8.1 Introduction to Deformation

Structural geology describes the arrangement of rocks and the structures that have been deformed from their original state. Rocks that remain in their initial orientation and state are called undeformed, whereas rocks that have changed orientation, location, and shape are called deformed.

The deformation of rock layers can make interpreting the geologic history of those layers more difficult. However, it can also tell us about the tectonic processes that may have occurred in the region. For these reasons, geologists must understand how deformation of rock layers occurs and how to deconstruct the often confusing structures that result from deformation. Then, the original position of the rock layers can be determined. Remember that these tectonic processes can occur on vastly different spatial-temporal scales and can vary based on the type and thickness of rocks that are being deformed, so no two faults or folds will be exactly alike.

To identify different types of deformation, you first need to be able to recognize bedding and foliation planes (layers). Look for distinct changes in colour, breaks between layers or foliation, changes in lithology, etc. When you look at an outcrop, first try to identify the bedding or foliation. This may be horizontal or tilted. Folded, tilted, faulted bedding and foliation planes are useful markers that allow you to investigate deformation structures in rocks. They can be continuous and followed for long distances or shorter if the depositional environment has local changes.

You can identify deformational features by tracing the bedding and foliation planes. Only two dimensions are often visible, and you need to infer the third dimension. So, look at the outcrop and note if the bedding/foliation planes are straight or curved. If they are curved, the rocks are deformed by bending or flowing (ductile) and are folded. If they are straight, then you are looking at brittle deformation. What controls brittle (breaking) versus ductile (bending) is the complex interaction between temperature and rock types. Remember how deeply you observed earthquakes occurring? Earthquakes are brittle deformations and occur at shallow depths and low temperatures. As the temperature increases, the rocks switch to ductile deformation, and there are no more earthquakes.

Figure 8.1 shows how rock type controls whether the rocks bend or break. The shales bend, whereas the sandstone breaks. Are the bedding planes discontinuous across a break in the rocks? If they are discontinuous, this typically indicates a brittle fault. In this case, follow a few bedding planes, and if they are continuous, you are seeing folds and ductile deformation.

8.2 Ductile Deformation

Folds form when layers or beds are bent or folded due to convergent plate tectonics (compression). They can be rounded or angular. The plane that marks the center of the fold is called the axial plane. The line that marks where the axial plane intersects the surface of Earth is called the hinge line. The areas on either side of the curved hinge zone stick out like arms or legs and are appropriately called limbs. There are often several folds connected (see Figure 8.2). If the bedding planes tilt down from the axial plane, this is an anticline (easy to remember since it sort of looks like an A). However, if the bedding planes tilt toward the axial plane, this is a syncline (Figure 8.2).

Folds vary in size and shape. Some cover hundreds of kilometres in width, and others measure a few centimetres or less. Folds are generally classified according to the orientation of their axial plane, which can be vertical, horizontal, or inclined at any intermediate angle. In addition, the angle between the axial plane and the fold limbs is important for classifying folds. The angle of the axial plane relative to horizontal is called plunge. So, in addition to the folds shown in 8.2, there are a variety of other folds shown in Figure 8.3. These include ductile folds:

• Symmetrical – vertical axial plane with the same dip for both fold limbs
• Asymmetrical – axial plane is tilted with a shallower dip on the left fold limb compared to the right fold limb
• Overturned – axial plane is tilted, and the left limb has a shallow dip, whereas the right limb with the stratigraphy overturned (upside down)
• Recumbent – horizontal axial plane with the same dip for both fold limbs

and brittle folds:

• Box – a rectangular fold in which the fold limbs are right angles. This fold has three limbs
• Fault-bend – tilted axial plane and angular fold limbs. These form over thrust faults (see next exercise)
• Chevron – tilted to vertical axial planes with straight limbs that abruptly bend at one point at their hinge zone.

Exercise 8.2 – Identifying Folds

Folds can be both brittle (bent) and ductile (rounded) features. Figure 8.4 shows six photos of folds. These start as large mountain-scale folds (A and B) and get smaller and more complicated (C to F). There are geological rock hammers for scale in images C and D.

