome. In this particular example, the uplifting force is created by an igneous intrusion (shown as red). Like anticlines, the oldest rocks in an eroded dome are found in the center and become progressively younger away from the center of the dome. The exception in this example, is the igneous intrusion, which created the dome, and is the youngest rock unit represented in the diagram.
Figure 14.40 is a sketch of the Black Hills Dome, in South Dakota. At its center is the Harney Peak Granite (pink/red), which has been radiometrically dated (U/Pb) to be about 1.7 billion years old. The heat from the intrusion metamorphosed the surrounding sedimentary rocks, to create a zone of gneiss, schist, and quartzite. The metamorphic zone surrounding the central granitic core is shown as orange in the sketch. It is called the central crystalline area. The intrusion of the granite, however, did not cause the uplift and the formation of the Black Hills Dome. Instead, about 70 million years ago (end of the Cretaceous Period), uplift provided by the Laramide Orogeny forced the granite and surrounding metamorphic rocks upward. This uplift also pushed the overlying sedimentary rocks upward, to create a mountain range that was estimated to be nearly 15,000 feet high. Those once lofty mountains have since eroded to produce the current Black Hills. FIGURE 14.40 The Black Hills Dome Image courtesy of the USGS
Basins (Figure 14.41) are created by downwarping of the crust. This can be accomplished by crustal extension, as is the case for normal faulting, like that seen in the Basin and Range Province. The competent basement rocks fault, but the overlying incompetent rocks bend to produce the basin. Like synclines, the oldest rocks in an eroded dome are around the outside of the structure, and the youngest rocks are found in the center. The Michigan Basin (Figure 14.42) is an FIGURE 14.41 A structural basin example of a structural basin. (The Illustration by Stan Celestian term structural, as in “structural basin” or “structural dome”, refers to the curvature of the strata of the feature. Most structural basins, like the Michigan Basin, are actually elevated areas.) The Michigan Basin was created during the continental collision of the North American Plate with the African Plate, during the Paleozoic Era. Uplift that created the Appalachians caused crustal thinning to the west. This thinning allowed the accumulating sediments to push the crust down, to form the basin. The lower portion of Figure 14.42 is a cross-section from point A to point B, on the geologic map view.
GEOLOGIC MAP KEY A
Upper Pennsylvanian Lower Pennsylvanian Upper Mississippian Lower Mississippian Devonian
B
Silurian G
Ordovician
B
A
FIGURE 14.42 Geologic map and cross-section of the Michigan Basin
Graphic by Stan Celestian
Age Relationships of Sedimentary Strata in Folded Rocks Eroded anticlines, synclines, domes and basins can be identified by how the rocks tilt below the Earth’s surface. However, in many cases, geologists can not see how these rocks tilt. To identify the type of structures, they rely on the relative age relations of the rocks at the surface. Blocks 14.43 A, B, and C demonstrate how this relationship reveals the underlying structure. The blocks that are being used were previous examples in this chapter, however, imagine walking along the surface of the blocks, unable to see the sides of the block diagrams.
A 7 5 3 2
6
B 6
5
4
C 5
6
7
3
4 1
A
B
C
Anticlines and synclines that have been eroded at the surface show a distinctive repeating patterns of the strata. Around the hinge line of a synclines (at A and C), note that the rocks become progressively older away from the axis. Around A, for example, rock unit 7 is the youngest followed by 6, then 5, then 4. Using the Law of Superposition, it can be determined that rock unit 1 is the oldest, and 7 is the youngest. Moving away from the hinge line of the anticline the rocks become progressively younger. (At B, rock unit 4 is older than 5, then 6 and 7.)
A dome can be considered a doubly plunging anticline. Like the rocks exposed at the surface of the eroded anticline, the exposed rocks of a dome are also the oldest in the center and progressively younger to the margins of the structure.
A basin can be considered a doubly plunging syncline. In this example it can be seen that the youngest rocks occupy the middle of the structure and the rocks become older and older as one observes the strata away from the center.
FIGURE 14.43 Determining the structure of folded rocks by observing surface outcrop patterns and the age of the rocks. Graphic by Stan Celestian
BRITTLE DEFORMATION — JOINTS Joints are fractures, or cracks, in rock. They differ from faults in that joints have no significant movement along the fracture. Columnar jointing in basalt has already been discussed, in a previous chapter. It is created by cooling and contraction. The decrease in volume of the liquid rock to create the solid rock, creates tensional forces that pull the cooling rock apart, but with no appreciable r e l a t i v e FIGURE 14.44 Joint sets in the Navajo Sandstone in Arches National Park, Utah Image courtesy of the USGS, The National Map movement. Joints can be created by large-scale earth movements, as in plate tectonics, or small scale movement, and by impacts. Joints normally do not occur as single fractures, but rather they occur in joint sets. Figure 14.44 is a satellite view of a portion of Arches National Park, Utah. It shows three prominent joint sets (as indicated by the red lines). These joints were created when the rock (sandstone) was uplifted and arched into a broad dome. Figure 14.45 shows the deeply-eroded joints at the Fiery Furnace area of the park. Although joints do not fragment the rock, as does the process of faulting, the joints do provide water with a ready-made channel for flow. Over many years, the water flowing over and through these joints has eroded them considerably. Figures 14.46 and 14.47 show a ground-level view of this area.
