Geometry and Geophysics | Magmatic Processes | Forearc Processes | Back-arc Processes | Orogeny
Subduction zones are regions of the Earth that are defined by deep trenches, lines of volcanoes parallel to the trenches, and zones of large earthquakes dipping from the trenches landward beneath the volcanoes. Plate tectonic theory recognizes that the earth’s surface is composed of a mosaic of interacting lithospheric plates, where the lithosphere consists of the crust (continental or oceanic) and associated mantle, for a total thickness of about 100km (60 miles). Oceanic plates are created at mid-ocean ridges (divergent or accretionary plate boundaries) by seafloor spreading and destroyed at convergent or destructive plate boundaries, the subsurface continuations of which are known as subduction zones (Fig. I-1; take a look at a global bathymetric and topographic map at http://ibis.grdl.noaa.gov/cgi-bin/bathy/bathD.pl) Continental crust is produced at subduction zones. At subduction zones, the oceanic lithosphere dives beneath another plate, which may be either oceanic or continental. Part of the material on the subducted plate is recycled back to the surface and the remainder is mixed back into the Earth's deeper mantle. This process balances the creation of lithosphere that occurs at the mid-ocean ridge system. The convergence of two plates occurs at rates of 1-10 cm/year, or 10-100 km (6-60 miles) per million years. Stress and phase changes in the upper part of the cold descending plate produce large earthquakes in a narrow band called the Benioff-Wadati zone, which is confined to the upper portion of the plate and can extend as deep as 700 km (420 mi). The plate is heated as it descends, and the resulting release of water leads to melting of the overlying mantle. This melt rises to produce the linear volcanic chains that are one of the most striking features of subduction zones.
The surface expression of each subduction system is known as an arc system, from the term ‘Island Arc’. Each arc system can be further divided into a forearc region, between the trench and the volcanic chain; a volcanic arc, the chain of active volcanoes running parallel to the trench; and a back-arc region, farthest removed from the trench (Fig.I-2). Subduction zones can be divided in two ways on the basis of the nature of the crust in the overiding plate and on the age of the subducting plate. The first classification yields two broad categories: those beneath an oceanic plate, as in the Mariana or Tonga trenches, and those beneath a continental plate, as along the west coast of South America (Fig. I-1). The first type is known as an ‘intra-oceanic convergent margin’ and the second is known as an "Andean-type convergent margin’. The second classification yields two end-member types: Chilean and Mariana (Fig. I-X). Chilean-type subduction zones consume young (<50 million years old) lithosphere that is hot and thin and therefore relatively buoyant. This buoyant lithosphere resists subduction and is associated with Wadati-Benioff zones that dip gently, have a lot of seismic activity, and are associated with back-arc compression. In some cases, the hot subducted slab may melt, as discussed below. Mariana-type subduction zones consume old lithosphere (>100 million years old) which is cold and thick and therefore of greater density than the underlying mantle. This lithosphere sinks readily into the mantle and is associated with Wadati-Benioff zones that dip steeply, have little seismic activity, and are associated with back-arc extension, as discussed below. Because the seafloor of the Pacific is much older in the Western Pacific than in the Eastern Pacific, most Mariana-type subduction zones are in the Western Pacific and most Chilean-type margins lie along the margins of the Eastern Pacific.
The subducting plate bends and begins its descent into the mantle at the oceanic trench. The bend of the plate increases until the slab dips into the mantle at angles of 10-70° beneath the volcanic arc; however, in some cases, as in the Marianas, the slab descends nearly vertically beneath the active arc. The cold oceanic plate cools the surrounding mantle and produces unusually low temperatures at great depths (Fig. I-2). The presence of this cold, dense slab results in very low heat flow values beneath the forearc and a positive gravity anomaly in the midpart of the forearc (Fig. I-2). The characteristic gravity low in the outer (toward the trench) part of the forearc results from the accumulation of lower-density sediment and fractured rock at the interface of the two plates.
Earthquakes are limited to the uppermost 100 km (60 miles) of the earth everywhere but in subduction zones, where they may occur up to 7 times greater depths. Earthquakes occur in the cold, brittle upper part of the oceanic lithosphere as it is subducted. The shallower earthquakes result largely from the accumulation of stress due to the movement of the two plates. Quakes deeper than 300 km (180 miles) are due to phase changes in minerals within the plate as it moves to higher pressures, the most important of which is the conversion of olivine (Mg2SiO4) to a denser structure. Shallow earthquakes occur 10-20 km (6-12 mi) below the top of the subducting plate, not right at the interface of the two plates, and define the Benioff-Wadati zone (Fig. I-XX). At greater depths, the descending slab no longer produces earthquakes. However, studies of the deeper mantle using seismic waves (seismic tomography) have found evidence that the subducting lithosphere can remain intact and be identified on a basis of variations in seismic velocities down to the base of the mantle (Fig. I-XXX). This is accomplished by identifying regions where seismic velocities are fast relative to the surrounding mantle, which corresponds to cold, subducted material (blue areas in Fig. I-XX and I-XXX).
