Detachment fault
A gently dipping normal fault with large displacements in extensional tectonics.
A detachment fault is a type of normal fault with a gentle dip, linked to large-scale extension of the Earth's crust. These faults can accommodate enormous displacements, often tens of kilometers, and they place relatively unaltered rock in the hanging wall directly above medium to high-grade metamorphic rock in the footwall, a structure known as a metamorphic core complex. They are sometimes referred to as denudation faults.
In continental settings, detachment faults are also called décollements, denudational faults, low-angle normal faults (LANF), or dislocation surfaces. Their low-angle nature has led to debate: some scientists argue they form at low angles from the start, while others believe they begin as steep faults and rotate over time. Evidence for rotation, such as tilted volcanic dikes, exists in places like the Yerington district of Nevada, though some researchers dispute whether those examples qualify as true detachment faults. One definition holds that the essential features of an extensional detachment fault are a low initial dip, regional scale, and large displacements—up to tens of kilometers. Examples of this initially low-angle type include the Whipple Mountains in California and the Mormon Mountains in Nevada. These faults originate deep in the crust, in zones of ductile flow where mylonitic gneisses form. At mid to lower crustal depths, shearing is ductile, but it becomes brittle closer to the surface. The footwall can carry mylonitic gneisses upward, where they become chloritized and brecciated. The hanging wall, made of extended and brittle crust, is often cut by numerous normal faults that either merge into the detachment fault at depth or end at its surface. Unloading of the footwall can cause isostatic uplift and doming of the underlying ductile material. Low-angle normal faulting is not predicted by standard Andersonian fault mechanics, but slip may be aided by high fluid pressure or the weakness of certain minerals. Detachment faults can also form by reactivating older thrust faults.
Oceanic detachment faults occur at spreading ridges where magma supply is insufficient to account for the full plate spreading rate. They produce long, dome-shaped features parallel to the spreading direction, called oceanic core complexes, in the footwall. Slip on these faults can range from tens to hundreds of kilometers, and the displacement ofte
- type
- Geological structure
- also_known_as
- Denudation fault, décollement, low-angle normal fault (LANF), dislocation surface
- key_characteristic
- Gently dipping normal fault with large displacements (tens of km)
- associated_structures
- Metamorphic core complexes, oceanic core complexes
- settings
- Continental extensional terrains and oceanic spreading ridges with limited magma supply
Lore & Background
Detachment faults are thought to have formed either as initially low-angle structures or by rotation of initially high-angle normal faults, modified by isostatic effects of tectonic denudation. The low-angle nature has sparked debate among scientists, centered on whether these faults started at low angles or rotated from initially steep angles. Examples include the Snake Range detachment system of the Basin and Range Province (active during the Miocene), the Nordfjord-Sogn detachment of western Norway (active during the Devonian Period), and the Whipple detachment in southeastern California.
In continental settings, detachment faults are also called décollements, denudational faults, low-angle normal faults (LANF), and dislocation surfaces. They initiate at depth in zones of intracrustal flow where mylonitic gneisses form. Shear along the fault is ductile at mid to lower crustal depths but brittle at shallower depths. The footwall can transport mylonitic gneisses from lower crustal levels to upper crustal levels, where they become chlorititic and brecciated. The hanging wall, composed of extended, thinned and brittle crustal material, can be cut by numerous normal faults that either merge into the detachment fault at depth or terminate at its surface.
Oceanic detachment faults occur at spreading ridges where magmatic activity is not enough to account for the entire plate spreading rate. They are characterized by long domes parallel to the spreading direction (oceanic core complexes of the footwall). Slip on these faults can range from tens to hundreds of km. Unlike many continental detachment faults, oceanic detachment faults are usually rolling hinge normal faults, initiating at higher angles and rotating to low angles. Their footwalls are much more influenced by magmatism than in continental settings, often created by 'continuous casting' with lithology dominated by gabbro and peridotite.
Reader's Guide
Detachment faults are significant because they accommodate large-scale extension in both continental and oceanic crust, revealing processes of crustal thinning and exhumation of deep rocks. In continental settings, they expose metamorphic core complexes and have sparked debate about the mechanics of low-angle normal faulting, which is not explained by Andersonian fault mechanics. Slip on these faults may be facilitated by fluid pressure and weakness of minerals in wall rocks, or they may initiate on reactivated thrust fault surfaces. Oceanic detachment faults occur at amagmatic spreading centers like the Southwest Indian Ridge, where they produce oceanic core complexes. Their footwalls are extensively hydrothermally altered and cannot be structurally restored because slip exceeds the thickness of oceanic crust. The study of detachment faults thus provides insight into extensional tectonics, crustal rheology, and the interplay between magmatism and faulting at divergent plate boundaries.
Did You Know?
- Detachment faults often have displacements of tens of kilometers.
- The Snake Range detachment system was active during the Miocene.
- Oceanic detachment faults occur at spreading ridges with limited magma supply, such as the Southwest Indian Ridge.
- Slip on oceanic detachment faults can range from tens to hundreds of kilometers.
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