Tectonic subsidence
Encyclopedia
Tectonic subsidence is the sinking
of the Earth's crust
relative to neighboring crust or other point of reference. Components of plate movement
create several environments in which subsidence occurs including, passive margin
s, aulacogen
s, fore-arc basins, foreland basin
s, intercontinental basins, and pull apart basin
s, and are largely accommodated by faulting. Three mechanisms: extension, cooling, and loading, are common in the tectonic environments in which subsidence occurs.
is accomplished by normal faulting or larger scale rifting
, where the lithosphere undergoes horizontal extension. The crust will stretch until faulting occurs, via systems of either normal faults, creating horsts and grabens, or listric faults. These fault systems allow the region to stretch, while simultaneously decreasing its thickness. As the crust is thinned it subsides over time, relative to the thicker undeformed crust, by isostasy
(maintenance of gravitational equilibrium).
will rise closer to the surface, heating the overlying plates by processes of conduction and advection
from igneous intrusions. Heating of the lithosphere decreases its density and uplifts the crust due to its buoyancy compared to the undeformed cooler crust. Once the thinning is complete the heated portion of the crust slowly cools and becomes heavier causing what is known as post-rift subsidence.
or orogenic processes which causes crustal depression and subsidence. Sediments from erosion
accumulate in lowest elevation possible due to gravity, these areas are called accommodation spaces. The rate and magnitude of accumulation controls the rate at which subsidence occurs. Orogeny
is the process of mountain building which creates large loads on the earth’s surface, causing flexural depressions in the adjacent lithosphere
.
Passive margins
, like the gulf coast of the United States, are transition regions from continental to oceanic plates where there is no active plate boundary. These margins are created by old successful rifts that have achieved break-up, overlaid by sediment. The formation of passive margins begins with extension of crust due to the rifting apart of two continental plates, as the plates move the crust thins and subsides. The asthenosphere, moving towards the surface as the crust thins, sequentially heats the overlying crust making it less dense. Accumulation of non-marine sediment forms alluvial fans in the low-lying area. Continuous rifting creates listric fault systems and further subsidence resulting in the creation of an oceanic basin and corresponding sea creating marine sediment deposits. Once the rifting ceases, the crust subsides further due to cooling, and continued sedimentation accumulates on top of the ocean basin increasing the subsidence by progressive loading.
, like the environment in Quebec, Canada, are a product of failed rifts in which the continent doesn’t completely split. Similar to formation of passive margins, subsidence occurs as a product of lithospheric thinning due to spreading forces of continental plates. As spreading occurs sedimentation occurs applying a load on the region. Once the tensional forces cease subsidence continues due to cooling.
Foreland basins
are flexural depressions created by large fold thrust sheets that form toward the undeformed continent, these occur regularly east of the Rocky Mountains. Foreland basin subsidence is attributed to the isostatic response of the lithosphere from an orogenic load, basin growth is controlled by load migration and corresponding sedimentation. There is increased subsidence in the adjacent basin as the load migrates along with delayed subsidence in the basin further foreland and is seen in higher magnitudes with broader basins. The basins give accommodation space for sediment eroded from the fold thrust sheet with thickening sediment layers toward the thrust belt and thinning layers away from the thrust belt creating differential subsidence.
s have relatively short lived subsidence that form from transtensional strike-slip faults. Moderate strike-slip faults create extensional releasing bends and opposing walls pull apart from each other. Normal faults occur which induces small scale subsidence in the area and ceases once the fault stops propagating. A larger scale version is a pull-apart basin, such as the Los Angeles Basin
, larger magnitude extension occurs causing lithospheric thinning, sediments fill the basin and cooling occurs after the fault ceases to propagate.
are formed when oceanic plate subducts under a less dense continental plate. As the two plates collide sediment is scraped off the subducting oceanic plate forming an accrectionary wedge between the subducting oceanic plate and a sliver of oceanic plate, like what occurred in the Central Valley of California. Between this wedge and the volcanic arc
there is a sea floor depression zone. There are several poorly understood mechanisms varying from basin to basin. Extension and extensional faulting occurs by relative motions between accretionary wedge and volcanic arc. Abnormal cooling effects from the subducting oceanic plate and crustal thinning due to tectonic erosion and underplating
are also thought to be occurring.
