Detachment Faults: Mechanisms of Large-Scale Crustal Extension
In the complex world of plate tectonics, detachment faults play a critical role in reshaping the Earth's crust. These are gently dipping normal faults—faults where the crust moves apart—associated with large-scale extensional tectonics. Unlike standard faults, detachment faults often involve massive displacements, sometimes reaching tens of kilometers, and serve as the primary mechanism for creating metamorphic core complexes.
A metamorphic core complex is a geological structure where a deep, highly metamorphosed section of the crust (the footwall) is brought into contact with shallower, less metamorphosed material (the hanging wall) due to intense stretching and thinning of the Earth's surface.
Key Facts
- Scale: Displacements can reach tens to hundreds of kilometers.
- Structure: They juxtapose unmetamorphosed hanging walls against metamorphic footwalls.
- Types: They occur in both continental settings and on the ocean floor.
- Formation: They may form as initially low-angle structures or through the rotation of steep faults.
- Terminology: Also known as denudation faults, décollements, or low-angle normal faults (LANF).
Continental Detachment Faults
Continental detachment faults, sometimes referred to as décollements or dislocation surfaces, are a subject of significant scientific debate. One of the primary controversies involves their initial geometry: did these faults begin at a low angle, or did they start as steep faults that subsequently rotated due to tectonic forces?
In the Yerington district of Nevada, evidence such as tilted volcanic dikes suggests that faults may indeed rotate from steep to low angles. However, some geologists maintain a stricter definition for detachment faults, requiring them to have an initially low angle of dip, a regional scale of development, and massive translational displacements.
Mechanics and Composition
In continental settings, these faults often initiate deep within the crust in zones of intracrustal flow, where mylonitic gneisses (rocks formed by intense ductile deformation) are created. The physical behavior of the fault changes with depth:
- Mid to lower crust: Deformation is ductile, meaning the rock flows like a very viscous fluid.
- Shallower depths: Deformation becomes brittle, characterized by fracturing and breaking.
As the footwall is transported upward, these mylonitic gneisses move from the deep crust to the upper crust, where they undergo changes such as becoming chlorititic or brecciated (fragmented). Meanwhile, the hanging wall—the upper block of crust—becomes thinned and brittle, often cut by numerous smaller normal faults that merge into the main detachment surface.
: View of Doso Doyabi, Snake Range, Nevada, which was formed by detachment faulting.The unloading of the footwall can trigger isostatic uplift, a process where the crust rises like a boat being lightened when cargo is removed, leading to the doming of ductile material beneath the surface.
Oceanic Detachment Faults
While continental detachments are driven by crustal stretching, oceanic detachment faults occur at spreading ridges where there is insufficient magma to account for the entire rate of plate spreading. These are found near divergent plate boundaries, such as the Southwest Indian Ridge.
Oceanic detachment faults are unique because they are typically rolling hinge normal faults. This means they initiate at higher angles and gradually rotate to low angles as the plates move apart. Because the slip on these faults can exceed the total thickness of the oceanic crust (sometimes hundreds of kilometers compared to a crust thickness of only ~6 km), they cannot be easily reconstructed by geologists.
Comparison of Continental and Oceanic Settings
The environment of the ocean floor significantly alters the mineralogy and formation of these faults. In oceanic settings, the footwall is often created through a process called "continuous casting," where new footwall material is constantly generated by mantle upwelling or melt from a magma chamber as slip occurs.
| Feature | Continental Detachment | Oceanic Detachment |
|---|---|---|
| Primary Mineralogy | Quartz and Feldspar | Olivine, Serpentine, Talc, and Plagioclase |
| Dominant Rock Types | Mylonitic Gneisses | Gabbro and Peridotite |
| Magmatic Influence | Lower | Higher (Continuous Casting) |
| Hydrothermal Alteration | Less Extensive | Extensive |
| Fault Rotation | Debated (may be initially low-angle) | Rolling hinge (rotates from high to low angle) |
Frequently Asked Questions
What is the difference between a hanging wall and a footwall?
In a fault system, the hanging wall is the block of rock that lies above the fault plane, while the footwall is the block that lies below it.
Why do detachment faults form in oceanic settings?
They form at spreading ridges when the supply of upwelling magma is too limited to fill the gap created by the diverging tectonic plates, allowing the crust to stretch via faulting instead.
Can detachment faults be found on land?
Yes, many famous examples exist on land, such as the Snake Range detachment in Nevada, the Whipple detachment in California, and the Nordfjord-Sogn detachment in Norway.
What causes the rotation of these faults?
In oceanic settings, they are often "rolling hinge" faults that rotate from steep to low angles. In continental settings, rotation may be driven by the isostatic effects of tectonic denudation (the wearing away of the Earth's surface).
What are mylonitic gneisses?
Mylonitic gneisses are rocks that have been transformed through intense, ductile deformation (flowing) deep within the Earth's crust, often found in the footwalls of detachment faults.