geodynamics Geodynamics is a subfield of geophysics dealing with dynamics of the Earth. It applies physics, chemistry and mathematics to the understanding of how mantle convection leads to plate tectonics and geologic phenomena such as seafloor spreading, ...

, delamination refers to the loss and sinking (foundering) of the portion of the lowermost

lithosphere 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 ...

from the

tectonic plate Plate tectonics (, ) is the scientific theory that the Earth's lithosphere comprises a number of large tectonic plates, which have been slowly moving since 3–4 billion years ago. The model builds on the concept of , an idea developed durin ...

to which it was attached.

Mechanism

The outer portion of the Earth is divided into an upper,

layer and a lower,

asthenosphere The asthenosphere () is the mechanically weak and ductile region of the upper mantle of Earth. It lies below the lithosphere, at a depth between c. below the surface, and extends as deep as . However, the lower boundary of the asthenosphere i ...

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 In classical mechanics, a particle is in mechanical equilibrium if the net force on that particle is zero. By extension, a physical system made up of many parts is in mechanical equilibrium if the net force on each of its individual parts is ze ...

because the underlying mantle lithosphere has a greater

density Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...

than the asthenosphere below.Bird, P. (1979). Continental delamination and the Colorado Plateau. Journal of Geophysical Research: Solid Earth (1978–2012), 84 (B13), 7561-7571. The difference in densities can be explained by thermal expansion/contraction, composition, and phase changes.Kay, R. W., & Mahlburg Kay, S. (1993). Delamination and delamination magmatism. Negative

buoyancy Buoyancy (), or upthrust, is the force exerted by a fluid opposing the weight of a partially or fully immersed object (which may be also be a parcel of fluid). In a column of fluid, pressure increases with depth as a result of the weight of t ...

of the lower continental crust and mantle lithosphere drive delamination.Meissner, R., & Mooney, W. (1998). Weakness of the lower continental crust: a condition for delamination, uplift, and escape. Tectonophysics, 296(1), 47-60. 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 lower lithosphere must be denser than the asthenosphere. * The intrusion of more buoyant asthenosphere making contact with the crust and replacing dense lower lithosphere must occur. The

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, causi ...

transition from mafic granulite facies to the denser eclogite

facies In geology, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with distinctive characteristics. The characteristics can be any observable attribute of rocks (such as their overall appearance, composition, or con ...

in the lower portion of the crust is the main mechanism responsible for creating negative buoyancy of the lower lithosphere. The lower crust undergoes a density inversion, causing it to break off of the upper crust and sink into the mantle.Rollinson, H. R. (2009). Early Earth systems: a geochemical approach. John Wiley & Sons. 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. 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. Potential energy that drives the delamination is released as the low density, hot asthenosphere rises and replaces the higher density, cold lithosphere. Separation of lowermost crust and lithospheric mantle is controlled by the effective

viscosity Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...

of the upper continental crust. These processes often occur in environments of

rifting In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear Fault (geology), downfaulted depression, called a graben, or more commonly ...

, plume erosion,

continental collision In geology, continental collision is a phenomenon of plate tectonics that occurs at Convergent boundary, convergent boundaries. Continental collision is a variation on the fundamental process of subduction, whereby the subduction zone is destroy ...

or where there is convective instability. Convective instabilities facilitate delamination. The

convection Convection is single or Multiphase flow, multiphase fluid flow that occurs Spontaneous process, spontaneously through the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoy ...

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.Nielsen, S. B., Paulsen, G. E., Hansen, D. L., Gemmer, L., Clausen, O. R., Jacobsen, B. H., ... & Gallagher, K. (2002). Paleocene initiation of Cenozoic uplift in Norway. Geological Society, London, Special Publications, 196(1), 45-65.

Other factors in delamination

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. * If the viscosity of the upwelling asthenosphere is greater than that of the mantle lithosphere, delamination will stop. * The

upwelling Upwelling is an physical oceanography, oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water from deep water towards the ocean surface. It replaces the warmer and usually nutrient-depleted sur ...

asthenosphere forms two chilled, solid boundary layers on the top and bottom of the sill layer. This reduces the thickness of the portion of the lowermost crust which behaves viscously. *

Subsidence Subsidence is a general term for downward vertical movement of the Earth's surface, which can be caused by both natural processes and human activities. Subsidence involves little or no horizontal movement, which distinguishes it from slope mov ...

of the lithosphere acts to increase the thickness of the portion of the lowermost crust which behaves viscously. 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.

Geologic effects

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 A mountain is an elevated portion of the Earth's crust, generally with steep sides that show significant exposed bedrock. Although definitions vary, a mountain may differ from a plateau in having a limited summit area, and is usually higher t ...

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 Volcanism, vulcanism, volcanicity, or volcanic activity is the phenomenon where solids, liquids, gases, and their mixtures erupt to the surface of a solid-surface astronomical body such as a planet or a moon. It is caused by the presence of a he ...

. Delamination may thus account for some volcanic regions that have been attributed to mantle plumes in the past.Foulger, G. R. (2011). Plates vs plumes: A geological controversy. John Wiley & Sons.

Relation to tectonic processes

Delamination is seen in convergence zones, especially where continental-continental collisions occur. For example, delamination is seen in the

Tibetan Plateau The Tibetan Plateau, also known as the Qinghai–Tibet Plateau or Qingzang Plateau, is a vast elevated plateau located at the intersection of Central Asia, Central, South Asia, South, and East Asia. Geographically, it is located to the north of H ...

, 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. 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 A granitoid is a broad term referring to a diverse group of coarse-grained igneous rocks that are widely distributed across the globe, covering a significant portion of the Earth's exposed surface and constituting a large part of the continental ...

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. 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.

Geologic examples

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. 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. It is possible that the Sierra Nevada (US) is the only place on Earth where dense material is currently being removed from the crust.

References