Thermal subsidence

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In geology and geophysics, thermal subsidence is a mechanism of subsidence in which conductive cooling of the mantle thickens the lithosphere and causes it to decrease in elevation. This is because of thermal contraction: as mantle material cools and becomes part of the mechanically rigid lithosphere, it becomes more dense than the surrounding material. Additional material added to the lithosphere thickens it and further causes a buoyant decrease in the elevation of the lithosphere. This creates accommodation space into which sediments can deposit, forming a sedimentary basin.

Geology The study of the composition, structure, physical properties, and history of Earths components, and the processes by which they are shaped.

Geology is an earth science concerned with the solid Earth, the rocks of which it is composed, and the processes by which they change over time. Geology can also refer to the study of the solid features of any terrestrial planet or natural satellite such as Mars or the Moon. Modern geology significantly overlaps all other earth sciences, including hydrology and the atmospheric sciences, and so is treated as one major aspect of integrated earth system science and planetary science.

Geophysics Physics of the Earth and its vicinity

Geophysics is a subject of natural science concerned with the physical processes and physical properties of the Earth and its surrounding space environment, and the use of quantitative methods for their analysis. The term geophysics sometimes refers only to the geological applications: Earth's shape; its gravitational and magnetic fields; its internal structure and composition; its dynamics and their surface expression in plate tectonics, the generation of magmas, volcanism and rock formation. However, modern geophysics organizations use a broader definition that includes the water cycle including snow and ice; fluid dynamics of the oceans and the atmosphere; electricity and magnetism in the ionosphere and magnetosphere and solar-terrestrial relations; and analogous problems associated with the Moon and other planets.

Subsidence the motion of a surface (usually, the Earths surface) as it shifts downward relative to a datum such as sea-level

Subsidence is the sudden sinking or gradual downward settling of the ground's surface with little or no horizontal motion. The definition of subsidence is not restricted by the rate, magnitude, or area involved in the downward movement. It may be caused by natural processes or by human activities. The former include various karst phenomena, thawing of permafrost, consolidation, oxidation of organic soils, slow crustal warping, normal faulting, caldera subsidence, or withdrawal of fluid lava from beneath a solid crust. The human activities include sub-surface mining or extraction of underground fluids, e. g. petroleum, natural gas, or groundwater. Ground subsidence is of global concern to geologists, geotechnical engineers, surveyors, engineers, urban planners, landowners, and the public in general.

Contents

Causes

Thermal subsidence can occur anywhere in which a temperature differential exists between a section of the lithosphere and its surroundings. There are a variety of contributing factors that can initiate thermal subsidence or affect the process as it is ongoing. [1]

Delamination

As endogenous and exogenous processes cause denudation of the earth's surface, lower, warmer sections of the lithosphere are exposed to relative differences in weight and density. This relative difference creates buoyancy. Isostatic uplift can then further expose the lithosphere to conductive cooling, causing a “rise and fall” phenomenon as warmer, less dense rock layers are pushed or buoyed up, then cooled, causing it to contract and sink back down. [2]

Denudation processes that cause the wearing away of the Earths surface by moving water, by ice, by wind and by waves, leading to a reduction in elevation and in relief of landforms and of landscapes

In geology, denudation involves the processes that cause the wearing away of the Earth's surface by moving water, by ice, by wind and by waves, leading to a reduction in elevation and in relief of landforms and of landscapes. Endogenous processes such as volcanoes, earthquakes, and plate tectonics uplift and expose continental crust to the exogenous processes of weathering, of erosion, and of mass wasting.

Buoyancy An upward force that opposes the weight of an object immersed in fluid

In physics, buoyancy or upthrust, is an upward force exerted by a fluid that opposes the weight of an immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the pressure at the bottom of a column of fluid is greater than at the top of the column. Similarly, the pressure at the bottom of an object submerged in a fluid is greater than at the top of the object. The pressure difference results in a net upward force on the object. The magnitude of the force is proportional to the pressure difference, and is equivalent to the weight of the fluid that would otherwise occupy the volume of the object, i.e. the displaced fluid.

Conduction

The conditions to create thermal subsidence can be initiated by various forms of uplift and denudation, but the actual process of thermal subsidence is governed by the loss of heat via thermal conduction. Contact with surrounding rock or the surface causes heat to leach out of a section of the lithosphere. As the lithosphere cools, it causes the rock to contract. [3]

Isostasy

When conduction causes a section of the lithosphere to contract and increase in density, it does not directly add mass to the rock. Instead, it causes the volume to decrease, increasing the mass of the section for a given area. The lithosphere is isostatic with the mantle; its weight is supported by the relative density of the surrounding rock. When a section cools and its density increases, it sinks, causing the relative elevation to decrease. This can create a basin in which sediments are deposited, which adds weight on top of the sinking section of lithosphere and increase the total mass of the section per unit area, causing it to sink further. [4]

Isostasy 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.

