Unit 9 Lab Crustal Deformation Fall 23 R1 sh

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UNIT NINE: CRUSTAL DEFORMATION Randa Harris INTRODUCTION The Earth is an active planet shaped by dynamic forces. Such forces can build mountains and crumple and fold rocks. As rocks respond to these forces, they undergo deformation, which results in changes in shape and/or volume of the rocks. The resulting features are termed geologic structures. This deformation can produce dramatic and beautiful scenery, as evidenced in Figure 1, which shows the deformation of originally flat (horizontal) rock layers. Figure 1. Rocks that have been deformed along the coast of Italy. Why is it important to study deformation within the crust? Such studies can provide us with a record of the past and the forces that operated then. The correct interpretation of features created during deformation is critical in the petroleum and mining industry. It is also essential for engineering. Understanding the behavior of deformed rocks is necessary to create and maintain safe engineering structures. When proper geological planning is not considered in engineering, disasters can strike. For example, the Vajont Dam was constructed at Monte Toc,
Italy in the early 1960’s. The place was a poor choice for a dam, as the valley was narrow, thorough geological tests were not performed, and the area surrounding the dam was prone to large landslides. In 1963, a massive landslide in the area displaced much of the water in the dam, causing it to override the top of the dam and flood the many villages downstream, resulting in the deaths of almost 2,000 people (Figure 2). Figure 2. An image of the Vajont reservoir shortly after the massive landslide (you can see the scar from the landslide on the right, and the dam is located in the foreground on the left). Learning Outcomes After completing this chapter, you should be able to: • Understand the different types of stress that rocks undergo, and their responses to stress • Demonstrate an understanding of the concepts of strike and dip • Recognize the different types of folds and faults, and the forces that create them • Use block diagrams to display geologic features • Create a geologic cross-section
Key Terms: Stress Strain Compressional forces Tensional forces Shear forces Contact Strike Dip Monocline Anticline Syncline Dome Basin Normal fault Reverse fault Strike-slip fault Horst & graben STRESS AND STRAIN Rocks change as they undergo stress , which is just a force applied to a given area. Since stress is a function of area, changing the area to which stress is applied makes a difference. For example, imagine the stress that is created both at the tip of high heeled shoes and the bottom of athletic shoes. In the high heeled shoe, the area is very small, so that stress is concentrated at that point, while the stress is more spread out in an athletic shoe. Rocks are better able to handle stress that is not concentrated in one point. There are three main types of stress: compression, tension, and shear. When compressional forces are at work, rocks are pushed together. Tensional forces operate when rocks pull away from each other. Shear forces are created when rocks move horizontally past each other in opposite directions. Rocks can withstand compressional stress more than tensional stress (see Figure 3).
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Figure 3. This is a picture of the Roman Forum. Why did the Romans use so many vertical columns to hold up the one horizontal beam? If the horizontal beam spanned a long distance without support, it would buckle under its own weight. This beam is under tensional stress, so it is not as strong. Applying stress creates a deformation of the rock, also known as strain . As rocks are subjected to increased stress and strain, they at first behave in an elastic manner, which means they return to their original shape after deformation (Figure 4). This elastic behavior continues until rocks reach their elastic limit (point X on Figure 4), at which point plastic deformation commences. The rocks may bend into folds, or behave in a brittle manner by fracturing (brittle behavior can be easily envisioned if you think of a hammer hitting glass), but regardless they do not return to their original shape when the stress is removed in plastic deformation. The resulting deformation from applied stress depends on many factors, including the type of stress, the type of rock, the depth of the rock and pressure and temperature conditions, and the length of time the rock endures the stress. Rocks behave very differently at depth than at the surface. Rocks tend to deform in a more plastic manner at depth, and in a more brittle manner near the Earth’s surface.
