List and describe the four major features associated with subduction zones.
In their ongoing quest to unravel the events that produce mountains, researchers examine ancient mountain belts as well as sites where orogenesis is currently active. Of particular interest are convergent plate boundaries, where lithospheric plates subduct. The subduction of oceanic lithosphere generates Earth’s strongest earthquakes and most explosive volcanic eruptions, and it plays a pivotal role in generating most of Earth’s mountain belts.
Features of Subduction Zones
Subduction zones can be roughly divided into four regions: (1) a volcanic arc, which is built on the overlying plate; (2) a deep-ocean trench, which forms where subducting slabs of oceanic lithosphere bend and descend into the asthenosphere; (3) a forearc region, which is located between a trench and a volcanic arc; and (4) a back-arc region, which is located on the side of the volcanic arc opposite the trench.
Perhaps the most obvious structure generated by subduction is a volcanic arc (Figure 1A). In settings where two oceanic slabs converge, one is subducted beneath the other, initiating partial melting of the mantle wedge located above the subducting plate. This molten rock rises and eventually leads to the growth of a volcanic island arc, or simply an island arc, on the ocean floor. Examples of active island arcs include the Mariana, Tonga, and Aleutian arcs in the Pacific.
By contrast, when oceanic lithosphere is subducted beneath a continental block, a continental volcanic arc results (Figure 1B). Continental volcanic arcs build on the topography of older, thicker continental blocks, resulting in volcanic peaks that may reach 6000 meters (nearly 20,000 feet) above sea level. The Cascade Range of the Pacific Northwest is a classic example.
Deep-ocean trenches are created where oceanic lithosphere bends as it descends into the mantle. Trench depth is strongly related to the age, and therefore the temperature and density, of the subducting oceanic slab. In the western Pacific, where oceanic lithosphere is cold and dense, oceanic slabs descend into the mantle at steep angles, producing trenches with average depths of about8 kilometers (5 miles) below sea level. A well known example is the Mariana trench, where the deepest area is an amazing 10,994 meters (36,069 feet) below sea level.
By contrast, the Cascadia subduction zone off the coasts of Washington and Oregon lacks a well-defined trench, partly because the warm, buoyant Juan de Fuca plate subducts at a very low angle. Trench depth is also related to the availability of sediments. A massive amount of sediment from the Columbia River basin fills most of what would otherwise be a shallow trench in this subduction zone—about 3 kilometers (2 miles) deep.
The forearc region of a subduction zone is located between a deep-ocean trench and the associated volcanic arc (see Figure 1). Here pyroclastic material from the volcanic arc, as well as sediments eroded from the adjacent landmass, accumulate. Ocean-floor sediments are also carried to forearc regions by subducting plates.
The amount of sediment carried to a forearc region varies. The forearc region adjacent to the Mariana trench, for example, contains minimal sediment, partially because of its distance from a significant source of sediment. By contrast, the forearc region adjacent to the Cascadia subduction zone is choked with sediment derived from the nearby outlet of the Columbia River.
In addition, forearc width can vary significantly. Where an oceanic slab subducts at a steep angle, the forearc region is quite narrow, but when the angle of subduction is low, the forearc tends to be broad.
Another site where sediments and volcanic debris accumulate is the back arc, which is located on the backside of the volcanic arc when viewed from the trench (see Figure 1). In these regions tensional forces tend to prevail, causing Earth’s crust to stretch and thin and resulting in the formation of a down-faulted basin.
The reason for this development is considered in the next section. Back-arc regions associated with volcanic island arcs tend to be long linear seas, such as the Sea of Japan and the Java Sea. In continental settings, the back-arc regions are located landward of the continental volcanic arc.
Here stretching of the crust usually results in subsidence, forming basins that quickly fill with volcanic ash and sediments derived from the growing volcanic structures.
Extension and Back-Arc Spreading
Because subduction zones form where two plates converge, it is logical to assume that large compressional forces deform the plate margins. However, convergent margins are not necessarily regions dominated by compressional forces. As mentioned above, tensional stresses act on the overlying plates along some convergent plate margins and cause extensio —stretching and thinning— of the crust. But how do extensional processes operate where two plates are moving together?
The age of the subducting oceanic slab is thought to play a significant role in determining the dominant forces acting on the overriding plate. When a relatively cold, dense slab subducts, it does not follow a fixed path into the asthenosphere (Figure 2A). Rather, it sinks vertically as it descends along an angled path. This causes the trench to retreat, or “roll back,” as shown in Figure 2B.
As the subducting plate sinks, it creates a flow in the asthenosphere called slab suction that “pulls” the upper plate toward the retreating trench. (Visualize what would have happened if you were in a lifeboat, unable to move away from the Titanic as it sank!)
Slab suction, in turn, produces tensional stress that elongates and thins the overriding plate, most often creating a basin in the region behind the volcanic arc (Figure 2C). Thinning of the crust results in upwelling of hot mantle rock and accompanying decompression melting.
Continued extension may initiate seafloor spreading, which increases the size of the newly formed basin.
Basins of this type within a back-arc region are termed back-arc basins (see Figure 2C). Seafloor spreading is currently enlarging the back-arc basins found landward of the Mariana and Tonga volcanic island arcs.