What type of faulting occurs with shear stress

In most cases both fracture and frictional sliding occur through shear deformation shear strain along a discrete surface. These relationship define the state of stress on the failure plane at the point of failure. Tectonic stresses in the crust arise due to density anomalies in the mantle, which drive convection and move the plates. However, if we are concerned with the stresses acting on faults in the crust, we can ignore the details of where these stresses come from and instead treat the tectonic stress as a horizontal normal stresses.

In this case, tectonic stresses either act in compression a positive normal stress or in extension a negative normal stress. To consider the total stress-state in the crust, we start from an assumption that the state of stress is equal to the lithostatic pressure and add the tectonic stress to get the stress component in the horizontal direction:.

Except in cases of very shallow rock less than about m , all the components of stress in the rock are positive due to the increase in the lithostatic pressure with depth. This means that the magnitudes of tectonic stresses are smaller than the magnitude of the lithostatic stress at depths greater than about m. While the magnitude of the horizontal stress is positive, it is the difference in stress magnitude between two orthogonal directions that determine whether a region experiences compression with reverse faults , extension with normal faults or shear strike-slip faults.

Remember the three principal stress are always orthogonal to each other. For this case think of squeezing playdoh between your fingers, it will extend outward in the direction of least resistance.

This first order description of the relationship between stresses and fault orientation is called the Andersonian Theory of Faulting. Existing faults and fractures can have any orientation in the crust, and almost all of these orientation will result in both shear stress and normal stress acting on the fault plane.

This is why the principal directions are normal stresses, and this is why we only need to know these three principal stresses to fully describe the state of stress. Near the surface of the Earth, it is likely that one of the principal stress directions aligns with the vertical, but the two horizontal principal stress can be in any orientation.

When looking at the figure below, imagine taking one of the planes and rotating it slightly. The stress component vector acting on that plane can now expressed in terms of component parallel to the fault plane and a component perpendicular to the fault plane. All other planes have both shear and normal stresses acting across the plane. The San Andreas Fault is a large strike-slip fault the runs most of the length of California.

It is the plate boundary between the Pacific Plate to the west and the North American plate to the East. When stress causes a material to change shape, it has undergone strain or deformation. Deformed rocks are common in geologically active areas.

Rocks have three possible responses to increasing stress illustrated in figure 3 :. Figure 3. With increasing stress, the rock undergoes: 1 elastic deformation, 2 plastic deformation, and 3 fracture. Under what conditions do you think a rock is more likely to fracture? What if the stress applied is sharp rather than gradual? Sedimentary rocks are important for deciphering the geologic history of a region because they follow certain rules.

You can trace the deformation a rock has experienced by seeing how it differs from its original horizontal, oldest-on-bottom position figure 4a. This deformation produces geologic structures such as folds, joints, and faults that are caused by stresses figure 4b. Using the rules listed above, try to figure out the geologic history of the geologic column below. Figure 4. Each layer is made of sediments that were deposited in a particular environment — perhaps a lake bed, shallow offshore region, or a sand dune.

Grand Canyon Supergroup rocks layers 12 through 15 have been tilted. Vishnu Basement Rocks are not sedimentary rocks 16 through The oldest layers are on the bottom and youngest are on the top. Rocks deforming plastically under compressive stresses crumple into folds figure 5. They do not return to their original shape. If the rocks experience more stress, they may undergo more folding or even fracture. Figure 5. Snow accentuates the fold exposed in these rocks in Provo Canyon, Utah.

Figure 6. At Colorado National Monument, the rocks in a monocline plunge toward the ground. Figure 7. When rocks arch upward to form a circular structure, that structure is called a dome. If the top of the dome is sliced off, where are the oldest rocks located? Figure 8. When rocks bend downward in a circular structure, that structure is called a basin figure 9. If the rocks are exposed at the surface, where are the oldest rocks located?

Figure 9. Basins can be enormous. This is a geologic map of the Michigan Basin, which is centered in the state of Michigan but extends into four other states and a Canadian province. A rock under enough stress will fracture. If there is no movement on either side of a fracture, the fracture is called a joint , as shown in figure Figure Granite rocks in Joshua Tree National Park showing horizontal and vertical jointing.

These joints formed when the confining stress was removed from the granite. If the blocks of rock on one or both sides of a fracture move, the fracture is called a fault figure Sudden motions along faults cause rocks to break and move suddenly.

The energy released is an earthquake. Slip is the distance rocks move along a fault. The third typical fault type is the strike-slip fault. Strike-slip faults are distinct from the previous two because they don't involve vertical motion. They form via shear stress. These are not as easy to recognize in cross-section unless there has been so much movement on the fault that there are completely different rock types on either side of the fault.

Most strike-slip faults are close to vertical with respect to the bedding. See in the animation below how the various fault types move.

Animation is silent and comes from IRIS. Each of these three types of faults is marked in a standard way on a geologic map. I've sketched those symbols below. Can you identify the type of faulting occurring at each plate boundary in the map below? Check your answer here. Have another look at Figure 1 from de Boer et al.

What type of faulting is being depicted on that map? Can you picture in three dimensions how the lithosphere is moving in that map? Think about it and compare your idea to my sketch and a captioned version. Skip to main content. Faults Print In the articles you just read, the authors assume you know something about faults: how they are classified, what kind of motion they experience, what sense of stress they feel, and how to recognize them on a map.

Fault categories The sense of stress determines the type of fault that forms, and we usually categorize that sense of stress in three different ways: compression, tension, and shear.

Handily, these three senses of stress also correlate with the three types of plate boundaries.