Active volcanoes are highly visible features of subduction zones. The volcanoes developed above subduction zones in east Asia, Australasia, and the western Americas surround the Pacific Ocean in the so-called Ring of Fire. At intra-oceanic convergent margins, these volcanoes may be the only component above sea level, leading to the name "island arc". The more general term volcanic arc refers to volcanoes built on either oceanic or continental crust. These active volcanoes associated with subduction zones occur 100-200 km (60-120 mi) above the Benioff-Wadati zone. This means that the distance of the volcanic front (the main line of active volcanoes) from the trench varies with the dip of the down-going plate. For example, in the Peru-Chile subduction zone the slab dips at an angle of 35° and the volcanoes are 250-500 km (150-300 mi) behind the trench, while in the Kermedec subduction zone, north of New Zealand, the slab dips at 65° and the volcanic front lies only 170 km (100 mi) behind the trench.
Arc volcanoes erupt a variety of lavas, including basalt [rocks with less than 53% silicon dioxide (SiO2)], andesite (53-63% SiO2), and dacite (greater than 63% SiO2). Basalt and andesite are the dominant volcanic products, though their relative proportions vary: arcs like those in the Oregon Cascades and the Marianas are dominated by basalt, while arcs in Japan and the Washington Cascades are more than half andesite. The eruptions at subduction-zone volcanoes are commonly explosive, because the magmas have both high amounts of silica, yielding very viscous magmas, and high contents of gases such as water. The eruptions produce both lavas and fragmental, or pyroclastic, material; the alternating layers of flows and ashes or breccias build steep-sided edifices known as composite volcanoes. Some magmas cool at depth to become plutonic rocks. The great masses of granitic rocks found today in the Sierra Nevadas of California formed this way, about 100 million years ago at the site of an ancient Andean-type convergent plate boundary.
Most subduction zone melts are generated in the mantle ‘wedge’ overlying the subducting plate, is discussed below. Pressure and heating release water and other volatile fluids from the cold, wet subducting plate as it passes below 100 km (60 mi). These fluids carry many of the more mobile elements with them (K, Rb, Ba, etc.) and leave less mobile elements (Zr, Ti, Nb, etc.) in their residues. As these fluids percolate upward, they lower the melting temperature of the overlying mantle by as much as 400°C (720°F), causing that mantle to melt. This effect is much the same as the freezing-point depression produced by adding antifreeze to water. The melts produced from the mantle are largely basalts, but with distinctive chemical compositions imparted to them by the mobile elements carried by fluids from the subducting plate. As these basaltic melts rise to the surface they fractionate (cool and lose some solid crystals), and they also react chemically with the oceanic or continental crust that they must pass through on the way to the surface. These reactions modify the composition of the original melt to yield a variety of different lava types at the surface. There are a very few volcanic arcs that erupt lavas whose chemistry indicates that they melted from the subducting slab itself; the rocks formed from these lavas are termed adakites after Adak Island in the Aleutians, where they were first described.
Convergent plate boundaries are where most of the crust that makes up the continents forms. Continental crust is distinct from oceanic crust in being thicker, older, and of more fractionated composition. When an intra-oceanic convergent plate boundary forms, the crust of the overriding plate is about 6 km (3.6 miles) thick. Magmatic thickening of the crust quickly yields a crust that is about 20km (12 miles) thick (Fig. I-XXXX). This is about halfway to the thickness of continental crust, which is obtained by collision between multiple sets of thickened intra-oceanic convergent margins taking place over many millions of years. This process can be seen today in Indonesia. The formation and accretion of intra-oceanic arc systems, operating over 4.5 billion years of earth history, is thought to have produced all of the continents.
The forearcs of subduction zones, between the trench and the volcanic arc, are where much of the mechanical interaction between the overriding and subducting plates occurs. Forearcs are classified as accretionary or nonaccretionary. Accretionary forearcs are those in which layers of sediment carried into the trench are scraped off the subducting plate and added to the upper plate as deformed, fault-bounded slices (Fig.I-3). The addition of successive slices of sediment causes the overriding plate to build outward and the older accreted material to be uplifted and rotated landward (Fig. I-3) to form accretionary prisms. The off-scraped material is commonly sand or silt deposited by density flows known as turbidity currents; these are high-velocity, water-supported flows which carry terrigenous material off the adjacent continent or volcanic arc into the trench (turbidites). The turbidites are deposited on top of older fine-grained pelagic (deep-ocean) sediments into the trench on the oceanic crust. As the plate moves downward, a low-angle fault develops such that most of the turbidites and some of the pelagic sediments are deformed and accreted to the upper plate, while the remaining sediment is subducted with the lower plate. The deformation and compaction of the accreted sediments release a large amount of water, which moves through the forearc, either as diffuse flow or as flow focused along the larger faults. These fluids play an important role in transporting soluble chemicals in the outer part of the subduction zone. Submarine vents associated with this fluid flow have been found in the forearcs of the Cascadia Trench and the Mariana Trench.
