In geodynamics, delamination refers to the loss and sinking (foundering) of the portion of the lowermost lithosphere from the tectonic plate to which it was attached.
In geodynamics, delamination refers to the loss and sinking (foundering) of the portion of the lowermost lithosphere from the tectonic plate to which it was attached.
The outer portion of the Earth is divided into an upper, lithosphere layer and a lower, asthenosphere layer. The lithosphere layer is composed of two parts, an upper, the crustal lithosphere and lower, the mantle lithosphere. The crustal lithosphere is in unstable mechanical equilibrium because the underlying mantle lithosphere has a greater density than the asthenosphere below. [1] The difference in densities can be explained by thermal expansion/contraction, composition, and phase changes. [2] Negative buoyancy of the lower continental crust and mantle lithosphere drive delamination. [3]
Delamination occurs when the lower continental crust and mantle lithosphere break away from the upper continental crust. There are two conditions that need to be met in order for delamination to proceed:
The metamorphic transition from mafic granulite facies to the denser eclogite facies in the lower portion of the crust is the main mechanism responsible for creating negative buoyancy of the lower lithosphere. [3] The lower crust undergoes a density inversion, causing it to break off of the upper crust and sink into the mantle. [4] Density inversions are more likely to occur where there are high mantle temperatures. This limits this phenomenon to arc environments, volcanic rifted margins and continental areas undergoing extension. [4]
The asthenosphere rises until it comes into contact with the base of the lower crust, causing the lower crust and lithospheric mantle to start to peel away. Slumping, cracking, or plume erosion facilitates the intrusion of underlying asthenosphere. [1] Potential energy that drives the delamination is released as the low density, hot asthenosphere rises and replaces the higher density, cold lithosphere. [2] Separation of lowermost crust and lithospheric mantle is controlled by the effective viscosity of the upper continental crust. These processes often occur in environments of rifting, plume erosion, continental collision or where there is convective instability. [1]
Convective instabilities facilitate delamination. The convection can simply peel away the lower crust or, in a different scenario, a Rayleigh–Taylor instability is created. Due to the instability in a local area, the base of the lithosphere breaks up into descending blobs fed by an enlarging region of thinning lithosphere. The space left behind by departing lithosphere is filled by upwelling asthenosphere. [5]
As delamination continues, more asthenosphere rises to replace the lower lithosphere as it sinks. This process causes three different changes to occur which can have an effect on the delamination process. [1]
If the freezing of the asthenosphere dominates (2) the system is stable, however if subsidence, and therefore separation of the lower lithosphere dominates (3) the system is unstable. Processes (2) and (3) compete with each other. [1]
Delamination of the lithosphere has two major geologic effects. First, because a large portion of dense material is removed, the remaining portion of the crust and lithosphere undergo rapid uplift to form mountain ranges. Second, flow of hot mantle material encounters the base of the thin lithosphere and often results in melting and a new phase of volcanism. Delamination may thus account for some volcanic regions that have been attributed to mantle plumes in the past. [6]
Delamination is seen in convergence zones, especially where continental-continental collisions occur. For example, delamination is seen in the Tibetan Plateau, which has formed from the collision of India with Asia. Observations which support delamination include sudden mafic volcanism and acceleration of uplift, occurring 14 to 11 Ma. [3]
Areas of extension are also associated with delamination. Negative buoyancy of the lower lithosphere drives delamination in both environments of collision and extension. During the collapse of a mountain belt, the thick crustal roots beneath what used to be a mountain disappear. The processes behind this disappearance are not clear. Granitic plutons formed by strong heat pulses have been associated with the disappearance of thick crustal roots. Delamination is a likely source for the heat pulses. [3]
The tectonic development of collapsed mountain belts is heavily debated. Some argue that delamination causes a second uplift along with crustal thickening, heating and volcanism. Others argue that delamination causes collapse and thinning of the crust. Some researchers postulate that the Sierra Nevada (California), Basin and Range Province and Colorado Plateau in the western US exemplify this. [3]
One example of the effects of lithosphere delamination is seen in the Sierra Nevada (US)², Basin and Range Province and Colorado Plateau in the western USA. [3] During crustal extension in the Basin and Range Province 10 million years ago, the upwelling of asthenosphere thinned the lithosphere. Heating caused by the rise of the warmer asthenosphere created a crustal lower-viscosity zone and delamination occurred on the flanks of the Basin and Range. Uplift of the Sierra Nevada mountain range in California and the Colorado Plateau has occurred on the flanks as a result of the loss of high density lower lithosphere. Eclogite xenoliths found within the crust in the region support the metamorphic phase change associated with the density inversion in the lower crust. [3] It is possible that the Sierra Nevada (US) is the only place on Earth where dense material is currently being removed from the crust. [4]
Plate tectonics is the scientific theory that Earth's lithosphere comprises a number of large tectonic plates which have been slowly moving since about 3.4 billion years ago. The model builds on the concept of continental drift, an idea developed during the first decades of the 20th century. Plate tectonics came to be accepted by geoscientists after seafloor spreading was validated in the mid-to-late 1960s.