Figure 4. A stress and strain diagram. As stress and strain increase, rocks first experience elastic deformation (and can return to their original shape) until the elastic limit is reached, depicted at point X. After this point, rocks will fracture or experience plastic deformation, so that their original shape is destroyed. STRIKE AND DIP To learn many of the concepts associated with structural geology, it is useful to use block diagrams. As you examine these blocks, note the different ways that you can view them. If you look at a block from along the side, you are seeing the cross-section view (like what you see along roads that have been cut through the mountains). If you look at the block from directly above it, you are looking at map view (Figure 5).
Figure 5. You are viewing the top block in this image in map view, viewed from directly above the block. The lower block is from the same rock layers, and you are viewing it in cross-section (or from the side). Note that in cross-section, you can see how the rock layers are tilted. As you think about how rocks have changed through the process of deformation, it is useful to remember how they deposited in the first place. We will briefly review some of the geologic laws that you learned earlier in the course. Sedimentary rocks, under the influence of gravity, will deposit in horizontal layers (Law of Original Horizontality). The oldest rocks will be on the bottom (because they had to be there first for the others to deposit on top of them), and are numbered with the oldest being #1 (Law of Superposition). The wooden block in Figure 6 displays how sedimentary rocks originally deposit. Figure 6. In the image above, different rock types are given different colors. The oldest rock, on the bottom, is given a #1. The youngest rock in this image is #4.
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Each of the boundaries between the colored rock units above represents a geologic contact , which is simply the surface between two different rock units. Earth’s rock layers are usually not this uncomplicated. Rock layers are often at an angle, not horizontal, indicating that changes have occurred since deposition. Examine Figure 7 to see tilted rocks. Figure 7. In the image above, beds have been tilted. Which color bed is the oldest? To determine this, it is useful to apply Occam’s Razor, which states that the simplest explanation is most likely the best. It is more likely that the gray bed on the left was the bottom bed during deposition, and therefore the oldest. In order to measure and describe layers like this, geologists apply the concepts of strike and dip. Strike refers to the line formed by the intersection of a horizontal plane and an inclined surface. Dip is the angle between that horizontal plane (such as the top of this block) and the tilted surface (the geologic contact between the tilted layers). In Figure 8 below, strike is a line on the horizontal plane created when the dipping layer intersects the Earth’s surface. The dip angle is measured from the horizontal surface to the dipping bed.
Figure 8. A demonstration of strike and dip. In this example, the beds are dipping to the southwest. Now, let’s apply this concept to the block of dipping beds that you just looked at (Figure 7). To determine strike, find where the dipping layer intersects the horizontal surface and draw a line parallel to this line of intersection on the top of the block (i.e. our horizontal surface). To determine dip, pretend that there is a drop of water between one bed and the next, for example, along the intersection of the pale blue bed and the red bed. In which direction would the water roll if it followed that contact? That is the direction of dip – towards the right in this case. The symbol for strike and dip is given along the top of the block. Note that the dip symbol (shorter line) is perpendicular to the strike symbol (Figure 9).
Figure 9. This image depicts the strike and dip for the pale blue bed. As all the beds are oriented in the same direction, they would all have the same strike and dip. GEOLOGIC STRUCTURES CREATED BY PLASTIC AND BRITTLE DEFORMATION Folds Folds are geologic structures created by plastic deformation of the Earth’s crust. To understand how folds are generated, take a piece of paper and hold it up with a hand on each end. Apply compressional forces (push the ends towards each other). You have just created a fold (bent rock layers). Depending on how your paper moved, you created one of the three main fold types (Figure 10).
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Figure 10. The three main fold types, from left to right, are monocline, anticline, and syncline (the anticline and syncline are both displayed in one block). A monocline is a simple fold structure that consists of a bend in otherwise horizontal rock layers. More commonly found are anticlines and synclines. An anticline fold is convex up and one in which the layered strata are inclined down and away from the center of the fold (if you drew a line across it, the anticline would resemble a capital letter A). A syncline is a concave upward fold in which the layered strata are inclined up (it resembles a smile). Parts of a fold include the axis (the hinge line), the axial plane, and limbs on either side of the axis (Figure 11). It is important to note that anticlines do not always represent mountains or high areas and synclines do not always represent basins or low areas. They are simply folded rock layers, and do not necessarily indicate topographic high and low points (see Figure 12).
