DEFINITION OF SUBDUCTION ZONE

Long narrow belts where an oceanic

lithospheric plate descends beneath another lithospheric plate

and enters the mantle in the processes of subduction. Subduction

zones are of two basic types, the first 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 few hundred

kilometers from the trench. As the oceanic plate enters

the trench it must bend, forming a flexural bulge up to few

thousand feet (a couple of hundred meters) high typically

about 100 miles (161 km) wide before the oceanic plate enters

the trench. The outer trench slope, on the downgoing plate, is

in most cases marked by a series of down-to-the-trench normal

faults. 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 be formed 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 more

mature systems have more intermediate volcanics and have

more of the volcanic edifice protruding above sea level. The

area between the arc and the accretionary prism is typically

occupied by a forearc basin, filled by sediments derived from

the arc and uplifted parts of the accretionary prism. Many

island arcs have back arc basins developed on the opposite

side of the arc, typically 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 may extend into

marginal seas, or close. Crustal thickening in continental

margin subduction systems is also aided by extensive magmatic

underplating.

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 a subduction zone 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 typically 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 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 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 rise to form a

mantle plume that rises through the mantle to the surface. In

this way, there is an overall material balance 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 ANDES; CONVERGENT PLATE MARGIN PROCESSES;

MANTLE PLUMES; MARIANAS TRENCH.

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