Fiery Furnace Overlook
FIGURE 14.45 The Fiery Furnace Overlook
Image courtesy of the USGS, The National Map
FIGURE 14.46 Well-eroded joint set at Arches National Park, FIGURE 14.47 A view from one of the “fins” at the Fiery Furnace, Arches National Park. Utah Photo by Stan Celestian Photo by Stan Celestian
Summary of the important factors influencing the deformation of rocks: - Rock type: Competent rocks like igneous rocks (granite, massive basalt layers, gabbro) will behave in a brittle manner and most commonly fracture rather than fold. Incompetent rocks like gypsum, salt, and clay-rich rocks will behave as plastic materials and bend or flow under an applied stress. - Pressure and direction of pressure: Compressional pressures tend to create folding in incompetent rocks and reverse faults or thrust faults in competent rock types. - Duration and intensity of pressure: A small applied stress acting over many centuries can cause folding in even competent rocks. A quick, strong stress can cause normally incompetent rocks to behave in a brittle manner. - Temperature: An increase in temperature generally weakens rocks. Even a very competent rocks like granite will behave in plastic manner if the temperature approaches its melting point.
KEY WORDS Anticline: a fold in which the limbs tilt down and away from the hinge line Asymmetrical fold: a fold where the axial plane is tilted slightly away from the vertical This results in one limb being tilted downward more the other. Axial plane: the planar center of symmetry for a fold Basin: a down-warped portion of the crust, where rock strata display a downward curvature Brecciated zone: an area along a fault plane, in which the rocks have been broken by the movement of the fault Brittle deformation: a breaking or fracturing of a rock, produced by a force acting on it Chevron fold: curve
a series of folds in which the limbs meet the hinge lines at a distinct angle, rather than a
Competent: refers to the relative strength of a rock The stronger (usually brittle) rock in a group of rocks. The weaker rocks are referred to as incompetent. Compressional Stress: a force acting to squeeze rocks together Deformation: the eventual change in shape of a rock body, due to stress Dip-Slip Fault: a fault with movement parallel to the dip of the fault plane; sSee Gravity fault, Normal, or Reverse fault. Dome: a crustal uplift as deduced from the upwarping of strata in a roughly circular or elongate manner Ductile Deformation: the change in shape (bending or flowing) of a rock body in a plastic manner, where no fracturing takes place; plastic deformation Extension: an expansion of the rock Fault: a fracture in rock, along which significant movement has taken place Fold: a deformation of rock into curves, due to compressional forces Footwall: it is the lower block of an inclined fault Graben: it is a down-dropped block, in a series of normal faults Gravity fault: a fault created by crustal extension. The hanging wall moves down relative to the footwall (gravity is the driving force); see Dip-slip fault or Normal fault Hanging wall: the upper block of an inclined fault Hinge line: the line of flexure for a fold
Horst: a block that stands higher than surrounding blocks in a set of normal faults Isoclinal fold: hinge line
a symmetrical fold in which opposite limbs tilt at the same angle away from the
Incompetent: a weaker or softer rock in a group of rocks. When a stress is applied, the incompetent rocks are more likely to deform. Stronger rocks are referred to as competent Joint: a fracture in a rock, along which no significant movement has taken place Klippe: an erosional outlier of a nappe Left-lateral movement: a descriptive term related to the movement along a strike-slip fault. Straddling the fault, the left-lateral strike-slip refers to the left block moving toward the observer. Limb: rock strata that diverge from the hinge line of a fold Monocline: a fold in which there is only one tilted limb Nappe: the main body of an overthrust block, part of a thrust fault Normal fault: a fault created by tensional forces in which blocks slip past one another along a plane of failure; the hanging wall moves down, relative to the footwall Orogeny: a mountain-building event created by crustal plate collisions Overthrust: the portion of a thrust fault that overrides the lower block of rock Overturned fold: a fold in which the axial plane has been significantly tilted from the vertical position; the limbs tilt in the same direction Plastic deformation: the change in shape (bending or flowing) of a rock body in a ductile manner, where no fracturing takes place; ductile deformation Plunging fold: a fold in which the axial plane tilts in a down from the horizontal Proterozoic: a period of time dating from about 570 million years ago to about 2.5 billion years ago Recumbent fold: a fold in which the axial plane is nearly horizontal Right-Lateral Fault: a descriptive term related to the movement along a strike-slip fault Straddling the fault, the right-lateral strike-slip refers to the right block moving towards the observer. Reverse Fault: a fault in which the hanging wall moves up, relative to the footwall Reverse faults are the result of compressional forces acting on brittle rock.
Shear stress: forces that are acting in parallel, but offset opposing directions Snickers® bar: chocolate
a tasty treat consisting of caramel, peanuts, and nougat, covered with milk
Strain: deformation caused by stress Stress: a force applied to a rock Strike-slip fault: a fault in which one block slides horizontally past the other, with very little vertical movement Symmetrical fold: a fold in which the limbs tilt equally away from the hinge line Syncline: a down-warped fold, in which the limbs tilt up and away from the hinge line Tensional stress: the rock
a force that acts outward, away from the center of a rock; creates extension of
Thrust fault: a low-angle reverse fault, where the hanging wall moves up and over the footwall Transform fault: a strike-slip fault that is associated with a plate boundary (All transform faults are strike-slip faults, because little or vertical displacement occurs. However, the term “transform fault is used exclusively for those strike-slip faults that occur along plate boundaries.)