1. For each photo, identify the type of fold shown and the direction of stress needed to create it. There is more than one fold in E and F.
2. Sketch the axial plane(s).
3. Indicate the age of the sedimentary units with “o” for oldest and “y” for youngest.
4. Describe the characteristics of the folds by filling out Table 8.2. Some characteristics include the dip of the axial plane, whether the dips of the layers are similar on both sides of the fold, symmetry of fold (dip of each limb relative to the axial plane), type of fold (anticline, syncline, inclined, recumbent, box, fault bend, etc.). Since folds form in convergent tectonic settings, you don’t need to determine the tectonic setting for these features

Table 8.2 – Fold features and identification

Photo #	Brittle or Ductile	Angle of Axial Plane	Axial Plane Curvature	Fold Symmetry	Type of Fold
A
B
C
D
E
F

8.3 Brittle Deformation

Let’s go over faults. First, ask yourself if you are looking at the Earth’s surface or a slice (cliff or outcrop along a road) through the Earth. Once you’ve done that, you will know if you are looking at either vertical (up and down) or horizontal (side to side) motion.

Vertical motion is called dip-slip. If the motion is mostly down, then this is a normal fault. If the motion is up, it is a reverse or thrust fault. The difficulty is deciding if the motion is up or down. To help with this, imagine standing with the fault above your head. Your feet are on a layer that doesn’t move.  Some call this the footwall. The rocks above your head are called the hanging wall, as they might come crashing down on you. Then, do you have to look up or down to find this same layer on the other side of the fault (Figure 8.5)? The difference between a thrust and reverse fault is how steep the fault is. If the fault is shallow, this is a thrust fault. If it is steep, this is a reverse fault. Reverse faults are common in divergent tectonic settings, whereas thrust and reverse faults are common in convergent tectonic settings.

Horizontal motion is called strike-slip or transform; these are common in transform plate boundaries. If you have a vertical section through a strike-slip fault, one side will move toward you and the other away.  From the top view, see if the features moved to the right or left. Imagine you’re on one side of the fault, standing on an object cut in half by the fault. Then, look across the fault and see if the other half of the object has moved to the left or right. It doesn’t matter which side of the fault you are on (see Figure 8.5).

8.4 Structures in Three Dimensions

Thinking about geology in three dimensions (3D) is difficult. If you practice, this is an ability that you can improve with time. If you can make 3D images of structures, you can figure out structures beneath you. Some geologists say that the structure you see at the surface is similar to what is underneath. Click here for a 3D image of a fold.  Is it the same from the top as well as the side?

Since it is difficult to do experiments to deform rocks, geologists investigate deformation using analogs and modelling studies. An easy analog to deformation is modelling clay. Start by making a few flat layers of different colours. Then use your hands as if they are two convergent plates and push on the two sides of your clay layers. This will make a fold, either an anticline or syncline. Other geoscientists use sandbox models to replicate folds and faults (Figure 8.9). If you like programming in Python, you can try out GemPy to make your own 3D geologic models. However, for this class, we’ll use a simpler web-based program to make your own models of geologic structures.

Often, geologists make geologic maps to show deformation features (more on that later). Now that satellites can provide images of the entire Earth’s surface, the first step in making a geologic map is to interpret satellite imagery. If you are going to work in an area covered by forests, another technique, LiDAR (Light Detection And Ranging), can be used to see below the canopy. For an example of folds shown with LiDAR, click here. Recently, geoscientists have turned to UAVs (unmanned aerial vehicles or drones) to get detailed imagery, creating either digital elevation models or hillshade maps of deformation features.

Geologists have borrowed the “Rule of Vs” to describe the geometry of mapped strata in terms of topography. The Rule also predicts the surface location of strata in areas of poor exposure. In the simplest case, where a geologic feature is a vertical plane, the feature is a straight line. In contrast, if the feature is horizontal, the feature follows the contour lines. Keep in mind that the topographical expression of bedding represents erosional remnants of rock layers that once extended above the present surface; a lot of rock has been removed to produce the topography you see today.

8.5 Structures on a Geologic Map

A geologic map is an important tool for many geologists. These maps show how rocks are distributed in an area, their relationships, and any geologic structures. In addition to the basic map information that you will learn about in Chapter 9 (or learned about in Chapter 5), the age and rock types are described in the legend. As you know from the map you made of a volcano, the colour of a unit may indicate its age. In addition, the pattern of the unit can indicate the rock type. There may also be information about faults and folds, indicating fold axes and fault types. Other important geologic map data are the strike and dip of various features.

As in many other labs, one reason for studying folds and faults is that they are important for understanding and finding economic resources. For this reason, many geologists love anticlines. If an anticline has an impermeable layer, such as shale, it can act as a barrier to fluids moving through the Earth. Consequently, fluids will stop at this layer, trapping water, oil, and gas. Also, fluids can carry strategic elements such as gold. The Hill End anticline in Australia was mined and produced over 60 tons of gold!

