Basically, geological faults are fractures that occur in the earth’s crust along which there has been an appreciable displacement of rocks or soils. These geological structures usually even reach up to 15 km in depth.
Although usually one geologic fault it is represented as a single plane, the truth is that this geological structure is a set of fractures that affects the rocks or soils, therefore, together they can be several kilometers long and wide.
Occasionally, small faults can be recognized in road cuts where the sedimentary strata have shifted a few meters.
Faults of this scale generally occur as individual discrete interruptions and are often of interest in the study of geotechnical and civil engineering works.
By contrast, large faults, such as the San Andreas fault in California, have offsets of hundreds of kilometers and consist of many interconnected fault surfaces.
These structures, best described as fault zones ( shear zones ), can be several kilometers across and are often easier to identify from aerial or satellite views than at ground level.
Parts of a geological fault
A geological fault will have a floor block (it is the rock block that remains stable), a top block (it is the rock block that moves) and a fault plane (it is the plane through which the rock block slides). rock with respect to the other block)
Types of geological faults
Now that we know that a geological fault is basically a zone of fractures that occur in the earth’s crust, we are going to see what are the different types of geological faults that occur in nature.
For this we will use the following table:
| slip faults | normal faults |
| Sabatons | |
| horsts | |
| Grabens | |
| reverse faults | |
| Tear or transcurrent failures | dextral failures |
| sinister faults | |
| Combined or mixed | It is a combination of the above, for example: reverse sinestral, normal dextral, etc. |
What the table means is that there are three main groups of failures, those that occur by sliding, tearing, which are also called transcurrent, and combined or mixed, which are a mixture of all the above types.
In fact, in nature a geological fault is never only normal or inverse, they are always combined, but for their study a kinematics is always defined as the main one.
So we are going to continue now to describe the types of faults that occur in nature and their distinctive characteristics.
Normal faults
The normal faults They are those that are formed by the action of tensile forces (extensive forces), that is, it is as if the efforts make the crust stretch, and fracture in such a way that a “block” of rocks or soils slides towards below with respect to another through a fault plane.
Identification features
- One block of rock or soil moves down a fault plane with respect to another.
- If there are layers of sedimentary rocks, we will realize that some layers will be below their similar layers on the other side of the fault plane.
- Fault planes typically have high angles, even greater than 60°, because the fault is formed with the help of gravity.
- They are associated with distensive geological settings, for example, the thinning of oceanic and continental crust at divergent plate rims.
It was mentioned that these faults are related to divergent plate boundaries, learn more about them by clicking here.
Reverse faults
The reverse faults are those that originate from the action of compressive stresses that act on the earth’s crust, that is to say, it is as if those stresses cause the crust to compress and fracture in such a way that a “block” of rocks slides towards up with respect to another through a fault plane.
Identification features
- One block of rock moves up a fault plane relative to another.
- If there are sedimentary rock strata, it is easy to see how the strata are higher than their similar ones on the other side of the fault plane.
- The fault planes in thrust faults are usually less than 45°, and this is because compressive stresses have to go against gravity to break the rocks.
- When the fault plane has very low angles (less than 20°), these faults are often called thrust structures.
- They are associated with compressive geological settings, for example, subduction zones and convergent plate boundaries.
- It is also typical that they appear accompanied by geological folds.
If you want to know more about the geological folds, which are structures that trap together with the geological faults, you can click here.
Convergent plate boundaries were discussed here, learn more about them by clicking here.
Tearing, transforming or transcurrent faults
The tear failures They are those that are mainly related to oblique compressive stresses that occur in the earth’s crust and cause the rock blocks to move laterally along a fault plane.
There are two types: dextral and sinister.
In this case it will be easy for you to learn about these faults because there is a very famous example, The San Andreas Fault is a tear fault that has been widely studied by geologists and scientists worldwide.
Identification features
- Blocks move laterally across a fault plane
- They are usually very large, so it is easier to identify them with satellite photographs.
- The fault planes are practically vertical or with angles of almost 90°, however, at depth they tend to become more horizontal.
- They are associated with transcurrent or transform plate boundary geologic settings.
Dextral failures
This is when a glitch block moves across a plane in such a way that it will always be the block on the right that is moving towards the viewer, while the block on the left is moving away.
Sinister faults
Unlike the hatchet glitch, here instead it will be the block on the left that always moves towards the viewer moving across a plane, while the one on the right moves away.
horst and record
The horst Y record they form associated with various normal fault systems that occur in the crust.
Thus, basically a graben is associated with the formation of valleys, basins or sunken topographic levels.
While the horst will be associated with higher topographic levels with respect to the grabens.
That is to say that for a graben to form, the soil or rock levels had to descend along fault planes that tend to intersect at depth.
Mixed or combined faults
As we mentioned before, in nature practically all geological faults are mixed or combined, that is, they present several kinematics that can be observed in the fault plane.
This means that if we observe a geological fault in the field, we will observe that, for example, it usually presents a dextral and at the same time normal transcurrent kinematics. So it is up to us to observe the possible combinations of fault kinematics.
An example that geologists are familiar with are the so-called bloom structures, which are formed by the action of tear-off faults, thrust faults, and normal faults.
How are geological fault lines formed?
For one to form geologic fault It is necessary for the earth’s crust to fracture and for the relative movement of the blocks or masses of rock (rock masses) that make up the crust to occur.
For this reason, geological faults are formed when the earth’s crust is affected by extensional stresses (tension), compressive stresses, and shear stresses, which cause the crust to fracture and cause rocks to move through a plane of failure.