Lab 8

This lab will cover the methods geologists use to describe geological structures, including strike and dip measurements, representations of geological structures on maps and how to construct geological cross-sections. 8.1.1 Learning Outcomes After completing this lab, you should be able to: • Demonstrate an understanding of the concepts of strike and dip • Interpret geologic features using block diagrams • Interpret a geologic map • Create a geologic cross-section from a geologic map • Recognize different types of folds and faults 8.1.2 Key Terms • Contact • Strike • Dip • Geological cross-section • Geological map • Monocline • Anticline • Syncline • Dome • Basin • Joints • Faults • Dip-slip faults • Foot wall • Hanging wall • Normal fault • Reverse fault • Thrust fault • Horst and graben • Strike-slip fault 8.2 STRIKE AND DIP To learn many of the concepts associated with structural geology, it is useful to look at block diagrams and block models. Block diagrams are images based on three-dimensional (3-D) block models, which are blocks of wood or paper with geological structures marked on them. Block models and block diagrams assist in visualizing how 3-D geological structures in the real world can be represented in two dimensions on a map or in a geological cross-section. As you examine the block diagrams in the figures in this section, note the different ways that you can view them: from above, or from the sides. If you look at a block from along the side, you are seeing the cross-section view. This is the view of geological structures you also see when you drive through the mountains and the roads have been cut through the rocks, exposing structures in the rock that you wouldn't see otherwise. If you look at the block from directly above, you are looking at the map or plan view (Figure 8.3). As you look at geological maps and drawings and try to figure out about how rocks have changed when they are tilted and/or deformed, it is useful to remember how they were deposited

To measure and describe the geometry of geological layers, 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. This line is called a strike line, and the direction the line points in (either direction, as a line points in two opposite directions) is the strike angle. Dip is the angle between that horizontal plane (such as the top of the block in figure 8.5) and the inclined surface (such as a geological contact between tilted layers) measured perpendicular to the strike line down to the inclined surface. A useful way to think about strike and dip is to look at the roof of a house (Figure 8.6). A house's roof has a ridge along the top, and then sides that slope away from the ridge. The ridge is like a strike line, and the angle that the roof tilts is the dip of the roof. Figure 8.7 illustrates strike and dip for tilted flat sedimentary layers. The line of strike is represented by the water line when a lake intersects with the rock along the shoreline (Figure Figure 8.6 | Strike and dip of a roof. The sloping roof of a building is a useful analogy to illustrate strike and dip. The ridge of the roof defines the strike of the roof. The roof dips away from the ridge with a characteristic angle (the dip angle). The inset in the top right corner of the figure shows the roof viewed from above with the strike and dip symbol superimposed on it (symbol explained in the text below). Source: Joyce M. McBeth (2018) CC BY-SA 4.0. Satellite image from © 2018 Google Earth.

Figure 8.9 | The Law of Vs are characteristic patterns that develop when inclined strata are cross-cut by streams or gullies in the landscape. In each paired set of images, the top image shows a block diagram and the bottom image shows the plan view for the same block diagram. Note that these examples (aside from panels I and J) are for a valley cutting across a flat landscape; some of these patterns would be di ff erent if there was a slope to the topography in addition to the valley. Images A and B in the panel show vertical beds; these do not generate a V shape when cut by a valley on a map, even if the topography is sloping. Image C and D show beds that are inclined and dipping toward the bottom of the page. Images E and F show beds that are inclined and dipping toward the top of the page. Images G and H show beds that are horizontal; in images I and J horizontal beds generate a V pattern when the stream valley is cutting through a slope. In both G/H and I/J, the edge of the bed is always parallel to topographic contours. Source: Lyndsay Hauber & Joyce McBeth (2018) CC BY 4.0, original work.

Figure 8.11 | Illustration of how to construct a cross-section from a geological map. A) geological map, with di ff erent rock units indicated by di ff erent colours, and age information given (see Figure 8.12 for code for rock unit ages). B) A blank sheet of paper is stretched along the bottom of the cross-section, and a mark is made at each geologic contact. Since the oldest bed is in the middle, this indicates that this structure is an anticline (also supported by the strike and dip symbols on the map), so beds have been drawn so that they dip away from each other. C) The paper is aligned with the cross- section diagram and the contact marks are transferred to the cross-section (as indicated by the black dashed lines on each end mark that connect up to the surface expression of the bed). Beneath the surface, contacts have been drawn in using a solid line. The beds above the surface that have since eroded have been drawn in with a dashed line. Note the parts of a cross-section diagram illustrated in C): L.O.T.S. Source: Joyce M. McBeth (2018) CC BY-SA 4.0, after Randa Harris (2015) CC BY-SA 3.0 view source