CHAPTER TABLE OF CONTENTS Chapter Introductory Picture: Asymmetrical folds of gypsum from the Castile Formation in New Mexico. Page 3 Stress and Strain and the Types of Stress Page 4 The Type of Deformation, Brittle Deformation and Plastic Deformation Described Page 5 Temperature and Time as Factors Controlling Deformation Page 6 The Stress and Strain of a Snickers Bar Page 7 Brittle Deformation, Footwall, Hanging Wall, Normal Faults Page 8 The Basin and Range Province Page 9 Reverse Fault Page 10 Thrust Fault and Strike Slip Fault (Right Lateral and Left Lateral) Page 11 The San Andreas Fault Page 12 Plastic Deformation - Folds, The Parts of a Fold Page 13 Types of Folds, Anticline, Syncline, Symmetrical, Asymmetrical, Overturned and Recumbent Page 14 Isoclinal Fold, Chevron Fold and Monocline Page 15 Plunging Anticlines and Synclines Page 16 Basins and Domes The Black Hills Dome Page 17 The Michigan Basin Page 18 Age Relationships of Sedimentary Strata in Folded Rocks Page 19 Joints at Arches National Park Page 20 Joint Pictures and Summary of the Important Factors Influencing the Deformation of Rocks Page 21 Key Terms
As you have seen, Earth is a dynamic planet. The forces that shape the crustal topography are ultimately traced back to the forces of plate tectonics. The shifting of the plates creates everything from mountain ranges along convergent boundaries, to the deep ocean trenches along subduction zones. The movement of crustal plates creates enormous stress on crustal rocks. These rocks often succumb to these stresses to create many different geologic structures. Stress is force exerted, typically measured in pounds per square inch. When the stress exerted on a rock exceeds it strength, the rock deforms, or strains. This strain (the resulting deformation of stress) is the topic of this chapter. Geologists have identified three different types of stress that create strain in rock, as shown in Figure 14.1a, b, c, and d): Compressional stress - A force acting to confine or squeeze together a rock. Tensional stress - A force acting to stretch or pull a rock apart. Shear stress - Most commonly, it is two forces that act in opposite but offset parallel directions to create a sliding or shearing force.
FIGURE 14.1a An Un-deformed Cube
FIGURE 14.1b Compression Compression results when opposing forces act toward the cube .
FIGURE 14.1c Tension Tensional forces result act out from the center of the cube, in opposite directions.
FIGURE 14.1d Shear Shear results when the opposing forces act toward the cube, but are offset from the center of the cube. Illustrations by Stan Celestian
Strain is the result of stress. In Figure 14.2, a deck of playing cards illustrates this relationship. The cards were pushed in the direction indicated by the left arrow (pointing right). The arrow on the right (pointing left) represents the opposing force, which in this case is the friction between the cards, and between the cards and the table top. The friction between the cards represents the strength of the card deck. The cards were stressed (as represented by the arrows). The result was a strain, or deformation, of the cards. In this simple example, the force holding the cards together was very small, just a little static friction and the force of gravity pushing down on the deck of cards. In nature, of course, the forces holding rocks together is much greater, and tied to the crystal structure of the minerals within the rock, as well as the boundaries between mineral grains, and possible cement holding grains together, as in the case of clastic sedimentary rocks. However, the forces of stress can be enough to strain the rocks that produce mountain ranges.
FIGURE 14.2 A shearing force acting on a deck of cards. Photo by Stan Celestian
The type of deformation is dependent upon: - Rock type - Pressure and direction of pressure - Duration and intensity of pressure - Temperature Some rock types yield to stress very easily, as shown by the beds of 2-million-year-old gypsum and clay beds along the San Andreas Fault near Palmdale, California, illustrated in Figure 14.3. The same forces acting in this area have created brittle failure elsewhere along the San Andreas Fault, as evident by the many earthquakes in the area. However, instead of breaking, these layers of gypsum and clay simply bend under the compressive stress, and did not produce a fault and subsequent earthquake. This type of deformation is called ductile or plastic deformation. Brittle deformation involves rupture of rock into smaller pieces. Brittle deformation is a common deformation style for glass, wood, china, and other rigid materials. In the FIGURE 14.3 Folded beds of gypsum and clay near Earth’s crust, most rocks at the surface will behave in a Palmdale, California. Photo by Stan Celestian brittle manner. The gypsum and clay beds at Palmdale are a notable exception. Conglomerate
Shale Siltstone FIGURE 14.4 Brittle deformation produces a fault plane. Photo courtesy of USGS; modified by Stan Celestian
Figure 14.4 is an example of a surface deposit of conglomerate, shale, and siltstone. These relatively weak rocks experienced brittle deformation in response to the stress applied to them. In this case, the fault that was created was the result of the tensional forces, indicated by the red arrows.
Temperature also influences the strength of rocks experiencing stress. In particular, a high temperature (but still below the rock’s melting point) will make the rock “softer”, and capable of plastic deformation. Even a brittle rock, like granite, can be made to flow, if the stress and temperatures are sufficiently high. Figure 14.5 is an example of a granitic gneiss that shows evidence of plastic flow.
FIGURE 14.5 Plastic deformation in a granitic gneiss. The left image shows flow banding in the gneiss. The right image is a close-up view of the upper left side of the left image, showing tighter folds resulting from compression. Photos by Stan Celestian
Figure 14.6 illustrates another example of a very plastic rock. In this example, a quartz-rich gneiss has been contorted to tight folds, by compressional forces. The gneiss must have been under high pressure, and near to the melting point of the rock, to produce this type of ductile deformation. Rock type, temperature, and pressure all play important roles in the deformation of rocks. Time, is another factor that can allow even small amounts of stress to create deformation, if FIGURE 14.6 Plastic deformation of a quartz-rich gneiss, near Bouse, Arizona. Photo by Stan Celestian the stress is constant, and allowed to act over long periods of time. Figure 14.7 is a diagram of a marble bench that has sagged, due to the downward force of gravity. This type of deformation is the result of millions of micro-fractures in the rock, that take place over hundreds of years. Another time-related deformation is the folding of incompetent rocks, near the surface. Figure 14.8 illustrates GRAVITY Soil C reep this deformation in a very weak schist exposed in a roadcut near Custer, Wyoming. The partiallyweathered schist is GRAVITY bending downslope, as the soil above exerts a FIGURE 14.7 The effect of time and gravity stress on the foliation on a marble bench Illustration by Stan Celestian of the schist. This process normally takes centuries to develop. FIGURE 14.8 Folding of schist due to prolonged influence of gravity induced soil creep Photo by Stan Celestian
THE STRESS AND RESULTING STRAIN OF A SNICKERS® BAR The deformation of a Snickers bar nicely illustrates the type of strain (deformation) created in different materials when the same stress is applied. The Snickers bar consists of tasty nougat, caramel, and peanuts, covered with milk chocolate. At room temperature, the chocolate coating is brittle and deforms by cracking, similar to a rock near the surface of the Earth. The caramel, however, behaves as a plastic, and it bends, rather than break. The sequence of photographs in Figure 14.9 illustrate the strain induced by the stress created by the hands pushing upward in the center (thumbs) and downward on the ends (forefingers). Figure 14.10 shows another Snickers bar that has been frozen, and is being slammed into the table-top. At about 0oF, the caramel behaves as a brittle material. The results of the rapid stress applied to the Snickers bar was that all of the components of the bar behaved as brittle material. This experiment illustrates the role of temperature in the deformation of different materials, including rocks.
