subduction zones

subduction zones Subduction zones form where an oceanic lithospheric plate is in collision with another plate, either oceanic or continental. The density of oceanic lithosphere is similar to that of the asthenosphere, and it can thus fairly easily be pushed down into the uppermost mantle. Subducted lithosphere remains cooler, and therefore denser, than the surrounding mantle for many millions of years; so once started, subduction tends to continue, driven in part by the weight of the subducting slab.

Palaeomagnetic studies have shown that the Earth's radius has not grown substantially since the Mesozoic, and certainly not enough to accommodate the amount of new oceanic lithosphere that has been created at mid-ocean ridges in the past 200 Ma. In plate-tectonic terms, subduction zones allow lithosphere to be ‘consumed’, thus balancing its production at ridges.

The most persuasive evidence for the existence of subduction zones is the narrow Benioff zones of earthquake epicentres dipping away from deep-sea trenches (see seismology and plate tectonics). The subducted slab can also be imaged by seismic tomography, which shows it as a zone characterized by high seismic velocity and low seismic attenuation, both of which can be attributed to its low temperature compared to that of the surrounding mantle.

Most subduction zones are marked at the surface by arcuate deep-sea trenches (see deep-sea trenches) and include an arc of active volcanoes that lies parallel to and several hundred kilometres behind the trench, above the dipping slab. Trenches and volcanic arcs tend to be convex toward the underriding plate. Geochemical analysis of arc lavas shows that their magmas originated at about the depth of the subducted slab immediately beneath them.

Ocean–ocean subduction zones

There are two kinds of subduction zones, depending on whether one or both of the colliding plates is oceanic. Ocean–ocean subduction zones are slightly simpler, and will be considered first. Figure 1a shows a schematic cross-section across a typical ocean–ocean subduction zone, although not all the features shown are always present. The various features and their origins are as follows, described in order of a traverse from the subducting plate through the subduction zone to the overriding plate.

A little over 100 km ‘ahead’ of the trench, on the subducting plate, there is a low bulge in the sea floor called the trench outer rise. This is formed as a result of elastic bending of the plate as it is subducted.

One of the major surface features of the subduction zone is the trench, the bottom of which marks the boundary between the two plates. The outer slope of the trench is fairly gentle, but is usually marked by active normal faulting which breaks the brittle upper part of the plate as it bends down. The trench floor is commonly flat and covered by sediments (turbidites) that have slumped off the walls.

The trench inner slope is usually steeper than the outer slope. It usually consists of sediments scraped off the subducting slab to form an accretionary wedge or prism, sometimes called the subduction complex. The accretionary prism can contain repeated slices of sediment transferred from one plate to another in this way. The degree of development of the accretionary prism is very variable, but it can build up to produce a significant ridge, which may even appear above sea level. In the Lesser Antilles, for example, the islands of Trinidad, Tobago, and Barbados are actually the top of the accretionary prism.

Most subduction zones contain a volcanic arc, parallel to the trench and situated about 150–200 km away from it on the overriding plate. Many of the volcanoes stand above sea level to form a volcanic ‘island arc’. The Lesser Antilles islands from Grenada to St Kitts represent the volcanic arc there. Arc volcanism results mainly from the melting of the asthenosphere immediately above the descending slab, together with the subducted sediments. Melting is facilitated by the large amount of water carried down as pore water and in hydrated minerals; this water lowers the mantle melting temperature.

Between the volcanic arc and the accretionary prism (where developed) is a ‘fore-arc basin’. This may contain sediments derived from the volcanic arc and possibly also from the accretionary prism.

Beyond the volcanic arc there is often a ‘back-arc basin’ or ‘marginal basin’. Most mature ocean–ocean subduction zones contain a back-arc spreading centre, which results from tensional forces on the overriding plate generated by the subduction process itself. Back-arc spreading centres are similar to mid-ocean ridges, and produce new oceanic lithosphere in back-arc basins by sea-floor spreading. However, the presence of water from the subducted lithosphere results in magmas that are chemically distinct from those produced at mid-ocean ridges. There are many back-arc basins in the western Pacific, including the Japan Sea and a string of basins from the South China Sea to the South Fiji and Lau basins.

Ocean–continent subduction zones

In many respects ocean–continent subduction zones (Fig. 1b) are similar to ocean–ocean ones, and can include an outer rise, trench, subducted slab, accretionary prism, fore-arc basin and volcanic arc. However, the arc magmas are here intruded through a thick continental crust, which may modify their chemistry. Perhaps more importantly, the weak continental lithosphere is readily deformed by compressional stresses so that extensive folding and thrusting may occur both in the fore-arc and in the back-arc regions. This can lead to the formation of complex mountain belts such as the Andes (which flank the Peru–Chile subduction zone), composed of both folded and faulted sediments and arc volcanoes. While there is evidence of back-arc extension in such belts, full sea-floor spreading does not seem to occur.

The accumulation of accretionary prisms and arc volcanic rocks at continent–ocean subduction zones represents one of the main mechanisms for the accretion of continents. For example, western North America and eastern Asia display well-developed concentric bands of crust, ageing towards the cratonic interior, that are thought to have been accreted in this way in successive episodes of subduction.

R. C. Searle

Bibliography

Kearey, P. and and Vine, F. J. (1996) Global tectonics. Blackwell Science, Oxford.