Folds: Bending to Earth's Will

Explore the different types of geologic folds and their geometries.

Benjamin Magnin

How Folds Form

Folds are the result of ductile deformation within the crust. As tectonic stress is applied to horizontal strata, enough stress may build up to eventually cause deformation and buckling of the crust. The best way to imagine this process is by taking a piece of paper and applying an inward force to both edges. As your hands press inward, notice how the paper begins to fold up, down, or both. This same process occurs within the Earth. As compressional stresses act on horizontal strata, rock beds, like the paper, begin to buckle and fold. Now take the same piece of paper and pull outward. Does a similar result occur as when you pressed inward? You will notice that the paper does not fold, and likewise rocks do not typically fold when under extensional/tensional stresses. Therefore, folds are ductile features that primarily result from compression. Next time you visit a location with folded rock layers, you know that the area must have undergone at least one compressive event.

Five Major Fold Types

Anticlines

Anticlines are a common fold type found in the Earth’s crust. When upright, the limbs (flat inclined surfaces) of these folds dip away from the axial surface (the plane intersecting the maximum curvature of the fold) to form a concave-downward structure. More generally, any type of fold that has this geometry is called an antiform. Anticlines, however, are more specific in that rock layers get younger away from the axial surface. An example of an anticline is shown in the map below.

Map 1: This structure is the Sheep Mountain anticline in Wyoming. The blue line down the center of the fold represents the axial trace, or the line that separates the two dipping limbs of the fold. If you look closely, you may be able to see flat irons, exposed bedding surfaces, dipping away from the center of the fold. The smaller set of arrows indicates the direction the beds dip on either side of the axial trace.

Syncline

Like anticlines, synclines are also a common fold type that often forms together with anticlines during compression. When upright, synclines are trough shaped with beds dipping toward the axial surface. Like an antiform, the term synform is used to describe any structure with this general geometry. The more specific term syncline, however, indicates that layers get younger toward the axial surface. It is important to specify antiform from anticline and synform from syncline as folds can be rotated so that overturned synclines become antiforms and overturned anticlines become synforms. Figure 1 shows an example of a syncline in the field.

Figure 1: Picture of the Sideling Hill syncline in West Virginia. Notice how the beds all dip toward the center of the fold. If you were to walk along the roadcut, the youngest beds would be in the center of the fold and the oldest toward the outside edges. Photo by Maxson (1995).  

Monocline

Monoclines are a special type of fold that contain only one limb. As you can see on the picture to the right, strata are horizontal on either side of the fold creating a stair-step geometry. While beds are not necessarily always horizontal on either side, this stair-step geometry is very characteristic of monoclines due to the single inclined limb (Fig. 2). These types of folds can be created by the reactivation of faults or the emplacement of migrating salt structures (Fossen 2016). 

Figure 2: Example of a monocline located along the San Juan River, Utah. Notice that the beds on the right and left of the picture exhibit near horizontal regional dip while the beds in the center are steeply inclined. These central beds represent the single limb of the monocline. Photo by Miller (n.d.).

Dome

In simple terms, a dome is a fold that looks like an upside-down bowl. Beds dip away in all directions from the dome center to create a circular geometry. Similar to an anticline, beds in the center of the dome are oldest and become progressively younger toward the outside edges (part A in the figure). If you were to construct a vertical cross-section in any orientation through the dome, you would always get an anticline!

Map 2: A dome located in western Wyoming. The blue marker indicates the center of the dome where the oldest rocks occur. If you look closely, you can see flat irons dipping away from the blue marker in all directions.  

Basin

A basin is similar to a dome in that it is a circular shaped fold. In this case, however, all beds dip toward the center of the fold with the youngest strata in the center and progressively older beds moving outward (part B in the photo on the right). If you were to draw a vertical cross-section in any orientation through a basin, you would get a syncline. A great example of a basin is the Michigan Basin (Fig. 3).

Figure 3: Geologic map of the Michigan Basin. Notice the circular succession of beds. These beds are all dipping inward toward central Michigan with the youngest units in the center. Photo by Gibson (2014).

Fold Constituents

  1. Fold Limbs: The fold limbs are the two surfaces of a fold, typically inclined, that are separated by a zone of maximum curvature. This and other features are illustrated on the right.
  2. Fold Hinge: The point of maximum curvature that separates the two fold limbs. If maximum curvature is better represented by an area than a point, then this area is referred to as the hinge zone. In three dimensions, the fold hinge can be represented as a line that follows the central axis of the fold. 
  3. Axial Plane: By connecting the fold hinges across beds, a plane can be drawn that is called an axial plane or axial surface. The axial plane separates the fold limbs of each successive bed. 

Fold Geometry

Cylindrical vs Noncylindrical Folds

One way to determine if a fold is cylindrical is to take an imaginary straight line and move it parallel to the fold hinge. If the fold can be traced using a straight line, then it is a cylindrical fold. Another way to imagine this is by wrapping paper around an empty paper towel roll. Cylindrical folds typically look as if they have been folded snuggly around a cylindrical object. Noncylindrical folds do not fit these criteria. Their hinge lines may be bowed or irregularly shaped and an imaginary straight line cannot be drawn parallel to itself to trace the fold.