Milk Chocolate
Caramel
Peanut
Nougat FIGURE 14.10 The deformation of a frozen snickers bar. Photos by Stan Celestian FIGURE 14.9 Finger-induced stress and resulting strain on a Snickers bar. Photos by Stan Celestian
(Note: All Snickers fragments were properly and safely recycled.)
Brittle Deformation Brittle deformation results from sudden failure of the rock. Much like the breaking of glass, the sudden rupture releases energy. For rocks, this results in a faults and earthquakes. Geologists have grouped the way in which rocks break into categories, based on the relative motion of the rocks on either side of the fault line. The concept of relative motion is shown in Figure 14.11. In the illustration, block B has moved down relative to block A. However, there are a variety of overall motions to be considered. Both blocks may have been moved downward, but block B has moved down more than block A. Conversely, the blocks may both have moved upward, but block A has moved up more than block B. Additionally, either block may have moved independently of the other. A B In describing the motion of blocks on either side of a fault line, a geologists uses the terminology of “relative motion” of the blocks. For example, block B has moved down relative to block A. Fault planes are rarely perfectly vertical. Hanging When blocks slip past each other along a B fault zone, one block is called the footwall, FIGURE 14.11 Relative Motion and the other is called the hanging wall. Illustration by Stan Celestian These terms originated in mines, many years ago. Fault zones are areas where mineral-laden water can precipitate many different types of economically important ore minerals. Many underground mines are thus along fault planes. In an underground mining operation, a drift (passageway along the ore body), is constructed. Miners drill into the Footwall walls of the ore body, in preparation for blasting and removal of ore material. The miner stands on the footwall and the hanging wall hangs overhead. In Figure 14.12 the miner is drilling into the footwall and the FIGURE 14.12 The hanging wall and hanging wall is overhead. The relative motion of the blocks along the fault footwall of an underground mine. Illustration by Stan Celestian zone are show by the arrows. In this example, the hanging wall has moved up relative to the footwall. A
Normal Faults (also called dip-slip or gravity gaults) are created by tensional forces, the extension (pulling apart) of the crust. These types of faults are called gravity faults because the downward pull of gravity is the driving force that moves the blocks one past the other. (Dip slip refers to the fact that the fault movement is parallel to the dip of the fault plane.) In a normal fault, the hanging wall moves down relative to the footwall, as shown by the arrows in Figure 14.13. The tensional forces necessary to produce normal faults can be found at divergent plate boundaries. Fault Scarp
Movement along a fault zone is not smooth, as usually pressure builds and the rock moves along the fault line, however, in some places more than other. Pressure continues to build, and the rocks slip along the fault again. The movement of one block of rock against another generates large amounts of friction, and the fault zone can consist of very broken (brecciated) rocks. The Footwall brecciation creates an increase in porosity and Hanging Brecciated permeability. These zones are often mineralized, Wall especially where one fault zone intersects FIGURE 14.13 Crustal extension and the development of normal another. faults.
Illustration by Stan Celestian
X
X
Y
Top View
Y B
A
C
FIGURE 14.14 Fault gouge breccia, intersecting fault planes, and mineralization Photo A is an example of a brecciated zone found along a fault plane in Nevada. A layer of chert was brecciated to produce what is known as a fault gouge breccia. The red is mercury ore, cinnabar, that has filled the pore spaces in the breccia. Photo and Graphics by Stan Celestian
Figure 14.14 illustrates a situation where two fault planes (X and Y) intersect. Each has its own brecciated zone, but where they intersect, a much more extensive brecciated zone is created. The United Verde Mine at Jerome, Arizona, was one such occurrence. About 10 million tons of ore were recovered from the mine. This yielded about 675 million pounds of copper, 20 million ounces of silver, and about 700 million ounces of gold. Many other mines in the area around Jerome also produced the same materials, but none could match the overall production of the United Verde Mine, primarily because of its unique geology. The Basin and Range Province A geologic province is a large area characterized by a similar geology. The Basin and Range Province of the western United States is dominated by normal faults, created by crustal extension. Figure 14.15 shows the location of the Basin and Range Province as well as portions of surrounding provinces. Figure 14.16 illustrates the crustal extension that created the Basin and Range Province. Block A illustrates the area before extension, and Block B illustrates the normal faults created by the extension. Notice also that the segments of the fault that are higher are called horsts, and the downthrown blocks are called grabens. The grabens are the basins, and the horsts are the ranges in the Basin and Range Province.
