Folds and faults are rock responses to the stresses and strains exerted on the Earth’s crust by gravity and tectonic plate motions. Simply put, folds occur when rocks bend, exhibiting ductile behavior. Faults occur when rocks break, exhibiting brittle behavior. Imagine trying to bend a plate of glass. Glass at room temperature can sustain only slight bending before it will fracture or break. However, if we heat the glass to a significant fraction of its melting temperature, it becomes more ductile and malleable, and you can bend and stretch it a great deal without breaking.
Most rocks are very brittle near the Earth’s surface, although weak rocks like clay-rich shales and evaporites (salt deposits) are ductile even at shallow depths. Strong rocks retain their brittle character to the greatest depth, but even they become weak and bend at the temperatures experienced at great depth. The more brittle a rock is, the less bending it can withstand before faulting or breaking. The more ductile the rock, the more it will bend, and even flow, in response to shear before finally breaking along a fault.
Three Principal Stresses
The geometry and orientation of faults and folds depend upon the stress field acting when they form. Below the Earth’s surface, there are three different stresses, which we call the principal stresses, whose relative magnitudes are controlled by gravity, rock density and horizontal tectonics. The vertical stress is due to gravity and increases with depth according to the density of the rock and sediment in the crust. The two horizontal stresses primarily result from tectonic plate motion. These horizontal stresses are perpendicular to each other and both are perpendicular to the vertical stress. The principal stresses are not all the same magnitude. One will be maximum, or most compressive, and one will be minimum, or least compressive, while the third will be intermediate. All are compressive, however, at any substantial depth below the Earth’s surface (>100 meters). Depending on the relative magnitudes of these principal stresses, different types of folds and faults can form.
Folds come in several varieties: slight bends are monoclines, which are merely ramps of inclined beds between two adjacent sets of layers that are roughly horizontal. U-shaped folds are called synclines—the rock layers warp downward in the center, and upside-down Us are called anticlines—the center of the fold is higher than the sides. It’s easiest to see folds in sedimentary (layered) bedrock. These folds can occur in isolation or in groups, and any given region can have one or multiple stages of folding over geologic history.
Faults can also be classified into a few types, depending primarily on whether the crust is stretching or shortening in the vertical and horizontal directions. In a normal fault, rock layers are being stretched horizontally until breaking. The fault plane itself represents a planar ramp, usually inclined about 60 degrees from horizontal. The block sitting on top of the fault is called the hanging wall. The block under the fault is called the footwall. In a normal fault, the hanging wall slides down the ramp relative to the footwall.
A number of structures are associated with normal faults. A graben is an elongated valley bounded on each side by normal faults, and these normal faults are inclined (dip) toward the central axis of the valley, such that their downthrown hanging walls form the valley topography. The blocks making up the walls of the valley and the neighboring elevated plateaus are the uplifted footwall blocks, called horsts.
In a reverse fault, the Earth’s crust is shortening horizontally, and the hanging wall moves up relative to the footwall. Reverse faults can usually be attributed to tectonic processes where plates are colliding or converging with one another. Normal faults are typically caused by the opposite tectonic motion – plates being pulled apart – but horizontal spreading of a great thickness of rock and sediment due to the forces of gravity can also be a cause.
The third type of fault, a strike-slip fault, is the one many people are familiar with, an infamous example being the San Andreas fault in California. In this type, two slabs or blocks of rock slide past each other horizontally without relative change of elevation between the two blocks, typically along a vertical fault plane.
Not all faults can be categorized as having only one type of motion. Either a normal or reverse fault can have some associated strike-slip shear. These kinds of faults are called oblique-slip.
Active faults, as their name suggests, are those that are currently producing or have recently produced displacement or slip. Slip does not necessarily generate earthquakes, however. Abrupt slip on a fault surface can generate seismic waves that travel through the rock, which are comparable to sound waves traveling through air. The vibration of these seismic waves can cause ground-shaking that can be felt by humans and possibly damage buildings and other structures. A large slip event (10’s of centimeters to meters) on a big fault (10’s of kilometers long) is likely to cause a felt earthquake that could result in damage, depending on resilience of the ground and the structures built upon it. However, if a fault is very small (less than a kilometer long), or the slip that occurs on the fault is minimal (a centimeter or less), the vibration caused may be very small and it may not result in any damaging ground shaking. Another reason slip may not cause an earthquake is if the motion is very slow. You may have heard that parts of the San Andreas fault are “creeping”, while others operate in a process called “stick-slip”. Stick-slip is where the fault can be frozen immobile for hundreds of years, and then suddenly the two sides jump past one another in slip events of a meter or more. For the creeping segments of the fault, the blocks are moving past one another so slowly that even though slip is being generated, no shaking results.
Faults that have not slipped during human history are typically described as inactive and have a negligible potential for earthquakes. However, some inactive faults can be reactivated by changes in stresses in the Earth’s subsurface. Recent earthquakes in some oil and gas basins are thought to be examples of such reactivated faults.
Images: “Agios Pavlos Kap Melissa” by Olaf Tausch licensed under the GNU Free Documentation License; “Graphic” by Top Energy Training, adapted from Dr. Jon Olson