Figure 4: Examples of cylindrical and noncylindrical folds. Notice the fold axis can be translated smoothly along the cylindrical fold. In the case of the noncylindrical fold, the hinge line is curved. Photo by Fossen (2016).

Plunging vs Nonplunging Folds

A plunging fold is a fold whose hinge line or fold axis is dipping in a given direction (Fig. 5). On the surface of the earth, beds wrap around and curve to create what geologists call a plunging nose. For an antiform, beds wrap around in the direction of plunge. For a synform, beds wrap opposite of the plunge direction (Fig. 6). 

Figure 5: A plunging antiform. Notice that the hinge line or fold axis is plunging away from the screen. Photo by Khese (2018).

Figure 6: Example of plunging folds. Notice the nose of the anticline wraps in the plunge direction, which is opposite for the syncline. Photo by GEOCONCEPTS01 (2019).

Symmetric vs Asymmetric Folding

Symmetric folds are amply named for their symmetric limbs. When looking down the hinge of the fold, the limbs should have the same length and angle of dip and should be bisected by the axial surface. Essentially, the limbs are mirror images of each other across the axial surface. For asymmetric folds, limbs are not mirrored and have differing dip angle and length (Fig. 7). Commonly, such asymmetry creates what geologists call a vergence direction. Vergence is determined by the sense of movement of the upper limb compared to the lower limb. In Figure 7, notice that the upper limb of the folds has been displaced to the right, on top of the lower limb. This is also reflected by the right-tilted or left-dipping axial surface. This relationship indicates a right-verging fold.

Figure 7: Diagram of asymmetric folds. Notice the varying dip angle and length of the limbs across the fold. Also, notice that the right-most antiform has its upper limb (left limb) displaced slightly overtop (to the right) of the lower limb. This indicates a right-verging antiform. Photo by HAMMADFLIP9 (2015). 

Sometimes larger folds contain smaller associated folds along their limbs and hinges. These are termed parasitic folds. The parasitic folds tend to be asymmetric on the limbs of the larger structure and are important because they can be used to determine the geometry of their larger “host” fold. Specifically, parasitic folds are unique in that they verge toward the core of the major fold due to shear (Fig. 8). This is a useful tool in the field for assessing the geometry of a regional structure.  

Figure 8: Diagram showing asymmetric parasitic folds on a larger antiform. Notice how the parasitic folds verge toward the core of the antiform on either limb. This vergence matches the shear direction felt by each limb. Photo by Fossen (2016).

Classification Based on Interlimb Angle

Another classification for fold types is based on the angle between the two limbs. An illustration of this classification is presented in Figure 9. The broadest folds (180⁰-120⁰) are termed gentle folds. When looking at an outcrop, folds that have distinctively obtuse angles would fall under this category. Folds with slightly smaller angles (120⁰-70⁰) are termed open folds. In outcrop, these folds look more or less to be at right angles. Folds with interlimb angles of 70⁰-30⁰ are termed tight folds. Folds that have distinctively acute angles in outcrop are generalized as tight folds. Lastly, folds with angles of 30⁰-0⁰ are termed isoclinal folds. These folds look extremely acute in outcrop to the point that both limbs are nearly parallel to each other. 

Figure 9: Fold classification based on the angle between two fold limbs. Photo by Wilkerson (pers. comm.).

Fold Kinematics: The Three Main Fold Processes

Buckling

Buckle folding occurs in a bed when the maximum shortening or compressive direction is parallel with that of an undeformed bed. As compression ensues, the bed will begin to buckle and form periodic folds (folds with more or less regular wavelengths). One way to imagine this process is by sliding a rug across the floor. Notice that as you push parallel to the length of the rug, the rug begins to buckle due to opposing friction with the floor or due to a collision with another object in the room. This same buckling occurs in rock due to tectonic stresses. Such buckle folding commonly occurs when a competent rock (resistant to deformation) is surrounded by a less competent rock. A contrast in viscosity, or resistance to flow, is required for strata to buckle (Fig. 10). Under some circumstances, viscosity differences are not enough to create buckling and smaller bedding irregularities must be present for buckling to initiate. 

Figure 10: Buckle folding of layered strata. Notice that only one of the layers (presumably the competent layer) buckles compared to the two black beds. Also recognize that the tectonic forces are parallel to the undeformed rock sequence. Photo by Ramberg (1987).

Figure 10 also shows that as the entire structure begins to fold the smaller, periodic buckle folds obey our earlier rule and verge towards the core of the antiform. During folding, beds shear past each other to accommodate for space in the core of the fold (Fig. 11). It is this sequence of events that creates the shear zones that develop the verging folds we come to expect in Figure 8.

Figure 11: Diagram illustrating shear during folding. Notice how shear zones are created along bedding contacts. This occurs as beds closer to the core of the fold try to escape the core to accommodate for space. If you placed smaller symmetric folds along these contacts, they would begin to verge toward the core of the larger folds. Photo by Hatcher (1995).