For the last 35 years, petroleum geologists have been using 3D imaging to understand geologic processes beneath the surface. Figure 8.10 shows two views of the subsurface below Hungary. On the right side of the figure, there are the sedimentary layers, which are relatively flat. These formed in small rift basins. In contrast, in the center of part b, you can see an anticline with several oil wells. So, the anticline formed after rifting. This is not evident from the geology at the surface.

Exercise 8.6 – Exploring Geologic Structures Using a Geologic Map

Use the geologic map titled Geologic Map of Wyndale and Holston Valley Quadrangles, Virginia (Fig. 8.11), to answer this question. Your instructor will provide a physical map in the lab. If you choose to do this online, use this link: Geologic Map of Wyndale and Holston Valley Quadrangles, Virginia (Bartlett and Biggs, 1980).

1. Find the fold axis that runs nearly along Jonesboro Road in the southwest part of the map. The bright orange unit Cco is exposed in the core of this fold. The legend for the map shows rock units from oldest at the bottom of the legend to youngest at the top. Use the age relationships and the relationships of dips on limbs of a fold to identify the type of fold (anticline or syncline, plunging or non-plunging). Check yourself: Does this match the symbol shown on the map?
   

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2. Note the scale on the map. Find the colour and symbol for the Chepultepec Limestone in the legend of the map. Now locate this unit along the northwest limb of the fold you identified in  10(a), along line A-A’. What is the apparent thickness (i.e., outcrop width), in metres, of the Chepultepec Limestone here?
   

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3. Find this same unit on the southeast limb of this fold. What is its apparent thickness, in metres, here?
   

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4. Why do the apparent thicknesses of this unit differ on the two limbs? [Hint: Look at the dips on the fold limbs on the map.]
   

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5. Sketch a geological cross-section of the fold along part of line A-A’ in the space provided on the next page: sketch only the portion that lies between the bend in A-A’ just southeast of the Bristol City – Washington County border and A’.

Draw here (Fig. 8.12). to answer Question 10(e). A(+) on the cross-section refers to the bend along line A-A’. The numbers on the y-axis are elevations above sea level; simplified topography has already been provided for you here. The cross-section could extend as deep as information can reasonably infer; for this question, use the depth indicated by the dashed-dotted line.

Steps to Make a Geological Cross Section: 

1. Locate the line for the cross-section. It is usually as close to perpendicular to strike as practical for the geology of the area.
2. Mark the points along the line from the map on a strip of paper, as you would for a topographic profile, only instead of marking locations of topographic contours, mark the locations of stratigraphic contacts, fault traces, and axial traces.
3. Transfer these points to your profile.
4. Use the dips from the map to determine the direction and amount of dip for the contacts. (Keep in mind: The real world will have some unevenness, and the precision of making field measurements of strike & dip is limited: where dips are variable within an area, make good judgments about average dips for plotting.)
5. Join the contacts into smooth, realistic lines/curves.
6. Label the geologic units with their symbols (and perhaps colours or patterns).
7. Draw appropriate axial traces for folds and fault traces for faults along the cross-section.
8. Check that the cross-section has horizontal and vertical scales.

Additional Information

Exercise Contributions

Lab 8: Why Do Some Rocks Fold And Others Break? was modified by Jessica Kristof and Ricardo L. Silva from the original Chapter 10: Deformation and Structures by Virginia Sisson, Daniel Hauptvogel, and Michael Comas in Hauptvogel et al., (2024) and the original Lab 9: Geologic Maps & Sections in Ferreira and Young (2018). 

References

Bartlett, C.S., Jr., and Biggs, T.H. (1980). Geology of the Abingdon, Wyndale, Holston Valley, and Shady Valley Quadrangles. Virginia: Virginia Division of Mineral Resources

Ferreira, K. and Young, J. (2018). GEOL 1340 The Dynamic Earth Lab Manual. Winnipeg, MB: Department of Geological Sciences, University of Manitoba

Hauptvogel, D., Sisson, V., and Comas, M. (2024). Investigating the Earth: Exercises for Physical Geology. Houston, TX: UH Libraries

Tari, G., D. Arbouille, Z. Schléder, and T. Tóth, 2020, Inversion tectonics: A brief petroleum industry perspective: Solid Earth, v. 11, no. 5, p. 1865–1889, doi:10.5194/se-11-1865-2020