STRUCTURAL FEATURES:

Folds, Faults and Geologic Maps

LEARNING OBJECTIVES

• Describe the types of stress that exist within the Earth’s crust
• Explain how rocks respond to stress
• Connect stress and strain to plate tectonic boundaries
• Understand the types of folds and faults
• Identify the features present on Geologic maps
• Measure and plot Strike and Dip on a geologic map
• Construct the location of layers in folded and faulted strata

SUPPLIES 

• Colored pencils, card stock, block models (provided), scissors, tape, protractor, silly putty, rubber bands, popsicle sticks, wooden blocks
• Geologic Map of Carlisle, PA
• Geologic Map of the eastern portion of PA with cross section and legend
• Geologic time scale

INTRODUCTION

The Earth is an active planet shaped by dynamic internal forces.  The internal heat convecting within the mantle breaks and moves the thin surface plates.  These forces push surface plates upward forming mountains (Himalayans mountains), downward producing deep ocean trenches (Marianna’s trench) and spreading to form new ocean floor (mid-Atlantic ridge).  Structural Geology is the study of how the Earth’s plates move and are deformed, often in a 3-dimensional manner.  Structural geologists help us understand the formation of folds in mountains and faults that produce earthquakes.  Geologic maps show the ages and orientation of surface and near surface rock layers, as well as folds, joints, faults, streams and weathered products.  Understanding these processes and their impact on Earth materials is an essential skill for engineers, energy production, seismologist, hydrologist and many more.

Rocks under dynamic forces, respond by stretching, folding and breaking. This deformation can produce hazardous, dramatic and beautiful scenery (figure 1).  Observing and understanding these geological structures helps us determine the movement of plates in the past and future.  

Figure 1. A fold in Sedimentary rocks: Limestone and Chert in Crete, Greece. Intense plate tectonic forces folded this rock unit. Image: CC BY-SA 4.0 S. Earle opentextbc.ca/physicalgeology2ed/

Crustal Deformation–Stress

Rocks are subject to stress mostly related to plate tectonic forces acting on them.   When stress is applied rocks change and deform.  Stress associated with simple burial of rocks is relatively equal in all directions. This is referred to as lithostatic stress and generally acts to compact the rocks into denser forms; for example, sedimentary shale being compressed into low grade metamorphic slate (figure 2).

Figure 2. When stresses are equal in all directions such as from simple burial of sediments and mostly sedimentary rocks such as shales and mudstones the forces are called lithostatic stress. Note equal size of stress arrows. Image: https://opentextbc.ca/geology/wp-content/uploads/sites/110/2015/07/image004.png

When stress is greater in one direction over another such as from mountain building forces, the rocks can deform in different ways depending on the strongest direction. Where plates are diverging the rocks are being thinned and pulled apart, this produces tensional stress.  Where plates converge the rocks are shortened, compacted and squeezed, this produces compressive stress.  At transform boundaries the plates move sideways in opposite directions producing shearing stress (figure 3). Stress is usually designated by vector arrows indicating the direction and strength. Longer vector arrows indicate greater force.

 Figure 3. Three stress types acting on rocks. Tensional, is a pulling stress, compressional stress results from converging forces and shear stress is in opposite directions. Image USGS.gov. https://opengeology.org/textbook/9-crustal-deformation-and-earthquakes/. https://earthquake.usgs.gov/learn/glossary/images/stress_types.gif

Tectonic Plate Boundaries

Forces and their corresponding stress type occur at specific plate boundaries. Look at the plate boundaries map and connect the force direction and plate boundary with the stress type (figure 4).

CIRCLE LOCATIONS OF THE THREE STRESS TYPES ON THE MAP BELOW.

Figure 4. Plate tectonics map. Image Wikimedia Commons Eric Gaba (Sting - fr:Sting), CC BY-SA 2.5 <https://creativecommons.org/licenses/by-sa/2.5>, via Wikimedia Commons

Strain

Strain is the corresponding response of the rock to applied stress. As rocks first respond to stress they initially behave elastically.  An elastic strain response is not permanent, like pulling a rubber band then letting go.  When the force (stress) is removed the rock returns to its original shape. If the forces continue over time the original rock may move past an elastic response and plastically deform.  Plastic deformation is permanent, like pulling on warm taffy or gum.  When rocks bend, they form folds. If conditions exceed the overall strength of the rock the strain response is brittle, and the rock breaks or fails. A brittle response produces faults (figure 5).