Figure 11. The axial plane and fold axis, along the center of the fold, and corresponding limbs on either side. Figure 12. This topographic high, along Interstate 68 and US 40 in Maryland, is a syncline.
Folds observed in cross-section look much different from map view. In map view, rather than seeing folds, you will only encounter beds that look like Figure 13. To help determine what type of fold you have, it is useful to determine the strike and dip of the beds you encounter. On Figure 13, determine the strike and dip for each location marked by an oval. Check yourself against Figure 14. Figure 13. A block diagram of an anticline and syncline. Looking from above you see map view, and from the side you observe cross-section view. Determine the strike and dip symbols that should go in the ovals.
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Figure 14. In this block, the strike and dip symbols, along with the symbols for anticline and syncline, have been drawn in for you. Note that on the anticline, the beds dip AWAY from the axis, and the anticline symbol is drawn along the axis, with arrows pointing away from each other. In the syncline, the beds dip TOWARDS each other, with the syncline symbol having arrows that point inwards. Once rocks are folded and exposed at the Earth’s surface, they are subjected to erosion, creating regular patterns. The erosion exposes the interiors of the folds, such that parallel bands of dipping strata can be observed along the fold axis. In an anticline, the oldest rocks are exposed along the fold axis, while it is the youngest rocks exposed at the fold axis in a syncline (Figure 15).
Figure 15. In the top block (A), you view a typical anticline and syncline. Look at the center of the folds. Are the beds that you see in the center of the anticline older or younger than the beds on either side of it? What about for the syncline? In the lower block (B), you see the top portion of the block removed, done so to simulate erosion of rock layers. Note the pale blue bed in the center of the anticline. It is older than the red bed on either side of it (since it is lower in side view – remember older beds are on the bottom). In the syncline, the black bed in the middle is younger than the yellow beds on either side of it. Fold Type Direction of dip of layers Age of beds at axis Anticline Away from axis Oldest Syncline Towards the axis Youngest
Imagine you take that same sheet of paper that you originally created a fold with. Create a fold again, but rather than holding it horizontally, plunge one end of your paper down into your desk surface. You have now created a plunging fold, which is essentially a tilted fold that creates a V shaped pattern on the surface (Figure 16). In an anticline, the oldest strata can be found at the center of the V, and the V points in the direction of the plunge of the fold axis. In a syncline, the youngest strata are found at the center of the V, and the V points in the opposite direction of the plunge of the fold axis. Figure 16. Both blocks are plunging folds. Note the V shape created in map view in a plunging fold. The fold on the left block is a plunging anticline (observe the end of the block to determine this) and the fold on the right block is a plunging syncline.
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Figure 17. Note this side view of a plunging anticline, to better see how the rock layers plunge into the Earth. Figure 18. These blocks depict map view for both plunging and non-plunging folds. The top block is a plunging fold, with the characteristic V-shape. The lower block shows map view of non-plunging folds. Similar to anticlines and synclines are domes and basins, which are basically the circular (or elliptical) equivalent of those folds. A dome is an upfold similar to an anticline, and a basin
is an area where the rocks are inclined downward towards the center, similar to a syncline (Figure 19). The key to identifying these structures is similar to identifying the folds. In a dome , the oldest rocks are exposed at the center, and rocks dip away from this central point. In a basin , the youngest rocks are in the center, and the rocks dip inward towards the center. Figure 19. The dome has older rocks in the center, with rocks dipping away from this point, while the basin has rocks dipping inwards and the youngest rocks in the center. L AB E XERCISES Part A – Folds 1. In the following diagram, which way do the beds dip relative to the fold axis? a. towards the axis b. away from the axis
c. the beds are horizontal c. none of the choices are correct 2. What is the name of the geologic structure depicted in this diagram? a. monocline b. dome c. basin d. anticline e. syncline 3. This geologic structure can further be defined as a: a. normal fold b. reverse fold c. shear fold d. plunging fold
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4. What type of forces created the structure depicted in this diagram? a. extensional b. compressional c. shear 5. What is the name of the geologic structure depicted in this diagram, shown in map view? a. monocline b. dome c. basin d. anticline e. syncline 6. In the following diagram, which way do the beds dip relative to the fold axis?