Trenches that lack a thick sequence of turbidite sands usually have nonaccretionary forearcs (Fig. I-3). The sediments that reach these trenches are entirely subducted, to be either under-plated to the upper plate at deeper levels or carried down and mixed back into the mantle. Some such forearcs are actually abraded by the down-going plate. Grabens (fault-bounded basins) develop in the lower plate as it bends into the trench; as the graben pass beneath the forearc, pieces of the forearc collapse into the graben and are carried down into the mantle. With time, the forearc is eroded back toward the volcanic arc. This process, subduction erosion, may be accelerated in some subduction zones when large seamounts or plateaus pass beneath the forearc, fracturing and weakening it. Fluids are released in nonaccretionary forearcs too, by the dehydration of the thin sedimentary sequence and the heating of altered volcanic rocks in the lower plate. In the Mariana and some other forearcs, these fluids move into the overlying shallow mantle and convert the water-free olivine (Mg2SiO4) in the mantle to less dense serpentine [Mg3Si2O5(OH)4]. This serpentine, because of its low density, rises into the forearc crust to form diapirs and seamounts (Fig. I-3).
The middle portion of the forearc is floored by igneous or metamorphic rocks that may be older continental crust, ocean crust trapped when the subduction zone began, or crust produced by older arc volcanoes. This crust is covered by a thick layer of volcanic sands, silts, or clays derived from the adjacent arc volcanoes, forming a forearc basin. These sediments are transported into the forearc basin either by wind or by turbidity currents.
Depending on the age of the seafloor being subducted, the regions behind the volcanic front may exhibit either extensional or compressional tectonics. Chilean-type subduction zones are characterized by strong coupling between the two plates; in effect, the upper plate moves outward over the lower plate and the upper plate experiences compression (Fig. I-X). These subduction zones typically have shallow dips and thick accretionary prisms and are characterized by high earthquake activity; their back-arc regions develop foreland fold and thrust belts, in which seaward-dipping faults stack up slices of crustal material (Fig. I-3). Mariana-type subduction zones show a very different type of interaction between the upper and lower plates. These typically have steeply dipping slabs; these steep dips result from the vertical sinking of the slabs. Old oceanic lithosphere is denser than the asthenosphere on which it sits and, once the lithosphere begins to subduct, will tend to sink vertically in addition to moving downdip. A consequence of this motion is that the slab rolls back; that is, the hinge at which it bends moves away from the volcanic arc. This change requires that the trench move as well and that the upper plate be stretched or extended. This extension is usually accommodated by the development of a small mid-ocean ridge spreading center just behind, or within, the volcanic arc. These small ocean basins are known as back-arc basins, and their development is episodic. They begin by splitting apart the weakest part of the arc system, the active volcanic chain. Sea-floor spreading in the basin moves one part of the arc away from the trench, where it eventually becomes extinct and forms a remnant arc. The other part of the volcanic chain remains active and moves with the forearc and the trench. The successive waxing and waning of back-arc sea-floor spreading has left a festoon of extinct volcanic chains and small oceanic basins behind some subduction zones, particularly in the western Pacific. Obviously, the Mariana arc is an outstanding example of a Mariana-type subduction zone. The southern IBM arc that the ‘Subduction Crew’ will study is characterized by a actively spreading back-arc basin.
One of the eventual consequences of subduction is orogeny or mountain building. Subduction zones are constantly building new crust by the production of volcanic material or by scraping off oceanic sediments. However, the development of the greatest mountain ranges-the Alps or the Himalayas-occur not during "normal" subduction, but during the death of a subduction zone, when it becomes clogged with a large continent or volcanic arc. Subduction zones destroy ocean crust and, as a result, cause ocean basins to eventually narrow and close. This closure requires that large blocks of relatively buoyant continental crust be carried closer and closer together. The eventual collision of such large blocks produces the folding and faulting that raises the great mountain ranges of Earth. The closure of an ancient ocean basin along a north-dipping subduction zone carried India into Asia about 50 million years ago, producing the orogeny that is still raising the Himalayas. An ongoing collision, still in the early stages, is carrying the edge of the Australian continent downward below the island of Timor along another north-dipping subduction zone. Collisions destroy subduction zones, sometimes with the result that buoyant crustal rocks, subducted to depths as great as 250 km (150 miles), rise back to the surface (Liou et al., 2000).
The collisions that produce orogenies also sometimes trap fragments of ocean crust from the forearc or from the down-going plate in the mountain belt. These fragments of crust, called ophiolites, are the only pieces of ancient ocean basins ever preserved in the geologic record. The nature of subduction zones is such that most oceanic lithosphere produced at the mid-ocean ridges is ephemeral, and is eventually returned to the mantle from which it was originally derived.
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