Figure 5. Deformation leading to plastic then brittle response. a) undeformed state, b) accumulation of elastic strain, and c) brittle failure and release of elastic strain.  The black arrows represent the direction of the stress forces. Image: Opengeology.org CC BY S.A.

Let’s Practice: Elastic, Plastic and Brittle Responses in Materials

Your Instructor has provided you with rubber bands, Silly Putty and wooden sticks.  Manipulate each of the three materials to experience the three strain responses.  Use figure 6 to relate the materials. 

Vary the rate and direction of the forces. Note how they respond.

Can you make the Silly putty and the stick behave elastically, plastically and then brittlely?

How did you vary your conditions - stress?

Figure 6. Different materials deform differently when stress is applied.  Material A has relatively little deformation under large amounts of stress before fracture.  B elastically deforms then brittlely fails. C significantly deforms plastically then fails brittlely.  Image opentextbc.ca CC BY S.A. S. Earle

PLASTIC FOLD RESPONSE

When a rock is compressed and behaves plastically it can produce folds.  This typically occurs when the rocks are warm, deeply buried, weak, and the forces are slow and spread over a long period of time.  An upward fold is called an Anticline, and a downward fold is a Syncline. The plane that splits the fold into two halves is the axial plane.  Folds can be symmetrical, even on both sides, or asymmetrical having one limb more steeply dipping than the other (figures 7 & 8).

Figure 7. Examples of Symmetrical, Asymmetrical and Overturned folds. Image CC BY S.A. 4.0 international license S. Earle. Opentextbc.ca/physicalgeology2ed/

Figure 8.  Folded Limestone one meter across showing both anticlinal and synclinal folds. Image CC BY S.A. 4.0 International license S. Earle. Opentextbc.ca/physicalgeology2ed/

BRITTLE FAULT RESPONSE

A rock that behaves in a brittle manner (breaks) produces faults (figure 9).  The fault types are unique to the specific directional stress. Fracturing or failure usually occurs when rocks are cold, or near the surface, and the forces applied are strong and occur over a shorter period of time. Fault type is specifically unique to the direction or type of forces that act on the rocks.  

Figure 9. Faulting white dashed line with arrow.  This is a right-lateral fault movement. Notice red dashed lines displacement and offset of pink intrusion. Image CC BY-SA. S Earle. Opentextbc.ca/physicalgeology2ed/

Rocks experiencing Compressional stresses at convergent plate boundaries can produce Reverse faults.  Rocks experiencing Extensional, pulling stress at Divergent boundaries can form Normal faults. Rocks that move past one another at Transform boundaries can produce Strike slip faults (figure 10).

Figure 10. The three main fault types with motion. Image CC BY S.A. 4.0 International license S. Earle. Opentextbc.ca/physicalgeology2ed/

Look at the images and determine if the response is brittle or plastic or both. Draw in the stress arrows.

Figure 11. Determine if the above images show Plastic or Brittle responses. Image CC BY-SA S. Earle. Opentextbc.ca/physicalgeology2ed/

Use the wooden blocks to create normal, reverse, and strike-slip faults.

MAKE DRAWINGS OF THE THREE FAULT TYPES WITH DIRECTIONAL ARROWS.

Geologic time

Rock type              

WHAT TYPE OF FAULT IS THIS?? Draw in the direction of stress.