Subsidence
Subsidence is the motion of a surface as it shifts downward relative to a datum such as sea-level. The opposite of subsidence is uplift, which results in an increase in elevation...
of the Earth's crust
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...
relative to neighboring crust or other point of reference. Components of plate movement
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
create several environments in which subsidence occurs including, passive margin
Passive margin
A passive margin is the transition between oceanic and continental crust which is not an active plate margin. It is constructed by sedimentation above an ancient rift, now marked by transitional crust. Continental rifting creates new ocean basins. Eventually the continental rift forms a mid-oceanic...
s, aulacogen
Aulacogen
In geology, an aulacogen is a failed arm of a triple junction of a plate tectonics rift system. A triple junction beneath a continental plate initiates a three way breakup of the continental plate. As the continental break-up develops one of the three spreading ridges typically fails or stops...
s, fore-arc basins, foreland basin
Foreland basin
A foreland basin is a depression 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...
s, intercontinental basins, and pull apart basin
Pull Apart Basin
250px|thumb|[[Cami Lake]] in [[Tierra del Fuego]] develops on a [[Patagonian Ice Sheet|glacially]] excavated pull apart basin along the [[Magallanes-Fagnano Fault]], hence its elongated form...
s, and are largely accommodated by faulting. Three mechanisms: extension, cooling, and loading, are common in the tectonic environments in which subsidence occurs.
Extension
Extensional tectonicsExtensional tectonics
Extensional tectonics is concerned with the structures formed, and the tectonic processes associated with, the stretching of the crust or lithosphere.-Deformation styles:...
is accomplished by normal faulting or larger scale rifting
Rift
In geology, a rift or chasm is a place where the Earth's crust and lithosphere are being pulled apart and is an example of extensional tectonics....
, where the lithosphere undergoes horizontal extension. The crust will stretch until faulting occurs, via systems of either normal faults, creating horsts and grabens, or listric faults. These fault systems allow the region to stretch, while simultaneously decreasing its thickness. As the crust is thinned it subsides over time, relative to the thicker undeformed crust, by isostasy
Isostasy
Isostasy is a term used in geology to refer to the state of gravitational equilibrium between the earth's lithosphere and asthenosphere such that the tectonic plates "float" at an elevation which depends on their thickness and density. This concept is invoked to explain how different topographic...
(maintenance of gravitational equilibrium).
Cooling
In areas of crustal thinning, the mantle may melt due to decompression and the asthenosphereAsthenosphere
The asthenosphere is the highly viscous, mechanically weak and ductilely-deforming region of the upper mantle of the Earth...
will rise closer to the surface, heating the overlying plates by processes of conduction and advection
Advection
Advection, in chemistry, engineering and earth sciences, is a transport mechanism of a substance, or a conserved property, by a fluid, due to the fluid's bulk motion in a particular direction. An example of advection is the transport of pollutants or silt in a river. The motion of the water carries...
from igneous intrusions. Heating of the lithosphere decreases its density and uplifts the crust due to its buoyancy compared to the undeformed cooler crust. Once the thinning is complete the heated portion of the crust slowly cools and becomes heavier causing what is known as post-rift subsidence.
Loading
Loading is adding weight over an area by sedimentationSedimentation
Sedimentation is the tendency for particles in suspension to settle out of the fluid in which they are entrained, and come to rest against a barrier. This is due to their motion through the fluid in response to the forces acting on them: these forces can be due to gravity, centrifugal acceleration...
or orogenic processes which causes crustal depression and subsidence. Sediments from erosion
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...
accumulate in lowest elevation possible due to gravity, these areas are called accommodation spaces. The rate and magnitude of accumulation controls the rate at which subsidence occurs. Orogeny
Orogeny
Orogeny refers to forces and events leading to a severe structural deformation of the Earth's crust due to the engagement of tectonic plates. Response to such engagement results in the formation of long tracts of highly deformed rock called orogens or orogenic belts...
is the process of mountain building which creates large loads on the earth’s surface, causing flexural depressions in the adjacent lithosphere
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...
.
Passive margins
Passive margin
A passive margin is the transition between oceanic and continental crust which is not an active plate margin. It is constructed by sedimentation above an ancient rift, now marked by transitional crust. Continental rifting creates new ocean basins. Eventually the continental rift forms a mid-oceanic...