a. towards the axis b. away from the axis c. the beds are horizontal c. none of the choices are correct 7. In the following diagram, which of the following is correct? a. the oldest beds are in the center b. the youngest beds are in the center c. the beds are all the same age d. There is now way to determine this. 8. In the following diagram, given that bed B is older than bed A, what is the name of the geologic structure?
a. monocline b. dome c. basin d. anticline e. syncline 9. In the following diagram, given that bed B is older than bed A, which way do the beds dip? a. the rocks dip inward towards the center b. the rocks dip outward away from the center c. the rocks are horizontal d. the rocks are vertical Faults
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As rocks undergo brittle deformation, they may produce cracks in the rocks. If no appreciable displacement has occurred along these cracks, they are called joints. If appreciable displacement does occur, they are referred to as faults. We will first examine dip-slip faults, in which movement along the fault is either up or down. The two masses of rock that are cut by a fault are termed the fault blocks (Figure 20). The type of fault is determined by the direction that the fault blocks have moved. Figure 20. Two fault blocks. The fault is the break in the block that separates the two fault blocks. Fault block movement is described based on the movement of the hanging wall, the fault block located above the fault plane. The other fault block, located beneath the fault plane, is called the foot wall. The term hanging wall comes from the idea that if a miner was climbing along the fault plane, she would hang her lantern above her head, along the hanging wall. Alternately, you can draw a stick figure straight up and down across the fault plane. Its head will be on the hanging wall and its feet will be on the foot wall (Figure 21).
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Figure 21. In this image, the head of the stick figure is on the hanging wall (in peach) and the feet of the stick figure are on the foot wall (in blue). When extensional forces are applied to the fault blocks, the hanging wall will move down, creating what is called a normal fault (an easy way to remember this is the phrase “It’s normal to fall down”). As this happens, crust is stretched out and lengthened (Figure 22). Figure 22. The hanging wall, on the right, has moved down relative to the footwall, resulting in a normal fault. Notice how close together the cross symbols were in Figure 20, compared to this figure, evidence for the lengthening of the block (along red line). When compressional forces are applied to the fault blocks, the hanging wall will move up, creating a reverse fault . This causes the crust to shorten in the area (Figure 23). A special
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type of reverse fault is a thrust fault. It is a low angle reverse fault (with a dip angle of less than 45 o ), and has a much thinner hanging wall. Figure 23. The hanging wall, on the right, has moved up relative to the footwall, resulting in a reverse fault. Note how much closer the cross symbols have become when compared to figure 20, evidence for the shortening of the block.
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Figure 24. An example of a normal fault near Somerset, UK. Note the fault line in red, and the hanging wall sediments (on the left side) that have moved downward. Used with permission by geography.org.uk – 2455274. Fault Type Type of Force Direction HW moved Length of Block Normal Extensional Down Lengthened Reverse Compressional Up Shortened Tensional forces acting over a region can produce normal faults that result in landforms known as horst and grabens. In horst and grabens , the graben is the crustal block that downdrops, and is surrounded by two horsts, the relatively uplifted crustal blocks (Figure 25). This terrain is typical of the Basin and Range of the western United States. Figure 25. This figure depicts an area that has been stretched by tensional forces, resulting in numerous normal faults and horst and graben landforms. The previous faults we observed were dip-slip faults, where movement occurred parallel with the fault’s dip. In a strike-slip fault , horizontal motion occurs (in the direction of strike, hence the name), with blocks on opposite sides of a fault sliding past each other due to shear forces. The classic example of a strike-slip fault is the San Andreas fault in California. These faults can be furthered classified as right-lateral or left-lateral. To determine this, an observer
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would stand along one side of the fault, looking across at the opposite fault block. If that fault block appears to have moved right, it is right-lateral; if it has moved left, it is left-lateral. Figure 26 displays a block diagram of the San Andreas fault. Figure 27 provides all three fault types for your review. Figure 26. A block diagram of the San Andreas Fault, a right-lateral strike-slip fault.