Figure 12. Ketobe Knob, in the San Rafael Swell, Utah. Image CC BY NC S.A. 4.0 international R. Schott https://www.flickr.com/photos/rschott/814080386/

TABLE 1. FILL-IN. connections between Strain – Plate Boundary Type and Fold or Fault

GEOLOGIC MAP FEATURES: Rock Type, Geologic Age, Structures

Geologic maps are a visual expression of the rock layers on the surface and below the surface. They are an essential tool for understanding the changes that have occurred to rocks over the immense span of geologic time – 4.6 billion years. The first geologic map was created by William Smith during the early 19th century (Figure 13). Geologic maps differ from topographic maps due to the addition of rock types, rock ages, and structural features.  Both topographic and Geologic maps attempt to represent a two-dimensional world in three-dimensions. One of the most important aspects of a geologic map is the ability to determine rock types and if a rock unit is horizontal, folded or faulted.  You can even determine the relative ages and chronological order of the rock sequences present.  Geologic maps use different colors and symbols to represent the different units of rock.  The term outcrop is used to represent the expression of rocks at the surface. Most geologic maps also draw a vertical line across the map to look at a cross section or cut away view at depth. This side view shows the arrangement of rock layers below the surface as well as the topography of the land surface in profile. This profile view is essential when trying to determine how rocks are folded or faulted.

Figure 13. The first Geologic map created by William ‘Strata’ Smith of Great Britain in 1815. This map was used to connect similar rock types over large regions. For more information a book by Simon Winchester called “The map that changed the World” chronicles the life of William Smith and the development of this map.  Image CC BY S.A. Wiki Commons public domain. Scan by the Library Foundation, Buffalo and Erie County Public Library. Image now at http://www.livescience.com/449-map-changed-world.html https://en.wikipedia.org/wiki/William_Smith_(geologist)#/media/File:Geological_map_Britain_William_Smith_1815.jpg

ROCK TYPES – Geologic Age

Looking carefully at a geologic map you see many colors that are used to represent different rock types and their geologic age.  Using the legend at the bottom or the side of your map you can see the names of the rock units or formations, with specific details about the rocks that appear on the surface. For ease in reading a geologic map the rock unit/formation is given a symbol that incorporates the geologic time period (age), name and an abbreviation of the rock type's name.  

Let’s Practice: Geologic Map of Pennsylvania

Using the Eastern half of the Geologic Map of Pennsylvania, provided by your instructor we will practice using the legend to find specific rocks, determining rocks of a certain age and interpret the rock type age symbols. Together let’s find the Silurian age Wills Creek Formation rocks on the legend.  Rocks are arranged by age. So, find the geologic time span called the Silurian. The small blue square has the designation “Sw” which represent the uppercase Silurian “S” for geologic age and the lower-case “w” for Wills Creek – rock type.  Rock designations typically use an uppercase first letter to identify the geologic time/age of the rock, and a lowercase letter or letters as an abbreviation of the rocks name or type.  The combination of color and pattern with the upper-case geologic age combined with the rock unit name makes finding and identifying structures on the map easier.  Let’s look on the map and find where that rock unit is exposed at the surface. You may need to refer to a geologic time scale if you are not fully comfortable with the units of time (figure 14).  Try a few more.

Use the legend to find specific rocks.

What is the youngest unit represented on this map section?

What types of rocks are these?

What is the oldest rock unit and type on this map?

Find the following on the map.

What are the Mississippian aged rock units?

Find a few on the map.

What rock unit is labeled with the symbol ‘Os’?

What age is the Tuscarora formation? Find it on the map.

Figure 14. The Geologic Time Scale. Image NPS Geologic Resources Inventory 2018. Public Domain https://www.nps.gov/subjects/geology/time-scale.htm

Symbols: Used to represent the orientation of rock layers

Horizontal layers:

Geologists take careful measure to record geological structures because they are critically important to understanding the geologic history of the region.  As you have been looking closely at geologic maps you may have noticed a variety of symbols along with the various colors and geologic time references.  One feature that is measured is the attitude or orientation of the layers (bedding).  If rock units are horizontal only one unit will appear on the surface.  This will appear as the same color rock spread out over a large area (figure 15).  If the surface is not flat due to erosion from a stream, the layers below will appear parallel to a stream features and repeating across from one another. Rivers eroding through horizontal layers form a very particular dendritic (tree branching) drainage and erosion pattern (figure 16).  Notice on the geologic summary map in figure 16 the large Pennsylvanian age pale blue rock type exposed on the Pennsylvania Geologic summary map, and the dendritic (branching tree-like) drainage pattern clearly exposed to the north.  This indicates that the rock units below are horizontal. The middle-swirled pattern indicates folded rock units, while the southeastern sections unique pattern tells us that the rocks where split and diverged in an ancient rift. Looking at the complex cross-section of a full geologic map of Pennsylvania you can see that the rocks to the west are horizontal while the rocks in the middle portion of the state are folded (figure 17).