8.4 GEOLOGIC STRUCTURES CREATED BY PLASTIC AND BRITTLE DEFORMATION 8.4.1 Folds Folds are geologic structures created by plastic deformation of Earth’s crust. To demonstrate 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 upon how your paper moved, you created one of the three main fold types (Figure 8.14). Map Symbol Explanation Strike & Dip Vertical strata Horizontal strata Anticline Syncline Plunging anticline Plunging syncline Strike-slip fault Normal fault (Ball and bar on downthrown block) Inverse fault ( ‘ teeth ’ on upthrown block) Figure 8.13 | Guide to common map symbols. Source: Modified from Randa Harris (2015) CC BY-SA 3.0.

and dip of each of the beds. On Figure 8.17, you can practice this: determine the strike and dip for each location marked by an oval. Check your answers using Figure 8.18. Once rocks are folded and exposed at Earth’s surface, they are subjected to erosion, creating certain patterns. The erosion exposes the interiors of the folds, and parallel bands of dipping strata can be observed along the fold axis. In an anticline, the oldest rocks are exposed along the fold axis, or core of the fold. In a syncline, the youngest rocks exposed at the fold axis, or core of the fold (Figure 8.19, Table 8.1). Table 8.1 So far, we’ve studied folds that contain a horizontal fold axis. These folds are shaped like ripples in water, with the axes of the folds lying in the tops and bottoms of the ripples. Some folds have a fold axis that plunges downwards, and these are called plunging folds. Let’s explore what beds might look like for a plunging fold. Take a piece of paper and create a fold by compressing the paper from either side. Tip the piece of paper along the fold axis so that the axis is no longer horizontal, and instead plunges in one direction. You have now created a plunging fold. Plunging folds create a V-shaped pattern when they intersect a horizontal surface (Figures 8.20, 8.21, and 8.22). 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. Fold Type Direction of dip of beds Age of beds in core Anticline Away from fold axis Oldest Syncline Towards fold axis Youngest Figure 8.18 | This block includes the strike and dip symbols and symbols for the anticline and syncline. Note that on the anticline, the beds dip AWAY from the fold axis, and the anticline symbol is drawn along the fold axis, with arrows pointing away from each other, indicating the dip direction of the limbs of the fold. In the syncline, the beds dip TOWARDS each other. Hence, for the syncline symbol, the arrows point inwards. Source: Randa Harris (2015) CC BY-SA 3.0. view source .

Domes and basins are somewhat similar to anticlines and synclines; they are basically the circular (or elliptical) equivalent of these folds. A dome is an upwarping of Earth's crust, which is similar to an anticline in terms of the age relationships of the rocks, and a basin is an area where the rocks have been warped downwards towards the center, with age relationships being similar to a syncline (Figure 8.23). The key to identifying these structures is similar to identifying 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. Here are some helpful hints to remember when constructing a cross-section for an area that includes folded strata: 1. Anticlines – these folds have the oldest beds in the middle, with beds dipping away from the fold axis. Plunging anticlines plunge towards the closed end of the V. 2. Synclines – these folds have the youngest beds in the middle, with beds dipping towards the fold axis. Plunging synclines plunge towards the open end of the V. 8.4.2 Faults As rocks undergo brittle deformation, they may fracture. If no appreciable lateral displacement has occurred along fractures, they are called joints . If lateral displacement occurs, these fractures are referred to as faults . In dip-slip faults , the 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 8.24). The type of fault is determined by the relative direction that the fault blocks have moved. Figure 8.23 | A dome (left) has older rocks in the center, with rocks dipping away from this point, while a basin (right) has rocks dipping inwards and the youngest rocks in the center. Source: Randa Harris (2015) CC BY-SA 3.0. view source.