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Figure 27. The three types of faults discussed in this lab. In the reverse/thrust fault, the hanging wall moves up. In the normal fault, the hanging wall moves down. The strike-slip faults depicts lateral movement. L AB E XERCISES Part B – Faults 10. In the following diagram, which side is the hanging wall, and which way has it moved?
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a. right, up b. right, down c. left, down d. left, up 11. In the following diagram, what is the name of the geologic structure? a. anticline b. syncline c. normal fault d. reverse fault e. strike-slip fault 12. What type of force created the structure in the following diagram?
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a. compressional b. planar c. shear d. extensional 13. In the following diagram, which side is the hanging wall, and which way has it moved? a. right, up b. right, down c. left, down d. left, up 14. In the following diagram, what is the name of the geologic structure?
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a. anticline b. syncline c. normal fault d. reverse fault e. strike-slip fault 15. What type of force created the structure in the following diagram? a. compressional b. planar c. shear d. extensional
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16. How can you further define the structure in the following diagram? a. left lateral b. right lateral c. plunging d. planar 17. In the following diagram, what is the name of the geologic structure? a. anticline b. syncline c. normal fault d. reverse fault
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e. strike-slip fault 18. What type of force created the structure in the following diagram? a. compressional b. planar c. shear d. extensional Google Earth 19. In Google Earth, go to 32 40 56.64N 56 30 7.55E and zoom to an eye altitude of ~25,000 feet. Based on their appearance in map view, how can you describe these structures? a. normal faults b. reverse faults c. horizontal folds d. monoclines e. plunging folds 20. In Google Earth, go to 38 34 23.94N 109 32 59.42W and zoom to an eye altitude of ~25,000 feet. This is the city of Moab, Utah, which has some beautiful geology. This area has a series of plunging folds that run northwest/southeast. Moab Valley, where the town is located, is along one of these folds, with the oldest rocks found in the middle of the structure. Which structure is it?
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a. plunging anticline b. plunging syncline c. plunging monocline Part C – Crustal Deformation and the National Parks Many of the incredible features that you observe in the national parks are the result of deformation. Today we are going to head to Arches National Park in Utah. 21. Go to 38 36 52.31N 109 37 14.67W. Look at the zoom-in-and-out controls in the upper right corner of the map - when you put the mouse over these, you should be able to see a drawing of a person. Click on the person icon and drag it over to the pinpointed spot to enter Street View, and examine the outcrop to the southeast (right) of the roadway. You'll find numerous faults that offset the sedimentary layers here. What type of faults are these? a. normal b. reverse c. strike-slip 22. Which way has the hanging wall moved in these faults? a. up b. down 23. What type of force created these faults? a. compressional b. shear c. extensional d. planar 24. Exit street view and g o to 38 37 4.9N 109 36 30.44W and zoom into an eye altitude of ~4,500 feet. This is a particular type of mass wasting (the movement of rock or soil downslope under the influence of gravity). What type of mass wasting does this appear to be? a. slump b. creep c. rock fall
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25. Go to 38 44 35.49N 109 30 24.16W and zoom into an eye altitude of ~7,500 feet. This area is jointed. What is the direction of the biggest, most obvious joints? Use the rule tool to measure them, and give the answer in headings. Remember that the bearing depends on where you click first and last, so if none of these answers make sense, try starting from the other end of the joint than you did originally. a. 50 degrees b. 41 degrees c. 214 degrees d. 94 degrees e. 78 degrees
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STUDENT RESPONSES 1. In the following diagram, which way do the beds dip relative to the fold axis? a. towards the axis b. away from the axis c. the beds are horizontal c. none of the choices are correct 2. What is the name of the geologic structure depicted in this diagram? a. monocline b. dome c. basin d. anticline e. syncline 3. This geologic structure can further be defined as a:
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a. normal fold b. reverse fold c. shear fold d. plunging fold 4. What type of forces created the structure depicted in this diagram? a. extensional b. compressional c. shear 5. What is the name of the geologic structure depicted in this diagram, shown in map view?