Figure 15. A block model (left) showing three horizontal layers.  Only the layer on the top is exposed unless an erosional event such a stream cut through it. Right image shows the geologic map of Pennsylvania western portion with horizontal layers exposed by stream weathering.  Image:

https://commons.wvc.edu/rdawes/courseinfo/Assignments/blk1234.gif. Image right. https://www.dcnr.pa.gov/Education/GeologyEducation/Pages/default.aspx

Figure 16. Summary geologic map of Pennsylvania. DCNR Public Domain. Image: https://www.dcnr.pa.gov/Education/GeologyEducation/Pages/default.aspx

Figure 17. Cross section of the state of Penssylvania.  Notice the horizontal flat layers to the west and the folded and faulted layers in the middle of the state. Image: http://elibrary.dcnr.pa.gov/GetDocument?docId=1751215&DocName=Map1_Geo_Pa

Inclined Strata

If the layers are no longer horizontal, we can infer that they have been tilted, folded or faulted by tectonic forces.  The orientation, tilt or fold can be expressed with two values: Strike and Dip (figures 18 & 19).  The strike represents the orientation of a horizontal line on the surface.  The second value represents the angle at which the surface dips from the horizontal.  It is always perpendicular to the strike. The dip direction of a plane is shown by the short line drawn perpendicular to the strike and pointing downhill. The number indicates the angle at which the layers dip into the earth.  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 green bed and the red bed (figure 19).

Figure 18. 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.  
Image: Joyce M. McBeth (2018) CC BY-SA 4.0. Satellite image from © 2018 Google Earth.

Figure 19. A depiction of the strike and dip of some tilted sedimentary beds. The dipping beds are shown partially covered with water so that you can visualize a horizontal line on the rock surface. The notation for expressing strike and dip on a map is also shown. Image CC BY-SA Karla Panchuk (2018) modified by S. Earle. Opentextbc.ca/physicalgeology2ed/. https://openpress.usask.ca/geolmanual/chapter/overview-of-strike-dip-and-structural-cross-sections/

Map symbols can be used for more than telling tilted, folded, vertical and horizontal strata (figure 20). Folds can tilt or plunge into the earth. Fault movement and type is also expressed.  Recall that faults can be Normal (tensional stress) and Reverse (compressional stress).

Figure 20. Common map symbols used on geologic maps.  Image CC BY-SA 3.0 R Harris oer.galileo.usg.edu/geo-textbooks/1

On the box model try to draw in the inclined layers. Use a protractor to include the strike and dip symbols.

Image credit: openpress.usask.ca.

Folded Strata

Folds are geologic structures created by plastic deformation of the Earth’s crust.  Anticlinal folds form when compressional stress directions dominate and produce layers that are inclined downward away from the center fold (figure 21). This produces a fold that resembles the letter ‘A’ in a cross-section or side view.  Anticlines show dip symbols away from the axis along with older rock units in the center with repeating patterns of younger rocks on either side (figure 22). They can be a single fold to form a monocline.  A synclinal fold is a concave upward fold in which the layers are inclined upward. This resembles a sink. Younger rock layers are on the inside with repeating older layers on either side. (figures 21 & 23).

