4.2: Folding

Anticlines, Synclines, and Monoclines

When a body of rock, especially sedimentary rock, is squeezed from the sides by tectonic forces, it may either bend or break. If the rock is cold and brittle, it is more likely to fracture and form joints or faults. If it is warm enough to behave plastically, it may fold.

An anticline is a fold that is convex upward, whose core contains relatively older rock (Figure \(\PageIndex{1}\)). If the ages of the folded rocks are unknown, then the fold is called an antiform. A syncline is a fold that is convex downward, whose core contains relatively younger rock (Figure \(\PageIndex{2}\)). If the ages of the folded rocks are unknown, then the fold is called a synform.

Figure \(\PageIndex{1}\): An anticline in cross-sectional view (left) and map view (right). The beds are convex up in an “A” shape. O = Ordovician, S = Silurian, D = Devonian. The youngest rocks are in the center of the fold. “Anticline” by Chloe Branciforte is licensed under CC BY 4.0.

Figure \(\PageIndex{2}\): A syncline in cross-sectional view (left) and map view (right). The beds are convex down in a “U” shape. O = Ordovician, S = Silurian, D = Devonian. The youngest rocks are in the center of the fold. “Syncline” by Chloe Branciforte is licensed under CC BY 4.0.

It is common to find a series of antiforms and synforms, although some sequences of rocks are folded into a single antiform or synform. A plane drawn through the crest of a fold in a series of beds is called the axial plane or axial surface of the fold (Figure \(\PageIndex{3}\)). The sloping beds on either side of an axial plane are limbs. An antiform or synform is described as symmetrical if the angles between each limb and the axial plane are generally similar, and asymmetrical if they are not. If the axial plane is sufficiently tilted that the beds on one side have been tilted past vertical, the fold is known as an overturned antiform or synform.

Figure \(\PageIndex{3}\): A fold consists of limbs that meet at the hinge zone. An axial surface bisects the fold along the hinge zone. The axial trace is where the axial surface intersects another surface, such as the top of a bed. "Labeled Antiform" by Karla Panchuk is licensed under CC BY-NC-SA 4.0.

A monocline is a step-like fold in rock strata, where horizontal or gently dipping layers transition to a zone of steeper dip (Figure \(\PageIndex{4}\)). Monoclines are often caused by deformation from the movement on a fault beneath the folded layers.

Figure \(\PageIndex{4}\): A block diagram of a monocline fold with horizontal or near-horizontal beds and a region of more steeply dipping beds in the center. “Monocline01.svg” by Kilom691 via Wikimedia Commons is licensed under CC BY-SA 3.0.

Describing Simple Folds

The tightness, or angle between the limbs of a fold (known as the interlimb angle), can range from gentle, with a large angle between limbs, to parallel or nearly parallel (Figure \(\PageIndex{5}\)). This very tight fold is called an isoclinal fold.

Figure \(\PageIndex{5}\): The angle between the limbs of the fold can be described as isoclinal (<5º), tight (5º-30º), closed (30º-70º), open (70º-120º), or gentle (120º-180º). “Interlimb angles.jpg” by Alejandro Guevara Alday is licensed under CC BY-SA 4.0.

The orientation of a fold’s axial plane can also vary. An upright fold is one which has a vertical axial plane. If that axial plane is horizontal, we call the fold recumbent. Any angle in between is considered inclined.

You can also describe the orientation of the fold hinge, or line of maximum curvature. If this line is horizontal then again, you have an upright fold. However, if that hinge line is in an orientation other than horizontal, the fold is considered “plunging”. Plunging folds create map patterns that look like the letter V (Figure \(\PageIndex{6}\)). In a plunging 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 plunging 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.

Folds can be of any size, and it is very common to have smaller folds within larger folds. Large folds can have wavelengths of tens of kilometers, and very small ones might be visible only under a microscope.

Figure \(\PageIndex{6}\): Plunging folds have hinges that are not horizontal, described by the plunge angle—the angle the hinge makes with a horizontal line. When a plunging fold intersects Earth's surface, it creates a V-shaped pattern. "Plunging Folds" by Karla Panchuk is licensed under CC BY-SA 4.0.

Domes and Basins

A dome is a symmetrical or semi-symmetrical upwarping of rock layers, shaped like an inverted bowl, similar to an architectural dome on a building.

A basin is the inverse of a dome, a bowl-shaped depression in rock strata. Some structural basins are also sedimentary basins,collecting large quantities of sediment over time. While sedimentary basins can form from folding, they are more commonly created during mountain building, forming between mountain blocks or through faulting. As the basin subsides, it can accumulate more sediment, with the added weight causing further subsidence in a positive feedback loop.

Figure \(\PageIndex{7}\): A dome (top) has the oldest units at the center. Beds dip out and away from the center in all directions. A structural basin (bottom) has the oldest units on the outside. Beds dip in toward the center. “Complex Folds” by Chloe Branciforte is licensed under CC BY 4.0.

Query \(\PageIndex{1}\)

Economical Significance of Antiforms and Domes

Antiforms and domes (described below) are particularly favorable locations for oil and natural gas to collect. It is estimated that about 80 percent of the world's petroleum is found in these anticlinal traps, and California is no different Because of the low density of petroleum, oil migrates upward out of the source rock until it becomes trapped by an impermeable surface.

Figure \(\PageIndex{8}\): Antiforms make excellent petroleum traps. Buoyant oil and natural gas make their way upward until they reach an impermeable layer and become trapped. “Anticline trap.svg” by MagentaGreen via Wikimedia Commons is licensed under CC BY-SA 3.0.

References

1. Riva, Joseph P. "Accumulation in reservoir beds". Encyclopædia Britannica. Retrieved December 10, 2015.
2. Branciforte, Haddad. GEOS: A Physical Geology Lab Manual for California Community Colleges.