A mantle is a layer inside a planetary body bounded below by a core and above by a crust. Mantles are made of rock or ices, and are generally the largest and most massive layer of the planetary body. Mantles are characteristic of planetary bodies that have undergone differentiation by density. All terrestrial planets, a number of asteroids, and some planetary moons have mantles.

Effects

Thermal subsidence can have an effect on island formation. Isostatic uplift can be balanced with thermal subsidence in response to erosion on islands without barrier reefs, which sink only when subjected to wave erosion. However, volcanic islands and seamounts with barrier reefs are shielded from wave and stream erosion, and thus the countervailing isostatic uplift is eliminated, causing them to subside and create an atoll. [5]

Coral reef Outcrop of rock in the sea formed by the growth and deposit of stony coral skeletons

A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups.

Erosion Processes which remove soil and rock from one place on the Earths crust, then transport it to another location where it is deposited

In earth science, erosion is the action of surface processes that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location. This natural process is caused by the dynamic activity of erosive agents, that is, water, ice (glaciers), snow, air (wind), plants, animals, and humans. In accordance with these agents, erosion is sometimes divided into water erosion, glacial erosion, snow erosion, wind (aeolic) erosion, zoogenic erosion, and anthropogenic erosion. The particulate breakdown of rock or soil into clastic sediment is referred to as physical or mechanical erosion; this contrasts with chemical erosion, where soil or rock material is removed from an area by its dissolving into a solvent, followed by the flow away of that solution. Eroded sediment or solutes may be transported just a few millimetres, or for thousands of kilometres.

Seamount A mountain rising from the ocean seafloor that does not reach to the waters surface

A seamount is a mountain rising from the ocean seafloor that does not reach to the water's surface, and thus is not an island, islet or cliff-rock. Seamounts are typically formed from extinct volcanoes that rise abruptly and are usually found rising from the seafloor to 1,000–4,000 m (3,300–13,100 ft) in height. They are defined by oceanographers as independent features that rise to at least 1,000 m (3,281 ft) above the seafloor, characteristically of conical form. The peaks are often found hundreds to thousands of meters below the surface, and are therefore considered to be within the deep sea. During their evolution over geologic time, the largest seamounts may reach the sea surface where wave action erodes the summit to form a flat surface. After they have subsided and sunk below the sea surface such flat-top seamounts are called "guyots" or "tablemounts"

Metamorphism

Thermal subsidence can drive metamorphism in rocks. The conduction of heat out of a section of lithosphere causes the rock to thicken and become more insulated to heat flowing in from the mantle; as this thicker section is buried by the descending column of lithosphere, it descends into surrounding rock layers with a higher relative geothermal gradient. This gradient can cause metamorphism in rocks, as seen in South Australia. [6]

Eustasy

Eustasy refers to a change in the relative sea level. It can have effects on the thermal subsidence during formation of geological features such as mountain ranges. Sea level often changes in response to the formation of glaciers on land; the weight of these glaciers or the absence thereof can influence the overall rate of thermal subsidence. [7]

Petroleum Formation

As the lithosphere cools and subsides, a sedimentary basin can be formed on top of the subsiding mass. The characteristics of the basement sediments can produce conditions conducive to the conversion of kerogen into petroleum. The gigantic Wilmington Oil Field in the Los Angeles basin was formed as a result of this process. [8]

Related Research Articles

Orogeny The formation of mountain ranges

An orogeny is an event that leads to both structural deformation and compositional differentiation of the Earth's lithosphere at convergent plate margins. An orogen or orogenic belt develops when a continental plate crumples and is pushed upwards to form one or more mountain ranges; this involves a series of geological processes collectively called orogenesis.

Sedimentary basin Regions of long-term subsidence creating space for infilling by sediments

Sedimentary basins are regions of Earth of long-term subsidence creating accommodation space for infilling by sediments. The subsidence can result from a variety of causes that include: the thinning of underlying crust, sedimentary, volcanic, and tectonic loading, and changes in the thickness or density of adjacent lithosphere. Sedimentary basins occur in diverse geological settings usually associated with plate tectonic activity. Basins are classified structurally in various ways, with a primary classifications distinguishing among basins formed in various plate tectonic regime, the proximity of the basin to the active plate margins, and whether oceanic, continental or transitional crust underlies the basin. Basins formed in different plate tectonic regimes vary in their preservation potential. On oceanic crust, basins are likely to be subducted, while marginal continental basins may be partially preserved, and intracratonic basins have a high probability of preservation. As the sediments are buried, they are subjected to increasing pressure and begin the process of lithification. A number of basins formed in extensional settings can undergo inversion which has accounted for a number of the economically viable oil reserves on earth which were formerly basins.