Orogeny is a mountain-building process that takes place at a convergent plate margin when plate motion compresses the margin. An orogenic belt or orogen develops as the compressed plate crumples and is uplifted to form one or more mountain ranges. This involves a series of geological processes collectively called orogenesis. These include both structural deformation of existing continental crust and the creation of new continental crust through volcanism. Magma rising in the orogen carries less dense material upwards while leaving more dense material behind, resulting in compositional differentiation of Earth's lithosphere. A synorogenic process or event is one that occurs during an orogeny.
A lithosphere is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the lithospheric mantle, the topmost portion of the upper mantle that behaves elastically on time scales of up to thousands of years or more. The crust and upper mantle are distinguished on the basis of chemistry and mineralogy.
Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the heavier plate dives beneath the second plate and sinks into the mantle. A region where this process occurs is known as a subduction zone, and its surface expression is known as an arc-trench complex. The process of subduction has created most of the Earth's continental crust. Rates of subduction are typically measured in centimeters per year, with rates of convergence as high as 11 cm/year.
Isostasy or isostatic equilibrium is the state of gravitational equilibrium between Earth's crust and mantle such that the crust "floats" at an elevation that depends on its thickness and density. This concept is invoked to explain how different topographic heights can exist at Earth's surface. Although originally defined in terms of continental crust and mantle, it has subsequently been interpreted in terms of lithosphere and asthenosphere, particularly with respect to oceanic island volcanoes, such as the Hawaiian Islands.
A convergent boundary is an area on Earth where two or more lithospheric plates collide. One plate eventually slides beneath the other, a process known as subduction. The subduction zone can be defined by a plane where many earthquakes occur, called the Wadati–Benioff zone. These collisions happen on scales of millions to tens of millions of years and can lead to volcanism, earthquakes, orogenesis, destruction of lithosphere, and deformation. Convergent boundaries occur between oceanic-oceanic lithosphere, oceanic-continental lithosphere, and continental-continental lithosphere. The geologic features related to convergent boundaries vary depending on crust types.
A mantle plume is a proposed mechanism of convection within the Earth's mantle, hypothesized to explain anomalous volcanism. Because the plume head partially melts on reaching shallow depths, a plume is often invoked as the cause of volcanic hotspots, such as Hawaii or Iceland, and large igneous provinces such as the Deccan and Siberian Traps. Some such volcanic regions lie far from tectonic plate boundaries, while others represent unusually large-volume volcanism near plate boundaries.
A craton is an old and stable part of the continental lithosphere, which consists of Earth's two topmost layers, the crust and the uppermost mantle. Having often survived cycles of merging and rifting of continents, cratons are generally found in the interiors of tectonic plates; the exceptions occur where geologically recent rifting events have separated cratons and created passive margins along their edges. Cratons are characteristically composed of ancient crystalline basement rock, which may be covered by younger sedimentary rock. They have a thick crust and deep lithospheric roots that extend as much as several hundred kilometres into Earth's mantle.