A
GRABEN
HORST
B Figure 14.15 The location of the Basin and Range Province. Map Courtesy of the USGS
Figure 14.16 Normal faulting in the Basin and Range Province Illustration by Stan Celestian
FIGURE 14.17 A map view (left) and a geologic map view (right) of a portion of the southwestern United States, showing the Basin and Range Province. The Geologic Map of the United States can be viewed by clicking on this hyperlink: http://pubs.usgs.gov/dds/dds11/kb.html Maps Courtesy of the USGS
Figure 14.17 illustrates the Basin and Range Province on a shaded relief map (left) and a geologic map (on the right). A geologic map is one in which the various colors represent different rock types. Nevada is especially endowed with Basin and Range geology. Reverse Fault (also called a dip-slip fault)
Hanging Wall Footwall
Before After
A reverse fault is the result of compression, and consequent brittle failure of the rock. In a reverse fault, the hanging wall moves up relative to the footwall. Figure 14.18 is an illustration of a high angle, reverse fault. (The high angle refers to the angle at which the fault plane tilts from the horizontal position. In this case, it is about 60o.) The illustration also shows the confining pressures, as indicated by the orange arrows; and the amount of shortening, by the before and after bars below the block diagram. The compressional forces needed to produce reverse faults are found along convergent plate boundaries.
Figure 14.19 illustrates a reverse fault in Iceland. In the photo, a geologist is measuring the amount of vertical displacement along the fault. In this situation, the rock below the glacial ice faulted. The movement was abrupt and caused the ice, which is normally plastic, to break in a brittle manner. This illustrates the concept that a sudden burst of force can cause even soft material to act in a brittle manner. FIGURE 14.18 A reverse fault The bars below the block diagram show the amount of crustal shortening caused by the compressional forces (orange arrows). Illustration by Stan Celestian
FIGURE 14.19 A reverse fault in glacial ice Photo courtesy of the USGS
Nappe Proterozoic Rocks
Chief Mt. (Klippe)
Thrust Fault (red line)
FIGURE 14.20 The Lewis Overthrust and the erosional remnant of Chief Mountain.
Cretaceous Rocks Illustration by Stan Celestian
Another example of a compressional fault is the low-angle reverse fault, also called a thrust fault. In northern Montana, Chief Mountain, is a remnant of a huge thrust fault. About 170 million years ago the North American Plate collided with a crustal block. That collision pushed a huge mass of old (~2 billion years old) Proterozoic rocks eastward for about 50 miles, over much younger (~100 million years old) Cretaceous-age sedimentary rocks. This massive low-angle thrust fault, called the Lewis Overthrust, is shown in Figure 14.20. The angle of the thrust fault plane averages about 10o. Figure 14.21 is a photograph of Chief Mountain, which is a classic example of a “ klippe” (German for cliff). Klippes are erosional remnants of “nappes” (French for tablecloth). The geological concept is that a wedge of continental crust was thrust up and over other crustal rocks. In the case of Chief Mountain, it was part of the nappe, the older Proterozoic rocks. In Figure 14.20, the thrust plane (fault plane) is represented by the red line just above the yellow Cretaceous rocks. The nappe is the main body of the overthrust rock. As it was thrust over the younger Cretaceous rocks, it was folded and contorted by the compression, much like a tablecloth being pushed over a tabletop. Millions of years of erosion has reduced the forward edge of the nappe to fragments, the largest of which are called klippes, like Chief Mountain.
Thrust Plane
FIGURE 14.21 The Chief Mountain Klippe
Photo by Stan Celestian
Strike-Slip Faults A strike-slip fault is one that moves in the direction of the fault plane’s surface trace. In Figure 14.22, the trace of the fault is shown as a turquoise line. Note that no vertical movement on the fault is shown. All of the movement is parallel to the fault trace (the strike of the fault). The relative motion of the faults is shown by the black arrows. There is a distinction between the two faults. The blocks on the left represent a right-lateral strike-slip fault, and the blocks on the right represent a left-lateral strike-slip fault.
Figure 14.x A Strike Slip (Transform) Fault.
Left Lateral
Right Lateral FIGURE 14.22 A strikes-slip (transform) fault Strike-slip faults are produced by shear forces as indicated by the blue arrows. These forces act in opposing directions but are offset. Illustration by Stan Celestian
The names of the fault are derived from their relative motion, with respect to a person straddling the fault. The fault is named a right-lateral if the right side moves toward the person. Conversely, the fault is named a left-lateral if the left side moves toward the person. From your perspective, as you view the image (opposite of the person straddling the fault) the names still apply. For example, in the right-lateral strike-slip fault, the right side is moving toward you.
ho Elk
Perhaps the most studied strike-slip fault in the world is the San Andreas Fault in California. Figure 14.23 shows offset of a small stream (Wallace Creek) by movement along the San Andreas Fault. There are many examples of this type of stream Wallace offsets, as well as rock outcrops along the Creek fault zone. The San Andreas Fault stretches about 780 miles across California. The San Andreas Fault is really a fault system. It represents a number of faults (including the Hayward and Calaveras Faults) that are more or less parallel to the San Andreas, and result from the same type of stress and strain, as the North American Plate slides past the Pacific Plate. An important aspect of the strike-slip fault is that there is very little vertical motion. One fault block slides FIGURE 14.23 Wallace Creek offset along the San Andreas Fault. Image Courtesy of the USGS, The National Map laterally (horizontally) past the other. The San Andreas Fault System is tens of miles wide in some locations and has been determined to be over 11 miles deep in some places. Movement along the fault system does not always produce destructive earthquakes. In some areas, like around El Centro, the fault movement is considered to creep. Thousands of small earthquakes slowly move one plate past the other over many years. rn
. Rd
FIGURE 14.24 Large fault systems in California Illustration Courtesy of the USGS
FIGURE 14.25 Branches of the San Andreas Fault System Illustration Courtesy of the USGS
Figure 14.24 illustrates the major faults in California. Note the areas where creep takes place (gray areas). Figure 14.25 shows the complex nature of the fault system near San Francisco. PLASTIC DEFORMATION - FOLDS Folds are rocks that have been deformed by compressional forces in a plastic, rather than in a brittle manner. Fold nomenclature is shown in Figure 14.26, which is an example of a type of fold called an anticline. al Axi e n Pla
Hinge Line
Lim
The hinge line is the surface expression of the axial plane. The axial plane divides the fold symmetrically. In simple folds, like this example, the axial plane is a flat, planar surface. In more complex folds, it can be curved.
b Lim b
FIGURE 14.26 Parts of a fold
The hinge line represents the fold axis. It is the line from which the fold tilts in opposite directions. In this example of an anticline, the hinge line is on the top of the crest.