Post-glacial rebound Rise of land masses that were depressed by the huge weight of ice sheets during the last glacial period

Post-glacial rebound is the rise of land masses after the lifting of the huge weight of ice sheets during the last glacial period, which had caused isostatic depression. Post-glacial rebound and isostatic depression are phases of glacial isostasy, the deformation of the Earth's crust in response to changes in ice mass distribution. The direct raising effects of post-glacial rebound are readily apparent in parts of Northern Eurasia, Northern America, Patagonia, and Antarctica. However, through the processes of ocean siphoning and continental levering, the effects of post-glacial rebound on sea level are felt globally far from the locations of current and former ice sheets.

Ridge push or sliding plate force 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.

Forearc The region between an oceanic trench and the associated volcanic arc

A forearc is the region between an oceanic trench and the associated volcanic arc. Forearc regions are found at convergent margins, and include any accretionary wedge and forearc basin that may be present. Due to tectonic stresses as one tectonic plate rides over another, forearc regions are sources for great thrust earthquakes

Diastrophism refers to deformation of the Earth's crust, and more especially to folding and faulting. Diastrophism can be considered part of geotectonics. Diastrophism comes from the Greek word meaning a twisting.

Tectonic uplift The portion of the total geologic uplift of the mean earth surface that is not attributable to an isostatic response to unloading

Tectonic uplift is the portion of the total geologic uplift of the mean Earth surface that is not attributable to an isostatic response to unloading. 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.

Back-stripping is a geophysical analysis technique used on sedimentary rock sequences - the technique is used to quantitatively estimate the depth that the basement would be in the absence of sediment and water loading. This depth provides a measure of the unknown tectonic driving forces that are responsible for basin formation. By comparing backstripped curves to theoretical curves for basin subsidence and uplift it is possible to deduce information on the basin forming mechanisms.

Rock cycle Transitions through geologic time among the three main rock types: sedimentary, metamorphic, and igneous

The rock cycle is a basic concept in geology that describes the time-consuming transitions through geologic time among the three main rock types: sedimentary, metamorphic, and igneous. As the adjacent diagram illustrates, each of the types of rocks is altered or destroyed when it is forced out of its equilibrium conditions. An igneous rock such as basalt may break down and dissolve when exposed to the atmosphere, or melt as it is subducted under a continent. Due to the driving forces of the rock cycle, plate tectonics and the water cycle, rocks do not remain in equilibrium and are forced to change as they encounter new environments. The rock cycle is an illustration that explains how the three rock types are related to each other, and how processes change from one type to another over time. This cyclical aspect makes rock change a geologic cycle and, on planets containing life, a biogeochemical cycle.

A cratonic sequence is a very large-scale lithostratographic sequence that covers a complete marine transgressive-regressive cycle across a craton. They are also known as "megasequences", "stratigraphic sequences", "sloss sequence", "supersequence" or simply "sequences". In plain English, it is the geological evidence of the sea level rising and then falling, thereby depositing layers of sediment onto an area of ancient rock called a craton. Places such as the Grand Canyon are a good visual example of this, apparent in the layers deposited over time.

Passive margin The transition between oceanic and continental lithosphere that is not an active plate margin

A passive margin is the transition between oceanic and continental lithosphere that is not an active plate margin. A passive margin forms by sedimentation above an ancient rift, now marked by transitional lithosphere. Continental rifting creates new ocean basins. Eventually the continental rift forms a mid-ocean ridge and the locus of extension moves away from the continent-ocean boundary. The transition between the continental and oceanic lithosphere that was originally created by rifting is known as a passive margin.

Foreland basin A structural basin that develops adjacent and parallel to a mountain belt

A foreland basin is a structural basin that develops adjacent and parallel to a mountain belt. Foreland basins form because the immense mass created by crustal thickening associated with the evolution of a mountain belt causes the lithosphere to bend, by a process known as lithospheric flexure. The width and depth of the foreland basin is determined by the flexural rigidity of the underlying lithosphere, and the characteristics of the mountain belt. The foreland basin receives sediment that is eroded off the adjacent mountain belt, filling with thick sedimentary successions that thin away from the mountain belt. Foreland basins represent an endmember basin type, the other being rift basins. Space for sediments is provided by loading and downflexure to form foreland basins, in contrast to rift basins, where accommodation space is generated by lithospheric extension.

