A detachment fault is a gently
dipping normal fault
In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...
associated with large-scale
extensional tectonics
Extensional tectonics is concerned with the structures formed by, and the tectonic processes associated with, the stretching of a planetary body's crust or lithosphere.
Deformation styles
The types of structure and the geometries formed depend on ...
. Detachment
faults often have very large displacements (tens of km) and juxtapose unmetamorphosed
hanging wall
In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...
s against medium to high-grade
metamorphic
Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causin ...
footwall
In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...
s that are called
metamorphic core complex
Metamorphic core complexes are exposures of deep crust exhumed in association with largely amagmatic extension. They form, and are exhumed, through relatively fast transport of middle and lower continental crust to the Earth's surface. During th ...
es. They are thought to have formed as either initially low-angle structures or by the rotation of initially high-angle
normal fault
In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...
s modified also by the
isostatic effects of tectonic
denudation
Denudation is the geological processes in which moving water, ice, wind, and waves erode the Earth's surface, leading to a reduction in elevation and in relief of landforms and landscapes. Although the terms erosion and denudation are used interch ...
. They may also be called denudation faults.
Examples of detachment faulting include:
*The
Snake Range detachment system of the
Basin and Range Province
The Basin and Range Province is a vast physiographic region covering much of the inland Western United States and northwestern Mexico. It is defined by unique basin and range topography, characterized by abrupt changes in elevation, alternating ...
of western North America which was active during the
Miocene
The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...
*The
Nordfjord-Sogn detachment
The Nordfjord—Sogn Detachment (NSD) is a major extensional shear zone up to 6 km in thickness, which extends about 120 km along strike from Nordfjord to Sognefjord, bringing Devonian continental coarse clastic sedimentary rocks into ...
of western Norway active during the
Devonian
The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...
Period
*The
Whipple detachment in southeastern California
Detachment faults have been found on the sea floor close to divergent plate boundaries characterised by a limited supply of upwelling
magma
Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...
, such as the
Southwest Indian Ridge
The Southwest Indian Ridge (SWIR) is a mid-ocean ridge located along the floors of the south-west Indian Ocean and south-east Atlantic Ocean. A divergent tectonic plate boundary separating the Somali Plate to the north from the Antarctic Plate t ...
. These detachment faults are associated with the development of
oceanic core complex
An oceanic core complex, or megamullion, is a seabed geologic feature that forms a long ridge perpendicular to a mid-ocean ridge. It contains smooth domes that are lined with transverse ridges like a corrugated roof. They can vary in size from 10 ...
structures.
Continental detachment faults
Continental detachment faults are also called
décollement
Décollement () is a gliding plane between two rock masses, also known as a basal detachment fault. Décollements are a deformational structure, resulting in independent styles of deformation in the rocks above and below the fault. They are ass ...
s, denudational faults, low-angle normal faults (LANF) and dislocation surfaces.
[Davis, G. A., & Lister, G. S., 1988. Detachment faulting in continental extension: Perspectives from the southwestern US Cordillera. Spec. Pap. Geol. Soc. Am, 218, 133-15]
/ref> The low-angle nature of these normal faults has sparked debate among scientists, centred on whether these faults started out at low angles or rotated from initially steep angles. Faults of the latter type are present, for example, in the Yerington
Yerington is a city in Lyon County, Nevada, United States. The population was 3,048 at the 2010 census. It is the current county seat of Lyon County, with the first county seat having been established at Dayton on November 29, 1861. It is nam ...
district of Nevada. There, evidence for rotation of the fault plane comes from tilted volcanic dikes. However, other authors disagree that these should be called detachment faults. One group of scientists defines detachment faults as follows:
'' "The essential elements of extensional detachment faults, as the term is used here, are low angle of initial dip, subregional to regional scale of development, and large translational displacements, certainly up to tens of kilometres in some instances." ''[
Detachments faults of this kind (initially low-angle) can be found in the ]Whipple Mountains
The Whipple Mountains ( Mojave: Avii Kur'utat; Chemehuevi: Wiyaatuʷa̱) are located in eastern San Bernardino County, California. They are directly west of the Colorado River, Parker Dam, and Lake Havasu; south of Needles, California; north of ...
of California and the Mormon Mountains
The Mormon Mountains are located in Lincoln and Clark counties in Nevada, between 16 and northwest of Mesquite on I–15 in the Virgin Valley, east of the Meadow Valley Mountains and Meadow Valley and northeast of Moapa Valley.''Overton, Nevada ...
of Nevada.[John, B. E., & Cheadle, M. J., 2010. Deformation and alteration associated with oceanic and continental detachment fault systems: Are they similar?.Geophysical Monograph Series, 188, 175-20]
/ref> They initiate at depth in zones of intracrustal flow, where mylonite, mylonitic gneiss
Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures an ...
es form. Shear along the fault is ductile at mid to lower crustal depths, but brittle at shallower depths. The footwall
In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...
can transport mylonitic gneisses from lower crustal levels to upper crustal levels, where they become chlorititic and brecciated
Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix.
The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of d ...
.[ The hanging wall, composed of extended, thinned and brittle crustal material, can be cut by numerous normal faults. These either merge into the detachment fault at depth or simply terminate at the detachment fault surface without shallowing.][ The unloading of the footwall can lead to isostatic uplift and doming of the more ductile material beneath.][
Low angle normal faulting is not explained by Andersonian fault mechanics.][Kearey, P., Klepeis, K.A., Vine, F.J. (2009) Global Tectonics (3rd edition). Wiley-Blackwell.] However, slip on low angle normal faults could be facilitated by fluid pressure, as well as by weakness of minerals in wall rocks. Detachment faults may also initiate on reactivated thrust fault surfaces.[
]
Oceanic detachment faults
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. They cannot be structurally restored, as slip on the fault exceeds the thickness of oceanic crust (~30 km compared to ~6 km, for example).[
While occurring at relatively amagmatic spreading centres, the footwalls of these detachment faults are much more influenced by magmatism than in continental settings. In fact, they are often created by ‘continuous casting’: new footwall is continually being generated by mantle or melt from a magma chamber as slip occurs on the fault.][ The lithology is dominated by ]gabbro
Gabbro () is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is che ...
and peridotite
Peridotite ( ) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high prop ...
, resulting in a mineralogy of olivine
The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...
, serpentine, talc
Talc, or talcum, is a Clay minerals, clay mineral, composed of hydrated magnesium silicate with the chemical formula Mg3Si4O10(OH)2. Talc in powdered form, often combined with corn starch, is used as baby powder. This mineral is used as a thi ...
and plagioclase
Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more prope ...
. This is in contrast to continental settings, where the mineralogy is dominantly quartz
Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...
and feldspar
Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldsp ...
. The footwall is also much more extensively hydrothermally altered
Metasomatism (from the Greek μετά ''metá'' "change" and σῶμα ''sôma'' "body") is the chemical alteration of a rock by hydrothermal and other fluids. It is the replacement of one rock by another of different mineralogical and chemical com ...
than in continental settings.[
In contrast to many detachment faults in continental settings, oceanic detachment faults are usually ]rolling hinge normal faults
Rolling is a type of motion that combines rotation (commonly, of an axially symmetric object) and translation of that object with respect to a surface (either one or the other moves), such that, if ideal conditions exist, the two are in contac ...
, initiating at higher angles and rotating to low angles.[
]
References
Further reading
*George H Davis, Stephen J Reynolds, (1996), ''Structural Geology of Rocks and Regions,'' 2nd Edition, John Wiley and Sons Inc. .
{{Structural geology
Structural geology