Earth Materials. John O'Brien
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varying degrees. This produces stresses in the cool interior of the subducted lithosphere that generate earthquakes (Figure 1.11) along an inclined seismic (Wadati‐Benioff) zone that marks the path of the subducted plate as it descends into the asthenosphere. The four largest magnitude earthquakes in the past 120 years occurred along inclined seismic zones beneath Chile (1909), Alaska (1964), Sumatra (2004), and Japan (2011). The latter two events produced the devastating 2004 Banda Aceh tsunami which killed some 300 000 people around the Indian Ocean region and the Fukushima tsunami which killed tens of thousands in eastern Japan and destroyed an atomic power plant.
What is the ultimate fate of subducted slabs? Earthquakes occur in subducted slabs to a depth of 660 km, so we know they reach the base of the asthenosphere transition zone. Earthquake records suggest that some slabs flatten out as they reach this boundary indicating that they may not penetrate into the lower mantle. Seismic tomography, which images three‐dimensional variations in seismic wave velocity within the mantle, has shed some light on this question, while raising many others. A consensus has emerged (Grand 2002; Hutko et al. 2006) that some subducted slabs become dense enough to sink all the way to the core–mantle boundary where they contribute material to the D″ layer. These slab remnants may ultimately be involved in the formation of mantle plumes, as proposed by Jeanloz (1993).
Subduction zones produce a wide range of distinctive Earth materials. The increase in temperature and pressure within the subducted plate causes it to undergo significant metamorphism. The upper part of the subducted slab, in contact with the hot asthenosphere, releases volatile fluids as it undergoes metamorphism which lowers melting temperatures and triggers partial melting. A complex set of melts rise from this region to produce volcanic‐magmatic arcs. These melts range in composition from basaltic–gabbroic through dioritic–andesitic and may differentiate or be contaminated to produce melts of granitic–rhyolitic composition. Granitic–rhyolitic melts may also be generated by partial melting of older continental crust heated by rising magma and volatiles. Melts that reach the surface produce volcanic arcs such as those that characterize the “ring of fire” of the Pacific Ocean basin. Mt. St. Helens in Washington, Mt. Pinatubo in the Philippines, Mt. Fuji in Japan and the many volcanoes in Central America and the Andes of South America are all examples of volcanic arc composite volcanoes that form over Pacific Ocean subduction zones. These volcanic arcs add to the volume of continental crust.
When magmas intrude the crust and solidify below the surface, they produce plutonic igneous rocks that add new continental crust to Earth. Most of the world's major batholith belts represent plutonic magmatic arcs, subsequently exposed by erosion of the overlying volcanic arc. In addition, many of Earth's most important ore deposits (Chapter 19) are produced in association with volcanic‐magmatic arcs over subduction zones.
Many of the magmas generated over the subducted slab cool and crystallize at the base of the lithosphere, thickening it by underplating. Underplating and intrusion are two of the major sets of processes by which new continental crust is generated. Once produced, the low density of continental crust prevents most of it from being subducted. This helps to explain its preservation potential and the great age that continental crust can achieve (>4.0 Ga).
Figure 1.12 Subduction zone depicting details of sediment distribution, sedimentary basins and volcanism in trench‐arc system forearc and backarc regions.
Areas of significant relief, such as trench‐arc systems are ideal sites for the production and accumulation of detrital (epiclastic) sedimentary rocks (Chapter 13). Huge volumes of detrital sedimentary rocks produced by the erosion of volcanic and magmatic arcs are deposited in forearc and backarc basins (Figure 1.12). They also occur with deformed abyssal sediments in the forearc subduction complex. As these sedimentary rocks are buried and deformed, they are commonly metamorphosed (Chapters 15 and 18).
Continental collisions
As ocean basins shrink by subduction, portions of the ridge system may be subducted. Once the ridge is subducted, growth of the ocean basin by sea‐floor spreading ceases, the ocean basin continues to shrink, and the continents, microcontinents or arcs on either side are brought closer together as subduction proceeds. Eventually they converge to produce a continental collision.
When a continental collision (Dewey and Bird 1970) occurs, subduction eventually ceases. This occurs because most continental lithosphere is too buoyant to be subducted to great depths for prolonged periods of time. Small amounts may be subducted in this tectonic setting, likely because the density contrasts between the leading edges of the two plates are small. The continental lithosphere involved in the collision may be part of a continent, a microcontinent or a volcanic‐magmatic arc complex. Typically the collision occurs over millions to tens of millions of years as irregularly shaped ocean basins close at different times. As convergence continues, the margins of both continental plates are compressed and shortened horizontally and thickened vertically in a manner roughly analogous to what happens to two vehicles in a head‐on collision. However, in the case of continents colliding at a convergent plate boundary, the convergence occurs over millions of years. The result is a severe horizontal shortening and vertical thickening which results in the progressive uplift of a mountain belt and/or extensive elevated plateau that mark the closing of an ancient ocean basin (Figure 1.13).
Long mountain belts formed along convergent plate boundaries are called orogenic belts. The increasing weight of the thickening orogenic belt causes the adjacent continental lithosphere to bend downward to produce foreland basins adjacent to the orogenic belt. Large amounts of detrital sediments derived from the erosion of the mountain belts are deposited in such basins. In addition, increasing temperatures and pressures within the thickening orogenic belt cause regional metamorphism of the rocks within it. If the temperatures become high enough, partial melting may occur to produce melts in the deepest parts of orogenic belts which rise to produce a variety of felsic igneous rocks.
Figure 1.13 (a) Ocean basin shrinks by subduction, as continents on two plates converge. (b) Continental collision produces a larger continent from two continents joined by a suture zone. Horizontal shortening and vertical thickening are accommodated by folds and thrust faults in the resulting orogenic belt.
A striking example of a modern orogenic belt is the Himalayan Mountain range formed by the collision of India with Eurasia over the past 40 Ma. The continued convergence of the Indian micro‐continent with Asia has resulted in shortening and regional uplift of the Himalayan Mountain Belt along a series of major thrust faults and produced the Tibetan Plateau. Limestones near the summit of Mt. Everest, Earth's highest mountain, were formed on the floor of the Tethys Ocean that once separated India and Asia. They were then thrust to an elevation of nearly 9 km as that ocean was closed and the Himalayan Mountain Belt formed by continental collision. The collision has produced tectonic indentation of Asia, resulting in mountain ranges that wrap around India (Figure 1.14). The Ganges River in northern India