Fault block movement is described based on the relative movement of the hanging wall , the block located above the fault plane, and the foot wall , the block located beneath the fault plane. The term hanging wall comes from the idea that if a miner was climbing along the fault plane, they would be able to hang their lantern above their head from the hanging wall. For beginners, it is often useful to draw a stick figure straight up and down across a cross-section of the fault plane to help identify which wall is the hanging wall. The head of the stick figure will be on the hanging wall and the feet of the stick figure will be on the foot wall (Figure 8.25). Figure 8.25 | In this image, the head of the stick figure is on the hanging wall (in mauve) and the feet of the stick figure are on the foot wall (in blue). Source: Joyce M. McBeth (2018) CC BY-SA 4.0, after Randa Harris (2015) CC BY-SA 3.0. view source. Figure 8.24 | Two fault blocks. The fault is the break in the block that separates the two fault blocks. Source: Randa Harris (2015) CC BY- SA 3.0. . view source.

When extensional forces are applied to the fault blocks (e.g., in tectonic environments where tectonic plates are pulling apart, such as along the Mid- Atlantic Ridge), the hanging wall block will move down with respect to the foot wall block. This creates a normal fault . An easy way to remember that the hanging wall drops in a normal fault is to use the mnemonic “It’s normal to fall down”. As this happens, the crust is lengthened (stretched apart) and thinned (Figure 8.26). When compressional forces are applied to the fault blocks (such as in a convergent plate boundary tectonic setting), the hanging wall block will move up relative to the foot wall block, creating a reverse fault . This causes the crust to shorten laterally but thicken vertically (e.g., Figure 8.27). A special type of reverse fault is a thrust fault . A thrust fault is a low angle reverse Figure 8.27 | The hanging wall block, at the top, has moved up relative to the foot wall block, at the bottom, resulting in a reverse fault. Note how much closer the cross symbols have become as compared to figure 8.24, evidence for the shortening of the block, due to compression. Also note that the crust has thickened in the region of the fault. Source: Randa Harris (2015) CC BY-SA 3.0. view source. Figure 8.26 | The hanging wall block, on the right, has moved down relative to the foot wall block, on the left, resulting in a normal fault. Notice how close together the cross symbols were in Figure 8.24, compared to this figure, which is evidence for the lengthening of the crust, due to extension. Also note that the crust has thinned in the region of the fault. Source: Randa Harris (2015) CC BY-SA 3.0. view source

fault (the dip angle is less than 30). Table 8.2 summarizes the characteristics of normal and reverse faults. Table 8.2 * hanging wall block movement relative to foot wall block Tensional forces acting over a region can produce normal faults that result in landforms known as horst and graben structures. In horst and graben topography, the graben is the crustal block that drops down relative to the crust around it. The graben is surrounded by two horsts; these are relatively uplifted crustal blocks (Figure 8.28). This terrain is typical of the Basin and Range province in the western United States. Fault type Type of force Direction* Length of block Crustal thickening or thinning Normal Extensional Down Lengthened thinning Reverse Compressional Up Shortened thickening Figure 8.28 | This figure depicts an area that has been stretched by tensional forces, resulting in numerous normal faults and horst and graben landforms. Source: Wikimedia User “Gregors” (2011) modified from USGS, Public Domain. View source .

In dip-slip faults (normal and reverse faults), the fault movement has occurred parallel to the fault’s dip, and the movement is characterized by both a vertical and horizontal change in position of the hanging wall relative to the foot wall. In a strike-slip fault , the movement is only horizontal along the fault plane in the direction of strike (hence the name), the fault blocks do not move vertically relative to one another. Also, faults that behave as purely strike-slip faults are usually vertical in their orientation. The blocks on opposite sides of a strike-slip fault slide past each other, and the movement is driven by shear forces acting on the fault blocks on either side of the fault. The classic example of a strike-slip fault is the San Andreas Fault in California, USA (Figure 8.29). Strike-slip faults can be furthered classified as right-lateral or left-lateral strike-slip faults. To determine whether a fault is left- or right-lateral, use the following test. Imagine an observer standing on one side of the fault looking across at the opposite fault block. If the fault block on the opposite side of the fault appears to have moved right relative to the observer, it is right-lateral; if it appears to have moved left, it is left-lateral. Figure 8.29 | A block diagram of the San Andreas Fault, a right-lateral strike-slip fault. Note that the positions of the blocks do not represent the current position of the crust on each side of the fault. Source: Randa Harris (2015) CC BY-SA 3.0. view source.

Figure 8.30 provides examples of all three fault types for your revie Figure 8.30 | The three types of faults discussed in this lab: A) strike-slip fault, B) normal fault, and C) reverse fault. Source: Wikimedia User “Karta24” (2008) after USGS, Public Domain. View Source