Lab 7- Rock Deformation Handout.docx (1).pdf

GEOL101 Dynamics of the Earth – Spring 2023 Name: Emily Thomson Laboratory 8: Structural Geology Section Introduction Structural geology is a subdiscipline of geology that investigates rock deformation (bending and breaking or folding and faulting) on both the small (microscopic) and large (mountains, oceans, and continents) scale. For the purposes of this lab we are going to focus on the large-scale structural features associated with folding and faulting of rocks and their identification. Structural geologists (you) seek to use their observations at the surface of the Earth to interpret what the structures look like below the surface, which is hard to observe directly. In this lab we will discuss two different ways of looking at the Earth, map view and cross-sectional view (Figure 1). Map view is what you see when you look straight down onto the surface of the Earth or as in figure 1A the top of a cake. Cross sectional view is what the crust looks like if you were to take a slice out of the Earth or cake (Figure 1B). We use cross sections to project the structures we observe at the Earth’s surface into the subsurface. Figure 1: A. Map view of the top of a cake as an analogy for map view of the Earth. B. A slice of the cake in A as an example of cross-sectional view. Without cross sections, we would not have a way to project what we observe on the surface of the Earth into the subsurface. Notice that the slice of cake has five horizontal layers with the oldest layer at the bottom and the youngest layer at the top. Strike and Dip In our discussion of geologic time in lab 7 we learned six principles of relative dating, one being original horizontality. The idea that all rocks on the Earth are originally deposited in essentially horizontal layers and that if the rocks are currently not horizontal then some younger event must have disturbed their original horizontality. Globally geologists have decided on a map symbol that allows them to display the orientation or attitude of a rock unit on a map. We call this symbol strike and dip and it’s used for non-horizontal or dipping rock units or beds (figure 2). 1

Figure 2. A. Map view of a geologic map with three units, one being the oldest and 3 being the youngest. Strike and dip symbol annotated. B. Cross sectional view of A. Notice that unit 1 is below units 2 and 3 which is consistent with unit 1 being the oldest. C. 3D block diagram that combines A and B. The strike bar is always parallel to the intersection of the dipping plane with horizontal (figure 2A) and the dip tick points in the direction the beds are dipping. Notice in Figure 2C that the strike bar is parallel to the contact between the orange and brown units or units 1 and 2 and the dip tick is pointing in the same direction that all the units (1, 2, and 3) are dipping. Without the strike and dip symbol in Figure 2A we would not be able to draw the cross section in Figure 2B. In this lab you will not have to measure or draw a strike and dip symbol, but you will need to use them to interpret structures. Folds: Anticline and Syncline Folds, no matter the type, form under compression like that found along convergent boundaries where two tectonic plates are moving towards one another. However, compression can occur along transform boundaries too and therefore folds can form here as well. There are several kinds of folds but in this lab, we are going to discuss two types. Anticlines : When an anticline forms a topographic high (hill) is present (Figure 3B) and the limbs dip away (figure 3C) from the axis of the fold. Eventually the hill will erode, exposing the older layers in the center of the anticline. The map symbol for an anticline is a line that runs down the axis of the fold with arrows pointing away from each other (Figure 3C). 2

Figure 3: A. Three horizontal units with 1 being the oldest and 3 being the youngest. The tree is growing on the Earth’s surface. B. Anticline forms under compression. Notice the topographic high or hill that forms. C. Over time the topographic high will erode to a flat surface exposing the older unit 2 in the center of the anticline. Strike and dip symbols with opposing dips indicate the limbs of the fold dip away from each other. Along the axis of the fold is the map symbol for an anticline. Notice the tree in C is gone and a new younger tree is growing. Synclines : These form along with anticlines under compression (Figure 4B). When a syncline forms a topographic low or basin is present and the limbs of the fold dip towards each other (figure 4C). As time progresses the high part of the limbs will erode, exposing older units on the outside and younger units in the center of the fold (Figure 4C). The map symbol for a syncline is a line down the axis of the fold with two arrows pointing towards each other (Figure 4C). Figure 4: A. Three horizontal units with 1 being the oldest and 3 being the youngest. The tree is growing on the Earth’s surface. B. Syncline forms under compression. Notice the topographic low or basin that forms. C. Over time the topographic high will erode to a flat surface leaving the youngest unit 3 in the center of the syncline. Strike and dip symbols with dips towards the center of the fold indicate the limbs of the fold dip towards each other. Along the axis of th fold is the map symbol for a syncline.. 3

