Subduction subduction zone

principal axes remain perpendicular, but some other lines will be lengthening with each increment, and others will be shortening. There are some orientations that experience shortening first, and then lengthening. This leads to some complicated structures in rocks deformed by simple shear; for instance, folds produced by the shortening, and then exten-sional structures, such as faults or pull-apart structures known as boudens, superimposed on the early contractional structures.

Natural strains in rocks deform initially spherical objects into ellipsoids with elongate (prolate) or flattened (oblate) ellipsoids. All natural strains may be represented graphically on a graph known as a Flynn Diagram, which plots a = (X/ Y) versus b = (Y/Z). The number k =

Intermediate axis Minor axis

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For k= 0, strain ellipsoids are uniaxial oblate ellipsoids or pancakes. For 0 > k > 1, deformation is a flattening deformation, forming an oblate ellipsoid. For k = 1 the deformation is plane strain if the volume has remained constant. All simple shear deformations lie on this line. For 1 > k > infinity, the strain ellipsoids are uniaxial prolate ellipsoids, or cigar shapes.

See also convergent plate margin processes; deformation of rocks; divergent plate margin processes; fracture; mélange; metamor-phism and metamorphic rocks; plate tectonics; transform plate margin processes.

FURTHER READING

Hatcher, Robert D. Structural Geology, Principles, Concepts, and Problems. 2nd ed. Englewood Cliffs, N.J.: Prentice hall, 1995. Kious, Jacquelyne, and Robert I. Tilling. u.s. Geological survey. This Dynamic Earth: The story of Plate Tectonics. Available online. uRL: http://pubs.usgs. gov/gip/dynamic/dynamic.html. Last modified March 27, 2007.

Twiss, Robert J., and Eldredge M. Moores. Structural

Geology. New York: macmillan Press, 1992. van der Pluijm, Ben A., and stephen marshak. Earth Structure, An Introduction to Structural Geology and Tectonics. Boston: WCB-McG^-HM, 1997.

Flynn diagram showing fields of prolate, oblate, and plane strain subduction, subduction zone subduction zones are long, narrow belts where an oceanic litho-spheric plate descends beneath another lithospheric plate and enters the mantle in the processes of subduction. Two basic types of subduction zones exist, the first of which being where oceanic lithosphere of one plate descends beneath another oceanic plate, such as in the Philippines and marianas of the southwest Pacific. The second type of subduction zone forms where an oceanic plate descends beneath a continental upper plate, such as in the Andes of south America. Deep-sea trenches typically mark the place on the surface where the subducting plate bends to enter the mantle, and oceanic or continental margin arc systems form above subduction zones a couple of hundred miles (a few hundred kilometers) from the trench. As the oceanic plate enters the trench it must bend, forming a flexural bulge up to a few thousand feet (a couple of hundred meters) high, typically about 100 miles (161 km) before the oceanic plate enters the trench. A series of down-to-the-trench normal faults marks the outer trench slope on the down-going plate, in most cases. Trenches may be partly or nearly entirely filled with sediments, many of which become offscraped and attached to the accretionary prism on the overriding plate. The inner trench slope on the overriding plate typically is marked by these folded and complexly faulted and offscraped sediments, and distinctive disrupted complexes known as mélanges may form in this environment.

In ocean-ocean subduction systems the arc develops about 100-150 miles (150-200 km) from the trench. Immature or young oceanic island arcs are dominated by basaltic volcanism and may be mostly underwater, whereas mature systems have more intermediate volcanics and have more of the volcanic edifice protruding above sea level. A forearc basin, filled by sediments derived from the arc and uplifted parts of the accretionary prism, typically occupies the area between the arc and the accretionary prism. Many island arcs have back-arc basins developed on the opposite side of the arc, separating the arc from an older rifted arc or a continent.

Ocean-continent subduction systems are broadly similar to ocean-ocean systems, but the magmas must rise through continental crust so are chemically contaminated by this crust, becoming more silicic and enriched in certain sialic elements. Basalts, andesites, dacites, and even rhyolites are common in continental margin arc systems. ocean-continent subduction systems tend to also have concentrated deformation, including deep thrust faults, fold/thrust belts on the back-arc side of the arc, and significant crustal thickening. other continental margin arcs experience extension and may see rifting events that open back-arc basins that extend into marginal seas, or close. Extensive magmatic underplating also aids crustal thickening in continental margin subduction systems.

oceanic plates may be thought of as conductively cooling upper boundary layers of the Earth's convection cells, and in this context subduction zones are the descending limbs of the mantle convection cells. once subduction is initiated, the sinking of the dense downgoing slabs provides most of the driving forces needed to move the lithospheric plates and force seafloor spreading at divergent boundaries where the mantle cells are upwelling.

The amount of material cycled from the lithosphere back into the mantle of the Earth in subduction zones is enormous, making subduction zones the planet's largest chemical recycling systems. Many of the sedimentary layers and some of the upper oceanic crust are scraped off the downgoing slabs and added to accretionary prisms on the front of the overlying arc systems. Hydrated minerals and sediments release much of their trapped seawater in the upper couple hundred miles (few hundred kilometers) of the descent into the deep Earth, adding water to the overlying mantle wedge and triggering melting that supplies the overlying arcs with magma. The material that is not released or offscraped and underplated in the upper couple hundred miles (few hundred kilometers) of subduction forms a dense slab that may go through several phase transitions and either flatten out at the 416-mile (670-km) mantle discontinuity or descend all the way to the core-mantle boundary. The slab material then rests and is heated at the core-mantle boundary for about a billion years, after which it may form a mantle plume that rises through the mantle to the surface. In this way, an overall material balance is maintained in subduction zone-mantle convection-plume systems.