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a. monocline b. dome c. basin d. anticline e. syncline 6. In the following diagram, which way do the beds dip relative to the fold axis? a. towards the axis b. away from the axis c. the beds are horizontal c. none of the choices are correct 7. In the following diagram, which of the following is correct?
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a. the oldest beds are in the center b. the youngest beds are in the center c. the beds are all the same age d. There is now way to determine this. 8. In the following diagram, given that bed B is older than bed A, what is the name of the geologic structure? a. monocline b. dome c. basin d. anticline e. syncline 9. In the following diagram, given that bed B is older than bed A, which way do the beds dip?
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a. the rocks dip inward towards the center b. the rocks dip outward away from the center c. the rocks are horizontal d. the rocks are vertical 10. In the following diagram, which side is the hanging wall, and which way has it moved? a. right, up b. right, down c. left, down d. left, up 11. In the following diagram, what is the name of the geologic structure?
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a. anticline b. syncline c. normal fault d. reverse fault e. strike-slip fault 12. What type of force created the structure in the following diagram? a. compressional b. planar c. shear d. extensional
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13. In the following diagram, which side is the hanging wall, and which way has it moved? a. right, up b. right, down c. left, down d. left, up 14. In the following diagram, what is the name of the geologic structure? a. anticline b. syncline c. normal fault
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d. reverse fault e. strike-slip fault 15. What type of force created the structure in the following diagram? a. compressional b. planar c. shear d. extensional 16. How can you further define the structure in the following diagram? a. left lateral
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b. right lateral c. plunging d. planar 17. In the following diagram, what is the name of the geologic structure? a. anticline b. syncline c. normal fault d. reverse fault e. strike-slip fault 18. What type of force created the structure in the following diagram?
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a. compressional b. planar c. shear d. extensional 19. In Google Earth, go to 32 40 56.64N 56 30 7.55E and zoom to an eye altitude of ~25,000 feet. Based on their appearance in map view, how can you describe these structures? a. normal faults b. reverse faults c. horizontal folds d. monoclines e. plunging folds 20. In Google Earth, go to 38 34 23.94N 109 32 59.42W and zoom to an eye altitude of ~25,000 feet. This is the city of Moab, Utah, which has some beautiful geology. This area has a series of plunging folds that run northwest/southeast. Moab Valley, where the town is located, is along one of these folds, with the oldest rocks found in the middle of the structure. Which structure is it? a. plunging anticline b. plunging syncline c. plunging monocline 21. Go to 38 36 52.31N 109 37 14.67W. Look at the zoom-in-and-out controls in the upper right corner of the map - when you put the mouse over these, you should be able to see a drawing of a person. Click on the person icon and drag it over to the pinpointed spot to enter Street View, and examine the outcrop to the southeast (right) of the roadway. You’ll find numerous faults that offset the sedimentary layers here. What type of faults are these? a. normal b. reverse c. strike-slip 22. Which way has the hanging wall moved in these faults?
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a. up b. down 23. What type of force created these faults? a. compressional b. shear c. extensional d. planar 24. Exit street view and go to 38 37 4.9N 109 36 30.44W and zoom into an eye altitude of ~4,500 feet. This is a particular type of mass wasting (the movement of rock or soil downslope under the influence of gravity). What type of mass wasting does this appear to be? a. slump b. creep c. rock fall 25. Go to 38 44 35.49N 109 30 24.16W and zoom into an eye altitude of ~7,500 feet. This area is jointed. What is the direction of the biggest, most obvious joints? Use the rule tool to measure them, and give the answer in headings. Remember that the bearing depends on where you click first and last, so if none of these answers make sense, try starting from the other end of the joint than you did originally. a. 50 degrees b. 41 degrees c. 214 degrees d. 94 degrees e. 78 degrees
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