, like the gulf coast of the United States, are transition regions from continental to oceanic plates where there is no active plate boundary. These margins are created by old successful rifts that have achieved break-up, overlaid by sediment. The formation of passive margins begins with extension of crust due to the rifting apart of two continental plates, as the plates move the crust thins and subsides. The asthenosphere, moving towards the surface as the crust thins, sequentially heats the overlying crust making it less dense. Accumulation of non-marine sediment forms alluvial fans in the low-lying area. Continuous rifting creates listric fault systems and further subsidence resulting in the creation of an oceanic basin and corresponding sea creating marine sediment deposits. Once the rifting ceases, the crust subsides further due to cooling, and continued sedimentation accumulates on top of the ocean basin increasing the subsidence by progressive loading.
Aulacogens
AulacogensAulacogen
In geology, an aulacogen is a failed arm of a triple junction of a plate tectonics rift system. A triple junction beneath a continental plate initiates a three way breakup of the continental plate. As the continental break-up develops one of the three spreading ridges typically fails or stops...
, like the environment in Quebec, Canada, are a product of failed rifts in which the continent doesn’t completely split. Similar to formation of passive margins, subsidence occurs as a product of lithospheric thinning due to spreading forces of continental plates. As spreading occurs sedimentation occurs applying a load on the region. Once the tensional forces cease subsidence continues due to cooling.
Foreland basins
Foreland basins
Foreland basin
A foreland basin is a depression 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...
are flexural depressions created by large fold thrust sheets that form toward the undeformed continent, these occur regularly east of the Rocky Mountains. Foreland basin subsidence is attributed to the isostatic response of the lithosphere from an orogenic load, basin growth is controlled by load migration and corresponding sedimentation. There is increased subsidence in the adjacent basin as the load migrates along with delayed subsidence in the basin further foreland and is seen in higher magnitudes with broader basins. The basins give accommodation space for sediment eroded from the fold thrust sheet with thickening sediment layers toward the thrust belt and thinning layers away from the thrust belt creating differential subsidence.
Intracontinental basins
Intracontinental basins are large areal depression that are tectonically inactive and not located in the proximity of any plate boundaries. These basins experience long-lived and slow subsidence and occur all over the world. It is generally thought that this subsidence is a product of long-term cooling since the break-up of Pangaea, however there is long standing debate. Other hypotheses state that the long term subsidence is a product of interaction of deformation around the basins’ edge. There are others who believe that subsidence is driven by more deep earth dynamics.Pull apart basins
Pull apart basinPull Apart Basin
250px|thumb|[[Cami Lake]] in [[Tierra del Fuego]] develops on a [[Patagonian Ice Sheet|glacially]] excavated pull apart basin along the [[Magallanes-Fagnano Fault]], hence its elongated form...
s have relatively short lived subsidence that form from transtensional strike-slip faults. Moderate strike-slip faults create extensional releasing bends and opposing walls pull apart from each other. Normal faults occur which induces small scale subsidence in the area and ceases once the fault stops propagating. A larger scale version is a pull-apart basin, such as the Los Angeles Basin
Los Angeles Basin
The Los Angeles Basin is the coastal sediment-filled plain located between the Peninsular and Transverse ranges in southern California in the United States containing the central part of the city of Los Angeles as well as its southern and southeastern suburbs...
, larger magnitude extension occurs causing lithospheric thinning, sediments fill the basin and cooling occurs after the fault ceases to propagate.
Forearc basins
Forearc basinsForearc
A forearc or forarc, also called arc-trench gap is a depression in the sea floor located between a subduction zone and an associated volcanic arc. It is typically filled with sediments from the adjacent landmass and the island arc in addition to trapped oceanic crustal material...
are formed when oceanic plate subducts under a less dense continental plate. As the two plates collide sediment is scraped off the subducting oceanic plate forming an accrectionary wedge between the subducting oceanic plate and a sliver of oceanic plate, like what occurred in the Central Valley of California. Between this wedge and the volcanic arc
Volcanic arc
A volcanic arc is a chain of volcanoes positioned in an arc shape as seen from above. Offshore volcanoes form islands, resulting in a volcanic island arc. Generally they result from the subduction of an oceanic tectonic plate under another tectonic plate, and often parallel an oceanic trench...
there is a sea floor depression zone. There are several poorly understood mechanisms varying from basin to basin. Extension and extensional faulting occurs by relative motions between accretionary wedge and volcanic arc. Abnormal cooling effects from the subducting oceanic plate and crustal thinning due to tectonic erosion and underplating
Underplating
Underplating is the accumulaton of partial melts at the base the crust where an ocean plate is subducting under continental crust. Underplating is the result of partial melts being produced in the mantle wedge above a subducting plate. The partial melting is induced by a lowering of the solidus by...
are also thought to be occurring.