An Intro to Structural Geology: Faults

A fault is a discontinuity between two blocks of rock that are able to move corresponding to one another. An area that is bound by multiple faults is called a fault zone. Below is a photo illustrating a fault zone.

In the case of an inclined fault, one of the blocks of rock is labeled as the hanging wall, and the other is labeled as the footwall. The hanging wall is the block that lies above the fault line, and the footwall lies beneath it.

Movement of the blocks due to loss of cohesion is what is known as slip. When slip occurs, it creates one of three types of faults:

• Normal Fault
• Reverse Fault
• Strike-Slip Fault

If the fault blocks are moving mostly upward/downward, that is known as a dip-slip fault, for example both normal and reverse faults. If the blocks are moving horizontally, that is a strike slip fault. There are also oblique slip faults, which have both strike and dip slip.

Displacement of fault blocks will typically happen in the center when looking in a cross-sectional view. Closer to the surface, there is less displacement than further down in the beds. When faults form in porous rock, individual deformation bands become so prominent that they overlap and link together, creating one large deformation band that results in loss of cohesion and faulting. This is shown in the image below.

Faults can also merge together and grow. When this happens, typically something that is knowns as a relay ramp forms along with it. A relay ramp is when one of two adjacent faults tip onto the other. Shown in the picture below, the "tipping" is what forms the ramp as the structure collapses and the two faults merge into one.

There are many surface indicators of the presence of faults. The fault scarp is where the fault plane is exposed at ground surface, with the fault line/trace being at the intersection of the fault and the ground. In some cases, the fault does not intersect the ground surface. This is known as a blind fault (Wilkerson 2019).

There are many folds that form as a result of faulting. These occur because the beds assume the geometry of the material beneath it. Being able to identify fault related folds can aid in determining if a fault is present.

Seismic/well data can also be used to locate a fault. If there is an offset/discontinuity in a seismic section, that is a strong indicator.

A slickenside is a smoothly polished rock surface that is formed due to slip (Wilkerson 2019). Slickenlines, also known as slip lineations, are also polished rock surfaces, but have a linear structure instead of planar. Some slickenline examples are:

• fault striations - scratches from the blocks moving and rubbing against each other
• fibrous slip lineations - a crack or space that was created during slip is filled and sealed with new material
• slickolites - vertically sheared styolite, which is a jagged discontinuity formed by pressure dissolution

The image to the left is an example of a slickenside, you can tell that the surface facing upwards is polished and smooth. The image below is an example of a slickenline, specifically a fault striations.

Fault slip, or net slip, is the actual direction and amount of displacement that occurs along a fault. However, separation is the apparent displacement in a certain direction (Wilkerson 2019). Separation is quantified in three ways:

Vertical Separation - The vertical distance that a bed has been displaced.

Throw - The distance of vertical offset that occurs during dip-slip movement.

Heave - The distance of horizontal offset that occurs during dip-slip movement.

Both the throw and the heave are measured perpendicular to the strike of the fault. Below is an image which labels these three quantifications.

Slip can also be identified by the presence of steps in slickensides. If one were to run their hand over the surface, it would feel smooth in the direction that the block moved. In the case of this image of a fault in Ontario, Canada, the block has shifted to the left.

On a micro scale, there are some other geologic features that could indicate slip has occurred. These include:

En echelon gashes - structures inside a body of rock caused by shearing. Rock will shear when it is under compressional or tensile stress.

• Pinnate fractures - joints that form adjacent to fault zones
• Riedal shears - small conjugate fractures
• Chatter marks - wedge shaped marks that form due to plucking of the opposite block
• Fold vergence/asymmetry - a fold is typically asymmetric in the direction of shear

• The cover image is from  https://www.thoughtco.com/fault-geography-glossary-1434722 
• Fossen, Haakon. “Structural Geology by Haakon Fossen.” Cambridge Core, Cambridge University Press, https://www.cambridge.org/core/books/structural-geology/4F99FE5530610FB31EAA89FB0E9865EF.
• "Footwall | Definition Of Footwall By Lexico". Lexico Dictionaries | English, 2019, https://www.lexico.com/en/definition/footwall.
• "What Is A Fault And What Are The Different Types?". Usgs.Gov, 2019, https://www.usgs.gov/faqs/what-a-fault-and-what-are-different-types?qt-news_science_products=0#qt-news_science_products.
• "Reverse Fault In Teran Wash". Arizona Geology Magazine, 2019, http://azgeology.azgs.arizona.edu/azgs/image-of-the-day/images/reverse-fault-teran-wash.
• "Stylolite". En.Wikipedia.Org, 2019, https://en.wikipedia.org/wiki/Stylolite.
• "Slickensides On A Fault Cutting Granite - Mountain Beltway". Mountain Beltway, 2019, https://blogs.agu.org/mountainbeltway/2012/04/26/slickensides-on-a-fault-cutting-granite/.
• "Shear (Geology)". En.Wikipedia.Org, 2019, https://en.wikipedia.org/wiki/Shear_(geology).
• "Chatter Mark". En.Wikipedia.Org, 2019, https://en.wikipedia.org/wiki/Chatter_mark.
• Fleuty M.J. (1987) Slikensides and slikenlines. In: Structural Geology and Tectonics. Encyclopedia of Earth Science. Springer, Berlin, Heidelberg
• Wilkerson, M. Scott (2019). Presentation on faults.  https://moodle.depauw.edu/pluginfile.php/356221/mod_resource/content/0/2019_08Faults.pdf