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Below is a fold found along the northwestern coast of Africa near Algeria. Use this screenshot from Google Earth to answer Questions 1 to 4. The fold axis is depicted with the white line, and some strikes and dips of both limbs of the fold are shown. Note that North is up in this image! 1. By examining the dip directions on the strike and dip symbols, which direction is the north limb (upper half) dipping: north, south, east, or west? dipping north 2. By examining the dip directions on the strike and dip symbols, which direction is the south limb (lower half) dipping: north, south, east, or west? dipping south 3. Based on your answers to Questions 1 and 2 what type of fold is this, syncline or anticline? it is an anticline fold 4. Would the limbs on this fold form a topographic high or a topographic low? topographic high because since it is anticline it forms a little hill 4

A fold found in the central portion of Iran is shown to the right. Use this screenshot from Google Earth to answer Questions 5 to 8. The fold axis is depicted by the black line, and some strikes and dips on both limbs of the fold are included. Note that North is up in this image! 5. By examining the dip directions on the strike and dip symbols, which direction is the west limb (left half) dipping: north, south, east, or west? left half is dipping west 6. By examining the dip directions on the strike and dip symbols, which direction is the east limb (right half) dipping: north, south, east, or west? Refer to Figures 3C and 4C for examples. right half is dipping east 7. Based on your answers to Questions 1 and 2, what type of fold is this, a syncline or anticline? this is a syncline 8. Would the limbs on this fold form a topographic high or a topographic low? topographic low As you may have imagined, anticlines and synclines often form together in what we call a fold system, where you have several folds, anticlines, and synclines together. These fold systems can occur at very small microscopic scales, at outcrop or hillside scale, and at regional scales. Figure 5 on the next page is a screenshot from Google Earth of a series of folds along the northwestern coast of Africa near Algeria. It’s important to mention that these folds are plunging (i.e., it dips into the Earth along its axis), but this fact has no bearing on how you answer the questions below. Fold symbols have been drawn along the axis of several folds along with several strike and dip symbols. Use Figure 5 to answer Questions 9 - 12. 5

Figure 5. Plunging fold system located along the northwestern coast of Africa near Algeria. Fold axes are outlined and labeled with map symbols, as are strike and dip symbols. Line DE is for Question 12. 9. How many anticlines are in Figure 5? 4 anticlines 10. How many synclines are in Figure 5? 3 synclines 11. Based on the fold pattern you observe in Figure 5 what type of fold would you hypothesize to be located to the north of fold G? And similarly, to the south of fold A? syncline for both fold G and A 12. Line D-E is drawn perpendicular (at a right angle) to the folds present in Figure 5. Starting at D and moving towards E, choose the pattern below that matches the fold sequence:. A. syncline, anticline, syncline, anticline, syncline, anticline, syncline B. anticline, anticline, syncline, syncline, anticline, anticline, syncline C. anticline, syncline, anticline, syncline, anticline, syncline, anticline 6

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D. syncline, syncline, anticline, anticline, syncline, syncline, anticline Faults: Dip slip and strike slip Dip slip faults are faults with up/down motion. For this lab we are going to only discuss two types, normal and reverse, but there are also thrust faults. We first learned about normal and reverse faults in Lab 7 on Geologic time. Normal faults occur under extension like that at a divergent boundary plate boundary and the hanging wall moves down relative to the footwall (Figure 6A). Reverse faults occur under compression like that along convergent and some transform plate boundaries. The hanging wall moves up relative to the footwall (Figure 6B). In Figure 6A and B, the bolded line is the fault plane the block below the fault plane is the footwall block and the block above the plane is the hanging wall block. Figure 6: A. 3D block diagram of a normal fault. The fault is the bold line. The hanging wall block (left side) moves down relative to the footwall block (right side) under compression. B. 3D block diagram of a reverse fault. Again, the bold line is the fault. Here the hanging wall block (left side) moves up relative to the footwall block (right side) under compression. 7