Graphic by Stan Celestian
Limbs are simply the flanks of the fold. They are the rock layers that diverge from the hinge line.
Folds represent crustal shortening, as a result of compressional forces.
Anticline
Anticline
TYPES OF FOLDS
C B
D
The most basic types of folds are anticlines and A synclines. Figure 14.27 illustrates these two types of folds. In an anticline, the limbs tilt away from Syncline the crest; and in a syncline, the limbs tilt toward the trough. (A method to remember the difference between the two is that the word anticline starts FIGURE 14.27 Anticlines and a syncline Layer A is the oldest, and D is the youngest. Graphic by Stan Celestian with an “A”, which is the shape of the fold.) Although folds are the result of compressional forces, those forces are normally not uniform in their direction or consistency. As a result, folds can take different shapes. Figure 14.28 displays the most common varieties of folds. The symmetrical fold is one in which the axial plane (blue line) is vertical and the layers tilt in opposite directions at SYMMETRICAL FOLD approximately the same angle.
A
An asymmetrical fold is one in which the axial plane is tilted from the vertical position. This results in the layers (limbs) tilting more on one side than the other. An overturned fold is similar to the asymmetrical fold, but the layers of rock have been forced over, to the point that they are actually tilted in the same direction. In this example, the beds are tilted toward the lower left of the diagram.
ASYMMETRICAL FOLD
B OVERTURNED FOLD
C RECUMBENT FOLD
D FIGURE 14.28 Common types of folds Graphic by Stan Celestian
A recumbent fold is one in which the axial plane is horizontal or nearly horizontal. In these folds (A, B, C and D), the compressional forces (shown by the orange arrows) have changed their relative positions. In A (symmetrical fold) the compressional forces are equal and in opposite directions. However, in block B, the compressional forces are not precisely opposite. The result is a slight shearing component to the compressional force. This is the cause of the asymmetry in the fold. The fold has been tilted over by the shearing force. The separation of the orange arrows (direction of compressional force) becomes greater in block C, creating an overturned fold. Block D represents a fold where the axial plane is almost horizontal, and the orange arrow separation is the greatest. Notice also the change in the shape of the axial plane from a straight line to a curved line in C and D. The axial plane is also being folded by the shear component of the compressional forces.
RECUMBENT FOLD
THRUST FAULT
FIGURE 14.29 A recumbent fold ceating a thrust fault
Illustration by Stan Celestian
In many cases, the shearing compressional forces may overcome the strength of the rock. The plastic strain may yield to brittle deformation, and result in a low-angele reverse fault (thrust fault). Figure 14.29 illustrates how the pressures that produced a recumbent fold can also create a thrust fault. ISOCLINAL FOLD
Isoclinal folds, Figure 14.30, are produced when a competent bed (stronger) is surrounded by weaker rock units. The compressional force folds all of the rocks, however, the competent bed retains its original thickness, while the incompetent beds are displaced. The isoclinals (iso - same, cline - tilt) limbs of the fold are the same angle on both sides of the axial plane. FIGURE 14.30 Isoclinal Folds GraphicFOLD by Stan Celestian CHEVRON
Chevron folds are very angular. Instead of having a smooth, rounded transition from one limb to another, across the axial plane, chevron folds are marked by abrupt angular transitions. These types of folds are found when competent rocks are found interbedded with incompetent rocks. Figure 14.31 is an example of a chevron fold, in which case the pink beds are a competent sandstone interbedded with shale (blue beds). Another type of fold is the FIGURE 14.31 Chevron Folds Graphic by Stan Celestian monocline. As the name implies the monocline (mono - one, cline - tilt) has only one side of a fold. Monoclines commonly form when a competent rock, deep below the surface, is faulted. The displacement of the upthrown block deforms incompetent rocks above, to create a fold. FIGURE 14.32 Monocline
Graphic by Stan Celestian
Figure 14.33 illustrates the geologic circumstances that can create a monocline. The basement rock in this illustration is a very competent granite, that has been faulted. The fault extends into the sedimentary rocks above, however, the sedimentary rocks behave more plastically and deform by folding, rather than faulting. The result of the fault in the granite is a monocline at the surface. Figure 14.34 is the Raplee Monocline, in southeastern Utah.
Incompetent Rocks (Sedimentary)
Fault
Competent Rock (Granite)
FIGURE 14.33 Monocline over fault Graphic by Stan Celestian
FIGURE 14.34 The Raplee Monocline near Mexican Hat, Utah Photo by Stan Celestian
The fault that provided the force for the creation of the monocline lies far below the surface. The faulting and subsequent monocline occurred during the Laramide Orogeny, during the latest part of the Mesozoic Era and early Cenozoic. Based on the tilt angle of the beds and the geology of other similar structures around the Colorado Plateau, it is believed that the monocline formed above a high-angle reverse fault, due to compressional forces. In Figure 14.34, note that the rocks on top of the ridge are fairly horizontal. They tilt toward the west (left side of the picture) and then become more level, as indicated by the rocks in the foreground. Anticlines and synclines represent strain produced by compressional forces. These folds can also be tilted from the horizontal — that is, their hinge lines can be tilted into the Earth’s surface. Such folds are called plunging anticlines or plunging synclines. Figure 14.36 illustrates folded rocks in which the hinge lines of an anticline and two synclines are tilted from the horizontal. The red lines represent the plunging hinge lines of the folds. Figure 14.35 represents a plunging anticline in which the blue plane represents the axial plane. On the block diagram in Figure 14.36, note the surface pattern of the folds. They trace out a zigzag pattern that is very indicative of plunging anticlines and synclines.