Most continental crust has been created in subduction zone-arc systems of various ages stretching back to the Early Archean.

See also accretionary wedge; Andes Mountains; convergent plate margin processes; mantle plumes; mélange; ocean basin; ophiolites; plate tectonics.

FURTHER READING

Kious, Jacquelyne, and Robert I. Tilling. U.S. Geological Survey. This Dynamic Earth: The Story of Plate Tectonics. Available online. URL: http://pubs.usgs. gov/gip/dynamic/dynamic.html. Last modified March 27, 2007.

Kusky, Timothy M., and Ali Polat. "Growth of Granite-Greenstone Terranes at Convergent Margins and Stabilization of Archean Cratons." In Tectonics of Continental Interiors, edited by Stephen Marshak and Ben van der Pluijm, Tectonophysics 305 (1999): 43-73.

Moores, Eldridge M., and Robert Twiss. Tectonics. New

York: W. H. Freeman, 1995. Skinner, Brian, and B. J. Porter. The Dynamic Earth: An Introduction to Physical Geology, 5th ed. New York: John Wiley & Sons, 2004. Stern, Robert J. "Subduction Zones." Reviews of Geophysics 40 (2002): 3.1-3.38.

subsidence Natural geologic subsidence is the sinking of land relative to sea level or some other uniform surface. Subsidence may be a gradual, barely perceptible process, or it may occur as a catastrophic collapse of the surface. Subsidence occurs naturally along some coastlines and in areas where ground-water has dissolved cave systems in rocks such as limestone. It may occur on a regional scale, affecting an entire coastline or it may be local in scale, such as when a sinkhole suddenly opens and collapses in the middle of a neighborhood. other subsidence events reflect the interaction of humans with the environment and include ground surface subsidence as a result of mining excavations, groundwater and petroleum extraction, and several other processes.

Compaction is a related phenomenon, where the pore spaces of a material are gradually reduced, condensing the material and causing the surface to subside. subsidence and compaction do not typically result in death or even injury, but they do cost Americans alone tens of millions of dollars per year. The main hazard of subsidence and compaction is damage to property.

Coastal subsidence, which is equated with a local sea level rise, can also result in more sinister long-term effects. Many coastal cities are experiencing slow subsidence so that surfaces once above sea level sink to many feet below sea level over hundreds of years. This phenomenon results in putting cities including Venice, New orleans, and many others below sea level. In the case of New orleans, the subsidence has caused the surrounding wetlands to have sunk below sea level, placing the city—now partly below sea level—much closer to the coast than when it was built. subsidence has therefore contributed greatly to the increased damage to the city from recent hurricanes, including Katrina and Rita in 2005, and continues to place the city at ever-increasing risk.

subsidence and compaction of the land (and relative rise of sea level) directly affect millions of people. Residents of New orleans live below sea level and are constantly struggling with the consequences of living on a slowly subsiding delta. Coastal residents in the Netherlands have constructed massive dike systems to try to keep the North sea out of their slowly subsiding land. The city of Venice, Italy, has dealt with subsidence in a uniquely charming way, drawing tourists from around the world. Millions of people live below the high-tide level in Tokyo. The coastline of Texas along the Gulf of Mexico is slowly subsiding, placing residents of Baytown and other Houston suburbs close to sea level and in danger of hurricane-induced storm surges and other more frequent flooding events. in Florida, sinkholes have episodically opened up swallowing homes and businesses, particularly during times of drought.

Flood depth estimation map of New Orleans, September 3, 2005 (NOAA)

The driving force of subsidence is gravity, with the style and amount of subsidence controlled by the physical properties of the soil, regolith, and bedrock underlying the area that is subsiding. Subsidence does not require a transporting medium, but it is aided by other processes such as groundwater dissolution that can remove mineral material and carry it away in solution, creating underground caverns that are prone to collapse.

Natural subsidence has many causes, all of which may operate in the coastal environment. Dissolution of limestone by underground streams and water systems is one of the most common, creating large open spaces that collapse under the influence of gravity. Groundwater dissolution results in the formation of sinkholes, large, generally circular depressions caused by collapse of the surface into underground open spaces.

Earthquakes may raise or lower the land suddenly, as in the case of the 1964 Alaskan earthquake where tens of thousands of square miles suddenly sank or rose 35 feet (11.5 m), causing massive disruption to coastal communities and ecosystems. Earthquake-induced ground shaking can also cause liquefaction and compaction of unconsolidated surface sediments, also leading to subsidence. Regional lowering of the land surface by liquefaction and compaction was widespread in the magnitude 6.9 Kobe, Japan, earthquake of 1995.

Volcanic activity can cause subsidence, as when underground magma chambers empty out during an eruption. In this case, subsidence is often the lesser of many hazards that local residents need to fear. subsidence may also occur on lava flows, when lava empties out of tubes or underground chambers. The eruption of Krakatua in Indonesia in 1883 was associated with rapid collapse of the coastal caldera, and the sea rushed into the exposed magma chamber, generating a huge tsunami that killed 36,000 people in nearby coastal villages.

some natural subsidence on the regional scale is associated with continental scale tectonic processes. The weight of sediments deposited along continental shelves can cause the entire continental margin to sink, causing coastal subsidence and a landward migration of the shoreline. Tectonic processes associated with extension, continental rifting, strike-slip faulting, and even collision can cause local or regional subsidence, sometimes at rates of several inches (7-10 cm) per year.

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