Earth's mantle is a layer of silicate rock between the crust and the outer core. It has a mass of 4.01×1024 kg (8.84×1024 lb) and thus makes up 67% of the mass of Earth. It has a thickness of 2,900 kilometers (1,800 mi) making up about 46% of Earth's radius and 84% of Earth's volume. It is predominantly solid but, on geologic time scales, it behaves as a viscous fluid, sometimes described as having the consistency of caramel. Partial melting of the mantle at mid-ocean ridges produces oceanic crust, and partial melting of the mantle at subduction zones produces continental crust.
Mountain formation refers to the geological processes that underlie the formation of mountains. These processes are associated with large-scale movements of the Earth's crust. Folding, faulting, volcanic activity, igneous intrusion and metamorphism can all be parts of the orogenic process of mountain building. The formation of mountains is not necessarily related to the geological structures found on it.
Tectonic uplift is the geologic uplift of Earth's surface that is attributed to plate tectonics. While isostatic response is important, an increase in the mean elevation of a region can only occur in response to tectonic processes of crustal thickening, changes in the density distribution of the crust and underlying mantle, and flexural support due to the bending of rigid lithosphere.
Mantle convection is the very slow creeping motion of Earth's solid silicate mantle as convection currents carry heat from the interior to the planet's surface.
Volcanic passive margins (VPM) and non-volcanic passive margins are the two forms of transitional crust that lie beneath passive continental margins that occur on Earth as the result of the formation of ocean basins via continental rifting. Initiation of igneous processes associated with volcanic passive margins occurs before and/or during the rifting process depending on the cause of rifting. There are two accepted models for VPM formation: hotspots/mantle plumes and slab pull. Both result in large, quick lava flows over a relatively short period of geologic time. VPM's progress further as cooling and subsidence begins as the margins give way to formation of normal oceanic crust from the widening rifts.
Cape Verde is a volcanic archipelago situated above an oceanic rise that puts the base of the islands 2 kilometers (1.2 mi) above the rest of the seafloor. Cape Verde has been identified as a hotspot and the majority of geoscientists have argued that the archipelago is underlain by a mantle plume and that this plume is responsible for the volcanic activity and associated geothermal anomalies.
Eclogitization is the tectonic process in which the high-pressure, metamorphic facies, eclogite, is formed. This leads to an increase in the density of regions of Earth's crust, which leads to changes in plate motion at convergent boundaries.
The lithosphere–asthenosphere boundary represents a mechanical difference between layers in Earth's inner structure. Earth's inner structure can be described both chemically and mechanically. The lithosphere–asthenosphere boundary lies between Earth's cooler, rigid lithosphere and the warmer, ductile asthenosphere. The actual depth of the boundary is still a topic of debate and study, although it is known to vary according to the environment.
The subcontinental lithospheric mantle (SCLM) is the uppermost solid part of Earth's mantle associated with the continental lithosphere.
Ridge push is a proposed driving force for plate motion in plate tectonics that occurs at mid-ocean ridges as the result of the rigid lithosphere sliding down the hot, raised asthenosphere below mid-ocean ridges. Although it is called ridge push, the term is somewhat misleading; it is actually a body force that acts throughout an ocean plate, not just at the ridge, as a result of gravitational pull. The name comes from earlier models of plate tectonics in which ridge push was primarily ascribed to upwelling magma at mid-ocean ridges pushing or wedging the plates apart.
The plate theory is a model of volcanism that attributes all volcanic activity on Earth, even that which appears superficially to be anomalous, to the operation of plate tectonics. According to the plate theory, the principal cause of volcanism is extension of the lithosphere. Extension of the lithosphere is a function of the lithospheric stress field. The global distribution of volcanic activity at a given time reflects the contemporaneous lithospheric stress field, and changes in the spatial and temporal distribution of volcanoes reflect changes in the stress field. The main factors governing the evolution of the stress field are:
Intraplate volcanism is volcanism that takes place away from the margins of tectonic plates. Most volcanic activity takes place on plate margins, and there is broad consensus among geologists that this activity is explained well by the theory of plate tectonics. However, the origins of volcanic activity within plates remains controversial.