Bending

The process of bending is nearly opposite to that of its buckling counterpart. Instead of forces acting parallel to bedding to create buckling, bending is caused by forces acting across or even perpendicular to bedding (photo on the right). These forces can be created by several different mechanisms. For example, salt diapirs or domes can move up buoyantly through the crust and deform beds above them (Fig. 12). The vertical force exerted by the rising salt structure causes beds to fold over top of the dome. Another type of bending is a fault-bend fold (Fig. 13). In this example, beds are thrusted along an irregular fault that is flat lying with an interior inclined ramp section. This type of fault is termed a flat-ramp-flat fault. This irregular geometry causes beds to bend as they pass over the fault ramp like a rug draping over a stair-step.

Figure 12: Bend folding due to a salt dome. Notice how the rising salt dome causes the overlying beds to fold. This has implications for petroleum exploration as the salt dome and bend folds act as a perfect trap for oil. Photo by Geology.com.

Figure 13: Diagram of a fault-bend fold. Notice the irregular geometry of the fault. The inclined interval that cuts across bedding is termed a ramp while the horizontal intervals that propagate parallel to bedding are called flats. Movement along this fault causes the displaced beds to fold or bend over the irregular fault surface. Photo by Cosgrove (2015).

Passive Folding

Passive folding occurs when beds passively flow (i.e. layering does not mechanically influence folding). In fact, bedding only symbolizes how the rock sequence deformed. The beds are simply passive markers, like the swirling lines of colors in taffy that show evidence of mixing (i.e. these lines did not influence the mixing process). A great example of passive folds are shear folds (Fig. 14). These folds form due to simple shear (imagine shearing a deck of cards), pure shear (imagine pressing down on a jelly sandwich causing the jelly to squish out), or a combination of the two. For the simple shear folds in Figure 14, notice how these folds develop stronger vergence with progressive shear. This type of mechanism can cause the verging parasitic folds (Fig. 8) in rocks that behave ductilely like shales or phyllites.  

Figure 14: Illustration of progressive shear folding. Notice that simple shear creates folds with a vergence direction. Photo by Fossen (2016).

Case Study: Baraboo, Wisconsin

Baraboo, Wisconsin, is a geologic site of structural complexity that demonstrates several of the concepts discussed in this article. In this section, you will see examples of fold geometries and kinematics in a real-world setting. 

Figure 15: Digital elevation model of the Baraboo Ranges. Town of Baraboo is shaded in yellow. Photo by Wilkerson (pers. comm.). 

The Baraboo area consists of two main ranges, the North Range and South Range (Fig. 15). Both ranges are composed of maroon quartzite, which provides topographic relief due to its resistance to erosion. Based on bedding and stratigraphic age relationships, it is commonly accepted that these ranges represent two limbs of a large syncline. The North Range quartzite limb dips subvertically to create a narrow trending range; whereas, quartzite in the South Range limb dips at a shallower angle to create a wider surface expression. Based on this information, we can make a general interpretation of the subsurface structure (Fig. 16). Looking at the cross section in Figure 16, you will notice that the syncline is asymmetric. Visually, the interlimb angle looks to be ~90⁰, suggesting an open fold. You may also notice that this asymmetric fold has a vergence towards the south (i.e., its northern limb appears to be slightly overturned over its southern limb). 

Figure 16: Cross section of the Baraboo syncline. The dark orange layer is Precambrian Rhyolite, the purple beds are younger Precambrian Baraboo Quartzite, the light gray bed is Precambrian Phyllite, and the dark grey bed is younger Precambrian Slate. Based on the geometric relationship, this structure represents a synform. Looking closer at the age relationships, you can see that the beds get progressively younger towards the middle of the fold, which fits the model of a syncline. Geologists would call this structure an asymmetric, south verging, open syncline. You should note the shear zone between the quartzite and phyllite and consider what this should mean if there are any parasitic folds. Photo by Marshak et al. (2016).

We will now concentrate on the contact between the quartzite and phyllite. The cross-sectional interpretation indicates a shear zone created by the displacement of inner beds out of the core of the syncline (top to the south shear on the southern limb and top to the north shear on the northern limb). Based on our previous discussion, if there are any parasitic folds on the limbs, we should see shear folds reflecting the limb shear direction. Do we see this in the Baraboo field area? Figure 17 shows parasitic, folded quartzite beds on the southern limb of the larger Baraboo syncline. Vergence direction of these parasitic folds indicate top to the south shear, which would be expected on the southern limb (Fig. 18)! 

Figure 17: Small, parasitic quartzite folds near the north entrance of Devil’s Lake (hand and fieldbook for scale). These parasitic folds are located on the southern limb of the greater Baraboo syncline. Photo by the author.  

Figure 18: Flipped sketch of outcrop in Figure 17 by Marshak et al. (2016). Notice how fold vergence indicates a top to the south shear direction. This matches the kinematics we should expect on the southern limb. 

Here you see perfect examples of how the concepts discussed previously in the presentation are expressed in the real world. Analysis of folds are significant to geologists because they reveal the deformational kinematics and history of a geologic region. Furthermore, they have direct implications for mineral and oil exploration as they can trap migrating ore or petroleum fluids to create economically feasible mineral and energy resources. 

References

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