FIGURE 14.35 Plunging anticline Graphic by Stan Celestian
FIGURE 14.36 Plunging anticlines and synclines Graphic by Stan Celestian
Figure 14.37 is a NASA radar image that shows the complex fold pattern produced when folds plunge. This same type of zigzag pattern is evident on Figure 14.38, which is The National Map shaded relief map of an area near Bloomsburg, in Central Pennsylvania.
FIGURE 14.37 A radar image of plunging anticlines and synclines of the Appalachian Mountains. Image Courtesy of NASA
FIGURE 14.38 Shaded relief map of Central Pennsylvania, showing the topography of plunging anticlines and synclines. Image Courtesy of the USGS, The National Map
Domes and Basins
FIGURE 14.39 A structural dome
Graphic by Stan Celestian
A dome is a broad uplift that creates a circular or elongated symmetrical fold. Figure 14.39 illustrates an idealized diagram of a dome. In this particular example, the uplifting force is created by an igneous intrusion (shown as red). Like anticlines, the oldest rocks in an eroded dome are found in the center and become progressively younger away from the center of the dome. The exception in this example, is the igneous intrusion, which created the dome, and is the youngest rock unit represented in the diagram.
Figure 14.40 is a sketch of the Black Hills Dome, in South Dakota. At its center is the Harney Peak Granite (pink/red), which has been radiometrically dated (U/Pb) to be about 1.7 billion years old. The heat from the intrusion metamorphosed the surrounding sedimentary rocks, to create a zone of gneiss, schist, and quartzite. The metamorphic zone surrounding the central granitic core is shown as orange in the sketch. It is called the central crystalline area. The intrusion of the granite, however, did not cause the uplift and the formation of the Black Hills Dome. Instead, about 70 million years ago (end of the Cretaceous Period), uplift provided by the Laramide Orogeny forced the granite and surrounding metamorphic rocks upward. This uplift also pushed the overlying sedimentary rocks upward, to create a mountain range that was estimated to be nearly 15,000 feet high. Those once lofty mountains have since eroded to produce the current Black Hills. FIGURE 14.40 The Black Hills Dome Image courtesy of the USGS
Basins (Figure 14.41) are created by downwarping of the crust. This can be accomplished by crustal extension, as is the case for normal faulting, like that seen in the Basin and Range Province. The competent basement rocks fault, but the overlying incompetent rocks bend to produce the basin. Like synclines, the oldest rocks in an eroded dome are around the outside of the structure, and the youngest rocks are found in the center. The Michigan Basin (Figure 14.42) is an FIGURE 14.41 A structural basin example of a structural basin. (The Illustration by Stan Celestian term structural, as in “structural basin” or “structural dome”, refers to the curvature of the strata of the feature. Most structural basins, like the Michigan Basin, are actually elevated areas.) The Michigan Basin was created during the continental collision of the North American Plate with the African Plate, during the Paleozoic Era. Uplift that created the Appalachians caused crustal thinning to the west. This thinning allowed the accumulating sediments to push the crust down, to form the basin. The lower portion of Figure 14.42 is a cross-section from point A to point B, on the geologic map view.
GEOLOGIC MAP KEY A
Upper Pennsylvanian Lower Pennsylvanian Upper Mississippian Lower Mississippian Devonian
B
Silurian G
Ordovician
B
A
FIGURE 14.42 Geologic map and cross-section of the Michigan Basin
Graphic by Stan Celestian
Age Relationships of Sedimentary Strata in Folded Rocks Eroded anticlines, synclines, domes and basins can be identified by how the rocks tilt below the Earth’s surface. However, in many cases, geologists can not see how these rocks tilt. To identify the type of structures, they rely on the relative age relations of the rocks at the surface. Blocks 14.43 A, B, and C demonstrate how this relationship reveals the underlying structure. The blocks that are being used were previous examples in this chapter, however, imagine walking along the surface of the blocks, unable to see the sides of the block diagrams.
A 7 5 3 2
6
B 6
5
4
C 5
6
7
3
4 1
A
B
C
Anticlines and synclines that have been eroded at the surface show a distinctive repeating patterns of the strata. Around the hinge line of a synclines (at A and C), note that the rocks become progressively older away from the axis. Around A, for example, rock unit 7 is the youngest followed by 6, then 5, then 4. Using the Law of Superposition, it can be determined that rock unit 1 is the oldest, and 7 is the youngest. Moving away from the hinge line of the anticline the rocks become progressively younger. (At B, rock unit 4 is older than 5, then 6 and 7.)
A dome can be considered a doubly plunging anticline. Like the rocks exposed at the surface of the eroded anticline, the exposed rocks of a dome are also the oldest in the center and progressively younger to the margins of the structure.
A basin can be considered a doubly plunging syncline. In this example it can be seen that the youngest rocks occupy the middle of the structure and the rocks become older and older as one observes the strata away from the center.
FIGURE 14.43 Determining the structure of folded rocks by observing surface outcrop patterns and the age of the rocks. Graphic by Stan Celestian
BRITTLE DEFORMATION — JOINTS Joints are fractures, or cracks, in rock. They differ from faults in that joints have no significant movement along the fracture. Columnar jointing in basalt has already been discussed, in a previous chapter. It is created by cooling and contraction. The decrease in volume of the liquid rock to create the solid rock, creates tensional forces that pull the cooling rock apart, but with no appreciable r e l a t i v e FIGURE 14.44 Joint sets in the Navajo Sandstone in Arches National Park, Utah Image courtesy of the USGS, The National Map movement. Joints can be created by large-scale earth movements, as in plate tectonics, or small scale movement, and by impacts. Joints normally do not occur as single fractures, but rather they occur in joint sets. Figure 14.44 is a satellite view of a portion of Arches National Park, Utah. It shows three prominent joint sets (as indicated by the red lines). These joints were created when the rock (sandstone) was uplifted and arched into a broad dome. Figure 14.45 shows the deeply-eroded joints at the Fiery Furnace area of the park. Although joints do not fragment the rock, as does the process of faulting, the joints do provide water with a ready-made channel for flow. Over many years, the water flowing over and through these joints has eroded them considerably. Figures 14.46 and 14.47 show a ground-level view of this area.