COLOR IN THE LAYERS ON THE BLOCK MODELS BELOW ADD SYMBOLS

Figure 21. The left box is a drawing of an anticlinal fold with the older lower layers exposed at the center axis of the fold, in the middle. The right box shows a downward synclinal fold with repeating layers exposed around the younger central rock layer. Image:  https://commons.wvc.edu/rdawes/courseinfo/Assignments/blk1234.gif

Figure 22. Anticlinal fold. Notice that older rock units are exposed in the center with repeating exposure of younger rocks in parallel bands. Image Public Domain original by P.S. Foresman. https://no.wikipedia.org/wiki/Fil:Anticline_(PSF).png

Figure 23. Syncline along interstate 8 and US 40 in Maryland Sideling Hill. Image: Acroterion, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons

FOLDS ON A GEOLOGIC MAP

Folds in cross section look much different than on a surface map view. In a map view the folds look like repeating units of rock type. This is seen in repeating colors on either side of a single central color.  To determine the type of fold you need to look at the strike and dip symbol directions and confirm that the rock ages match.  Synclinal folds show dip symbols toward the center axial line and expose younger rock units in the center with repeating older layers on either side. Anticlinal folds have strike and dip symbols pointing away from the central point (axis) and have younger rock units in the center and repeating older units on either side (figure 24).

Figure 24. Box model showing anticline and synclinal folds with repeating color rock units on either side of the axis. Image: Phil Stoffer’s geology café. public domain  http://www.geologycafe.com/images/folds.jpg

Let’s practice on the Carlisle PA Geologic Map

Look on the geologic map for a symbol that may look like little “T”’s with numbers (figures 19 & 20).  The long top part of the T parallels the strike, which represents the trend of that plane of rocks, the shorter perpendicular dip section contains a number which equals the degrees of dip angle, as the layer dips into the earth.

USE THE LEGEND: Find Triassic – Newark Group Gettysburg Shale. Find where this unit is exposed on the map (lower right).

What is the symbol for this rock type? 

Find a few strike and dip symbols. How much do they dip?

Use the cross-section at the bottom of the map to compare the dip angle.

Use the cross section to identify and find an Anticlinal fold.  

What rock units are shown?

Confirm that this is an Anticlinal fold using your knowledge of strike and dip symbols and age relationships.

What are the oldest rocks exposed on this map?

Figure 25.  Carlisle, PA geologic map. Image: https://ngmdb.usgs.gov/Prodesc/proddesc_736.htm

Faulted Strata

Recall that rocks that behave in a brittle manner break, producing faults. Rocks experiencing Compressional stresses at convergent plate boundaries can produce Reverse or thrust faults.  Rocks experiencing Extensional, pulling stress at Divergent boundaries can form Normal faults. Rocks that move past one another at Transform boundaries can produce Strike slip faults (figure 26). Faults on a geologic map often show sharp divisions between rocks of very different ages.  Reverse and thrust faults can push older lower rock units up and on top of younger units.  Strike and dip symbols are used to show direction and angle of the dipping fault plane. 

 Figure 26. Faulted Strata: Normal (top), Reverse and Strike-slip (bottom) faults represented in block model form.  Follow the black arrows to see the displacement of the layers. Image USGS.gov public domain/parks – TL Thornberry-Ehrlich Colorado SU https://www.nps.gov/articles/faults-and-fractures.htm

Figure 27. Try to draw in and color the layers.  Make sure to represent the fault and the dip angle of the fault. Image: CC BY SA openpress.usask.ca

Let’s Practice on the Carlisle, PA Map Again

1. Identify the movement of the Triassic Border fault (lower portion). Determine if this is a Normal or Reverse fault.

2. The Cambrian Antietam Quartzite (left on map) has an unusual orientation.  Draw the symbol and explain.

3. What is the age of this rock unit?

4. What is the age range of rocks represented on this map?

5. Use the legend with the fault and strike and dip symbols to find examples of each type.

On the diagram below, use a protractor to measure the dip of the beds and draw in the strike and dip symbols. Note geologic ages of rocks. Observe the rock’s orientation before erosion.

Image CC BY-SA 3.0 R Harris. OER.galileo.usg.edu/geo-textbooks/1

Box models: Cut out and fold the box models provided by your instructor. Complete the missing sides. Add strike and dip symbols where requested.

Image 1: CC BY SA 3.0 openpress.usask.ca Joyce McBeth after Randa Harris. Images 2, 4: Timothy Davis. Image 3: Karen Tefend oer.galileo.usg.edu/geo-textbooks/1.