In geology, a forebulge is a flexural bulge in front of a load on the lithosphere. The load causes the lithosphere to flex by depressing the plate beneath it. Because of the flexural rigidity of the lithosphere, the area around the load is uplifted by a height that is 4% of that of the depression under the load. The load and the resulting flexure place stress on the mantle, causing it to flow into the area around the loaded area. The subsidence of the area under the load and the uplift of the forebulge continue until the load is in isostatic equilibrium, a process that takes on the order of 10,000 to 20,000 years. Because of the coupling with the mantle, the rate of forebulge formation and collapse is controlled by mantle viscosity.

Delamination (geology)

In geophysics, delamination refers to the loss and sinking (foundering) of the portion of the lowermost lithosphere from the tectonic plate to which it was attached.

Erosion and tectonics

The interaction between erosion and tectonics has been a topic of debate since the early 1990s. While the tectonic effects on surface processes such as erosion have long been recognized, the opposite has only recently been addressed. The primary questions surrounding this topic are what types of interactions exist between erosion and tectonics and what are the implications of these interactions. While this is still a matter of debate, one thing is clear, the Earth's landscape is a product of two factors: tectonics, which can create topography and maintain relief through surface and rock uplift, and climate, which mediates the erosional processes that wear away upland areas over time. The interaction of these processes can form, modify, or destroy geomorphic features on the Earth’s surface.

Magmatic underplating Trapping of basaltic magmas during their rise to the surface at the Mohorovičić discontinuity or within the crust

Magmatic underplating occurs when basaltic magmas are trapped during their rise to the surface at the Mohorovičić discontinuity or within the crust. Entrapment of magmas within the crust occurs due to the difference in relative densities between the rising magma and the surrounding rock. Magmatic underplating can be responsible for thickening of the crust when the magma cools. Geophysical seismic studies utilize the differences in densities to identify underplating that occurs at depth.

Tectonic subsidence is the sinking of the Earth's crust on a large scale, relative to crustal-scale features or the geoid. The movement of crustal plates and accommodation spaces created by faulting create subsidence on a large scale in a variety of environments, including passive margins, aulacogens, fore-arc basins, foreland basins, intercontinental basins and pull-apart basins. Three mechanisms are common in the tectonic environments in which subsidence occurs: extension, cooling and loading.

A river anticline is a geologic structure that is formed by the focused uplift of rock caused by high erosion rates from large rivers relative to the surrounding areas. An anticline is a fold that is concave down, whose limbs are dipping away from its axis, and whose oldest units are in the middle of the fold. These features form in a number of structural settings. In the case of river anticlines, they form due to high erosion rates, usually in orogenic settings. In a mountain building setting, like that of the Himalaya or the Andes, erosion rates are high and the river anticline's fold axis will trend parallel to a major river. When river anticlines form, they have a zone of uplift between 50-80 kilometers wide along the rivers that form them.

Mountains are widely distributed across the surface of Io, the innermost large moon of Jupiter. There are about 115 named mountains; the average length is 157 km (98 mi) and the average height is 6,300 m (20,700 ft). The longest is 570 km (350 mi), and the highest is Boösaule Montes, at 17,500 metres (57,400 ft), taller than any mountain on Earth. Ionian mountains often appear as large, isolated structures; no global tectonic pattern is evident, unlike on Earth, where plate tectonics is dominant.

References

  1. Bradley, Dwight. “Subsidence in Late Paleozoic basins in the northern Appalachians.” Tectonics, vol. 1, issue 1, p. 107-123. February 1982.
  2. Avigad, Dov, Zohar Gvirtzman. “Late Neoproterozoic rise and fall of the northern Arabian-Nubian shield: the role of lithospheric mantle delamination and subsequent thermal subsidence.” Tectonophysics, vol 477, issues 3-4, p. 217-228. November 2009.
  3. Watts, A. B. “Isostasy and Flexture of the Lithosphere.” Department of Earth Sciences, Oxford University, Cambridge Free Press 2001.
  4. Watts, A. B. “Isostasy and Flexture of the Lithosphere.” Department of Earth Sciences, Oxford University, Cambridge Free Press 2001.
  5. Menard, H. W. “Insular Erosion, Isostasy, and Subsidence.” Science magazine, vol. 220, p. 914-918. May 1983.
  6. Sandiford, Mike, Martin Hand, Sandra McLaren. “High geothermal gradient metamorphism during thermal subsidence.” Earth and Planetary Science Letters, volume 163, issues 1-4, p. 149-165, November 1998.
  7. Bond, Gerard, Michelle Kominz, William Devlin. “Thermal subsidence and eustasy in the Lower Paleozoic miogeocline of western North America.” Nature 306, 773-779. December 1983.
  8. Turcotte, D. L., D. C. McAdoo. “Thermal subsidence and petroleum generation in the southwestern block of the Los Angeles basin, California.” Journal of Geophysical Research, vol. 84, issue B7, p. 3460-3464. July 1979.