Figure 7: Cross sectional view of a fault in Utah cutting through sandstone units. The fault can be seen in the image as it offsets the sedimentary layers. 13. Examine the outcrop in Figure 7 above and locate the fault. If you drew a line along the faulty surface, which two letters would this line most likely intersect? D and I 14. In Figure 7, which side of the fault is the hanging wall block: Left or right? the left side 15. Did the hanging wall block in Figure 7 move up or down relative to the foot wall? it moves up relative to foot wall 16. What type of fault is depicted in Figure 7? It is a reverse fault 17. Based on your answer to Question 15, did this fault form under compression or extension? compression 8

Figure 8: Cross sectional view of a fault cutting sandstone units. The fault can be seen in the image as it offsets the sedimentary layers. 17. Examine the outcrop in Figure 8 above and locate the fault. If you drew a line along the fault’s surface, which two letters would this line most likely intersect? A and G 18. Which side of the image in Figure 8 is the hanging wall block? The right side 19. In Figure 8, does the hanging wall block move up or down relative to the footwall block? It moves down 20. Under what conditions would a fault, like the one depicted in Figure 8 occur? Extension or compression? caused by extension 21. What type of fault is shown in Figure 8? Normal Fault 9

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Strike slip faults are faults with side to side motion. Because the motion of strike slip faults is side to side, cross sectional view is not helpful to understand the direction of motion (unlike dip slip faults). For this purpose of this lab we will rely on map view to categorize strike slip faults as either left lateral or right lateral. We also need some kind of feature, like a road, or stream, or fence to be offset (Figure 9). As depicted in figure 9A, if a stream has been offset along a right lateral strike slip fault then the stick figure’s right foot would move back relative to their left foot as they have one foot on each side of the fault. If the stick figure were to turn and face in the opposite direction their right foot would still move back relative to their left. Along a left lateral strike slip fault, the stick figures left foot would move back relative to their right with movement along the fault (Figure 9B). Also, notice inFigure 9B that the stream is offset to the left. Figure 9: A. Cartoon block diagram of a right lateral strike slip fault. Notice that the stream is offset to the right as the arrows indicate. B. Cartoon block diagram of a left lateral strike slip fault. If you were to stand over the fault while there was an earthquake your left foot would move back relative to your right foot. 10

Figure 10. Aerial image of a fault in China. Notice how the landscape changes along the fault. 21. Figure 10 is an aerial image of a fault in China. Notice the landscape and how certain features are being offset by a fault. Identify the fault orientation. Which two letters would this line most likely intersect? C and M 22. Based on how the landscape is offset what type of strike slip fault can be found here? How do you know? Use evidence in the picture to justify your answer. Left lateral strike slip fault because the left side goes downwards 11

Figure 11. Aerial image of a fault in Southern California. Notice the pattern of the streams as they cross the fault. 23. Figure 11 is a picture of a fault in Southern California. Using a pen or pencil, draw in the trace of the fault across the picture. It wont let me draw a line but it intersects at points K and E 24. What type of strike slip fault is this based on Figure 11? How do you know? Use evidence in the picture to justify your answer. It is a right lateral strike slip fault because the right side goes downward 25. Which of the three major plate boundaries, convergent, divergent, or transform, commonly show strike slip faults? What is the name of the closest strike slip fault along a major plate boundary and why is that relevant to San Diego? Think back to the plate tectonics lab. transform plate boundaries commonly show strike slip faults. The San Andreas fault and it is relevant to San Diego because it is on the fault and we’re impacted by it the most because it can cause powerful earthquakes. 12

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