Fiery Furnace Overlook
FIGURE 14.45 The Fiery Furnace Overlook
Image courtesy of the USGS, The National Map
FIGURE 14.46 Well-eroded joint set at Arches National Park, FIGURE 14.47 A view from one of the “fins” at the Fiery Furnace, Arches National Park. Utah Photo by Stan Celestian Photo by Stan Celestian
Summary of the important factors influencing the deformation of rocks: - Rock type: Competent rocks like igneous rocks (granite, massive basalt layers, gabbro) will behave in a brittle manner and most commonly fracture rather than fold. Incompetent rocks like gypsum, salt, and clay-rich rocks will behave as plastic materials and bend or flow under an applied stress. - Pressure and direction of pressure: Compressional pressures tend to create folding in incompetent rocks and reverse faults or thrust faults in competent rock types. - Duration and intensity of pressure: A small applied stress acting over many centuries can cause folding in even competent rocks. A quick, strong stress can cause normally incompetent rocks to behave in a brittle manner. - Temperature: An increase in temperature generally weakens rocks. Even a very competent rocks like granite will behave in plastic manner if the temperature approaches its melting point.
KEY WORDS Anticline: a fold in which the limbs tilt down and away from the hinge line Asymmetrical fold: a fold where the axial plane is tilted slightly away from the vertical This results in one limb being tilted downward more the other. Axial plane: the planar center of symmetry for a fold Basin: a down-warped portion of the crust, where rock strata display a downward curvature Brecciated zone: an area along a fault plane, in which the rocks have been broken by the movement of the fault Brittle deformation: a breaking or fracturing of a rock, produced by a force acting on it Chevron fold: curve
a series of folds in which the limbs meet the hinge lines at a distinct angle, rather than a
Competent: refers to the relative strength of a rock The stronger (usually brittle) rock in a group of rocks. The weaker rocks are referred to as incompetent. Compressional Stress: a force acting to squeeze rocks together Deformation: the eventual change in shape of a rock body, due to stress Dip-Slip Fault: a fault with movement parallel to the dip of the fault plane; sSee Gravity fault, Normal, or Reverse fault. Dome: a crustal uplift as deduced from the upwarping of strata in a roughly circular or elongate manner Ductile Deformation: the change in shape (bending or flowing) of a rock body in a plastic manner, where no fracturing takes place; plastic deformation Extension: an expansion of the rock Fault: a fracture in rock, along which significant movement has taken place Fold: a deformation of rock into curves, due to compressional forces Footwall: it is the lower block of an inclined fault Graben: it is a down-dropped block, in a series of normal faults Gravity fault: a fault created by crustal extension. The hanging wall moves down relative to the footwall (gravity is the driving force); see Dip-slip fault or Normal fault Hanging wall: the upper block of an inclined fault Hinge line: the line of flexure for a fold
Horst: a block that stands higher than surrounding blocks in a set of normal faults Isoclinal fold: hinge line
a symmetrical fold in which opposite limbs tilt at the same angle away from the
Incompetent: a weaker or softer rock in a group of rocks. When a stress is applied, the incompetent rocks are more likely to deform. Stronger rocks are referred to as competent Joint: a fracture in a rock, along which no significant movement has taken place Klippe: an erosional outlier of a nappe Left-lateral movement: a descriptive term related to the movement along a strike-slip fault. Straddling the fault, the left-lateral strike-slip refers to the left block moving toward the observer. Limb: rock strata that diverge from the hinge line of a fold Monocline: a fold in which there is only one tilted limb Nappe: the main body of an overthrust block, part of a thrust fault Normal fault: a fault created by tensional forces in which blocks slip past one another along a plane of failure; the hanging wall moves down, relative to the footwall Orogeny: a mountain-building event created by crustal plate collisions Overthrust: the portion of a thrust fault that overrides the lower block of rock Overturned fold: a fold in which the axial plane has been significantly tilted from the vertical position; the limbs tilt in the same direction Plastic deformation: the change in shape (bending or flowing) of a rock body in a ductile manner, where no fracturing takes place; ductile deformation Plunging fold: a fold in which the axial plane tilts in a down from the horizontal Proterozoic: a period of time dating from about 570 million years ago to about 2.5 billion years ago Recumbent fold: a fold in which the axial plane is nearly horizontal Right-Lateral Fault: a descriptive term related to the movement along a strike-slip fault Straddling the fault, the right-lateral strike-slip refers to the right block moving towards the observer. Reverse Fault: a fault in which the hanging wall moves up, relative to the footwall Reverse faults are the result of compressional forces acting on brittle rock.
Shear stress: forces that are acting in parallel, but offset opposing directions Snickers® bar: chocolate
a tasty treat consisting of caramel, peanuts, and nougat, covered with milk
Strain: deformation caused by stress Stress: a force applied to a rock Strike-slip fault: a fault in which one block slides horizontally past the other, with very little vertical movement Symmetrical fold: a fold in which the limbs tilt equally away from the hinge line Syncline: a down-warped fold, in which the limbs tilt up and away from the hinge line Tensional stress: the rock
a force that acts outward, away from the center of a rock; creates extension of
Thrust fault: a low-angle reverse fault, where the hanging wall moves up and over the footwall Transform fault: a strike-slip fault that is associated with a plate boundary (All transform faults are strike-slip faults, because little or vertical displacement occurs. However, the term “transform fault is used exclusively for those strike-slip faults that occur along plate boundaries.)
