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Geological structures

In this article, we will discuss geological structures that results from rock deformation.
© Luleå University of Technology

In the previous video, you learned about basic principles of rock deformation. In this article, we will discuss geological structures that results from rock deformation.

They influence whole mountain ranges, from the largest peaks that rise high above you, all the way down to the nanoscale where they occur as disturbances of the crystal lattices of minerals. Maybe even more important for this course, geological structures are also efficient traps for metal-bearing fluids. This makes them particularly interesting for understanding where metals accumulate in the earth’s crust.

One of the most common and easy to visualise geological structures are folds. Folds are normally associated with convergent geological settings where compressional forces are at play and the crust is shortened. During crustal shortening, due to the decrease in the amount of space that a rock package can take up, and at some point, the rocks must move vertically to account for the shortening they are subjected to. The same processes are at play when you take a sheet of paper lying flat on a table and push the outer sides towards each other. During folding, an upward folding curve and a downward folding curve form, which geologists call antiform and synform respectively. If you encounter antiforms or synforms in the field, there’s a high chance that some of the rock layers have been eroded. In this case, you will probably find older rock layers as you walk in the direction of the center in an antiform, whereas in a synform you will see younger rock layers towards its center. If this is the case, you can call the antiform an anticline and the synform a syncline.

Eroded anti and syncline illustration Figure 1: Photograph of a partially eroded anticline (left) and syncline (right).

Rock deformation doesn’t stop there. Rocks also break and cause faults. This occurs during crustal shortening when rock properties prevent further folding. It also occurs during extension, when rocks are stretched so much that they can’t hold together anymore. And lastly, it occurs in transform settings where the two processes above are combined. Faults consist of two components, the hanging wall and the footwall. The hanging wall is the block overlaying the fault plane and the footwall is the underlaying block. Faults that formed during crustal shortening are often of a low angle where the hanging wall moved up in relation to the footwall. We call them reverse faults or thrust faults. Faults that formed during extension are generally of a steep angle where the hanging wall moved down relative to the footwall, and we call them normal faults. Finally, faults that formed in transform settings are often strike slip faults where the footwall and hanging wall moved sideways relative to each other. Understanding faults and brittle deformation is very important for the mining sector since these structures allow hydrothermal fluids to transport long distances and often hold metals.

Transform boundaries Figure 2: Transform fault movement that causes strike slip faults in the affected rocks.

Veins are fissures in host rocks filled with minerals. Because they form in the host rock, they are younger than the rock and occur where some space is available. The spaces in which veins form can range from tiny fractures to big crustal scale structures where thousands of veins form. Sometimes the fluid creates its own space to invade by forcing the rock walls to open due to the fluid pressure. When more than one deformation phase occurred in the past, veins can show relative age relations because the older veins are cut off by the younger ones. They also tend to fold, break, or both during renewed compression or stretch during extension. Vein minerals can consist of relocated host rock minerals such as quartz or carbonate, but also be deposited from intruding mineralised solutions.

These are particularly interesting for exploration geologists because they often contain metalliferous elements such as gold, copper, zinc, lead, cobalt, and a wide range of other metals depending on the geological environment.

Black coal white quartz Figure 3: Younger quartz vein (biggest one in the middle) cutting off older quartz veins.

© Luleå University of Technology
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