Detachment fault

A detachment fault is a special type of geological structure. It is a gently dipping normal fault. These faults occur during large-scale extensional tectonics. This means the Earth's crust is being pulled apart. These structures are important because they move massive amounts of rock. They can displace material by tens of kilometers. They often bring deep, metamorphic rocks to the surface. This process creates what geologists call metamorphic core complexes.

To understand how they work, we must look at the two sides of the fault. The upper side is called the hanging wall. This part consists of extended and thinned crustal material. It is brittle, meaning it breaks easily. Because it is brittle, the hanging wall is often cut by many smaller normal faults. These smaller faults either merge into the main detachment fault at depth. Or, they simply end when they reach the detachment surface. The lower side is called the footwall. The footwall can transport mylonitic gneisses from the deep crust to the upper crust.

The movement along these faults happens in different ways depending on depth. At mid to lower crustal depths, the shear is ductile. Ductile means the rock flows more like a thick liquid than a solid. These areas often contain mylonitic gneisses. However, at shallower depths, the movement becomes brittle. In these upper layers, the rocks can become chlorititic or brecciated. This means the rocks are chemically changed or broken into fragments. As the footwall is unloaded, it can lead to isostatic uplift. This causes the ductile material beneath to bulge upward into a dome.

Scientists have a long-standing debate about how these faults begin. Some researchers believe detachment faults start as low-angle structures. These can be found in the Whipple Mountains of California. Others argue that they begin as steep, high-angle normal faults. These steep faults then rotate until they become low-angle structures. We see evidence for this rotation in the Yerington district of Nevada. In that area, tilted volcanic dikes suggest the fault planes once sat at a different angle. Some scientists also suggest that fluid pressure might help these faults slip.

Detachment faults are found in both continental and oceanic settings. Continental detachment faults are also known as décollements or dislocation surfaces. They can be found in the Basin and Range Province of western North America. The Snake Range detachment system was active during the Miocene. Another example is the Nordfjord-Sogn detachment in Norway, which was active during the Devonian Period. These continental faults are mostly made of minerals like quartz and feldspar. They are part of the complex way continental crust stretches and thins over time.

Oceanic detachment faults occur on the sea floor near divergent plate boundaries. These are areas where tectonic plates pull apart. They appear at spreading ridges where there is a limited supply of upwelling magma. These faults are often "rolling hinge" faults. This means they start at high angles and rotate to low angles. The slip on these oceanic faults can be enormous. It can range from tens to hundreds of kilometers. This amount of movement actually exceeds the typical thickness of the oceanic crust, which is about 30 kilometers.

The environment of an oceanic detachment fault is very different from a continental one. The footwalls in the ocean are much more influenced by magmatism. They are often created by a process called "continuous casting." This happens when new footwall is constantly generated by the mantle or melt from a magma chamber. The rocks here are dominated by gabbro and peridotite. This results in a mineralogy of olivine, serpentine, talc, and plagioclase. These footwalls are also more extensively altered by hydrothermal activity. This creates unique oceanic core complex structures on the deep sea floor.