Recall from earlier this semester that where oceanic lithosphere subducts beneath oceanic lithosphere, a volcanic island arc and related tectonic features develop. Subduction of oceanic lithosphere beneath continental lithosphere, on the other hand, results in the formation of a continental volcanic arc and mountainous topography along the margin of a continent. Acting like a conveyor belt, oceanic lithosphere may also bring volcanic island arcs and other crustal fragments to a subduction zone. These crustal elements are generally too buoyant to subduct to any great depth, and they become welded to the overriding plate, which may be another small crustal fragment or a continent. If subduction continues long enough, it can ultimately lead to the closure of an ocean basin and the ensuing collision of two continents.

Island Arc–Type Mountain Building

Island arcs result from the steady subduction of oceanic lithosphere under oceanic lithosphere, which may continue for  million years or more. Periodic volcanic activity, the emplacement of igneous plutons at depth, and the accumulation of sediment that is scraped from the subducting plate gradually increase the volume of crustal material capping the upper plate (Figure 1). Some large volcanic island arcs, such as Japan, owe their size to having been built on fragments of continental crust that have rifted from a large landmass or to the joining of multiple island arcs over time.

The continued growth of a volcanic island arc can generate mountainous topography consisting of nearly parallel belts of igneous and metamorphic rocks. This activity, however, is viewed as just one phase in the development of Earth’s major mountain belts. As you will see later, some volcanic arcs are carried by subducting plates to the margin of large continental blocks, where they become involved in large-scale mountain-building episodes.

Andean-Type Mountain Building

Andean-type mountain building is characterized by subduction beneath a continent rather than oceanic lithosphere, as in the Andes Mountains of South America. Subduction along these active continental margins is associated with long-lasting magmatic activity that builds continental volcanic arcs. The result is crustal thickening, with the crust reaching thicknesses of more than 70 kilometers (43 miles).

The first stage in the development of Andean-type mountain belts occurs along passive continental margins. The east coast of the United States provides a modern example of a passive continental margin where sedimentation has produced a thick platform of shallow-water sandstones, limestones, and shales (Figure 2A). At some point, the forces that drive plate motions change, and a subduction zone develops along the margin of the continent. This subduction zone may form because the oceanic lithosphere has become so old and dense that it begins to sink of its own accord. Alternatively, strong compressional forces may help initiate subduction.

Building Volcanic Arcs 

Recall that as oceanic lithosphere descends into the mantle, increasing temperatures and pressures drive volatiles (mostly water and carbon dioxide) from the crustal rocks. These mobile fluids migrate upward into the wedge-shaped region of mantle between the subducting slab and upper plate. At a depth of about 100 kilometers (about 60 miles), these fluids reduce the melting point of hot mantle rock sufficiently to trigger partial melting (Figure 2B); recall from earlier this semester that this process is called flux melting.

Partial melting of the ultramafic mantle rock (Figure 3) generates magmas with basaltic (mafic) compositions. Because these newly formed basaltic magmas are less dense than the rocks from which they originated, they will rise buoyantly. In continental settings, basaltic magma often ponds beneath the less dense rocks of the crust. The hot basaltic magma may heat these overlying crustal rocks sufficiently to generate a silica-rich magma of intermediate and/or felsic (granitic) composition that can rise to form the continental volcanic arcs characteristic of an Andean-type subduction zone.

Emplacement of Batholiths 

Because of its low density and great thickness, continental crust significantly impedes the ascent of molten rock. Consequently, much of the magma that intrudes Earth’s crust never reaches the surface; instead, it crystallizes at depth to form massive collections of igneous plutons called batholiths. The result of this activity is thickening of Earth’s crust.

Eventually, uplift and erosion exhume the batholiths. The American Cordillera contains several large batholiths, including the Sierra Nevada batholith of California, the Coast Range batholith of western Canada, and several large igneous bodies in the Andes. Most batholiths consist of intrusive igneous rocks that range in composition from granite to diorite.

Development of an Accretionary Wedge 

During the development of volcanic arcs, unconsolidated sediments that are carried on the subducting plate, as well as fragments of oceanic crust, may be scraped off and plastered against the edge of the overriding plate, like a wedge of soil scraped up by an advancing bulldozer. The resulting chaotic accumulation of deformed and thrust-faulted sediments and scraps of ocean crust is called an accretionary wedge (Figure 2B).

Some of the sediments in an accretionary wedge are muds that accumulated on the ocean floor and were carried to the subduction zone by plate motion. Additional materials in an accretionary wedge are derived from an adjacent continental volcanic arc and consist of volcanic debris and products of weathering and erosion.

Where sediment is plentiful, prolonged subduction may thicken a developing accretionary wedge so that it protrudes above sea level. This has occurred along the southern end of the Puerto Rico trench, where the Orinoco River basin of Venezuela is a major source of sediments. The resulting wedge emerges to form the island of Barbados.

Forearc Basins 

As an accretionary wedge thickens, it acts as a barrier to the movement of sediment from the volcanic arc to the trench. As a result, sediments begin to collect between the accretionary wedge and the volcanic arc. This region, which is composed of relatively undeformed layers of sediment and sedimentary rocks, is called a forearc basin (refer to Figure 2C). Subsidence and continued sedimentation in forearc basins can generate a sequence of nearly horizontal sedimentary strata that can attain thicknesses of several kilometers.

Sierra Nevada, Coast Ranges, and Great Valley

California’s Sierra Nevada, Coast Ranges, and Great Valley are excellent examples of the tectonic structures that are typically generated along an Andean-type subduction zone. These structures were produced by the subduction of a portion of the Pacific basin (the Farallon plate) under the western margin of California (refer to Figure 2B). The Sierra Nevada batholith is a remnant of the continental volcanic arc that was produced by many intrusions of magma over a span of more than 100 million years. The Coast Ranges were built from the vast accumulation of sediments (accretionary wedge) that collected along the continental margin.

Beginning about 30 million years ago, subduction gradually ceased along much of the margin of North America, as the spreading center that produced the Farallon plate entered the California trench. The uplifting and erosion that followed removed most of the evidence of past volcanic activity and exposed the core of crystalline igneous and associated metamorphic rocks that make up the Sierra Nevada (Figure 2C). The Coast Ranges were uplifted only recently, as evidenced by the young, unconsolidated sediments that currently blanket portions of these highlands.

California’s Great Valley is a remnant of the forearc basin that formed between the Sierra Nevada and the accretionary wedge and trench that lay offshore. Throughout much of its history, portions of the Great Valley lay below sea level. This sediment-laden basin contains thick marine deposits and debris eroded from the adjacent continental volcanic arc.

• The type of convergent margin determines the type of mountains that form. Where one oceanic plate overrides another, a volcanic island arc forms. Where an oceanic plate subducts under a continent, Andean-type mountain building occurs.

• In either case, release of water from the subducted slab triggers melting in the overlying mantle wedge, generating basaltic magmas that rise to the base of the continental crust where they often pond. The hot basaltic magma may heat the overlying crustal rocks sufficiently to generate a silica-rich magma of intermediate or felsic (granitic) composition.

• Sediment scraped off the subducting plate builds an accretionary wedge. Between the accretionary wedge and the volcanic arc is a relatively calm site of sedimentary deposition, the forearc basin.

• The geography of central California preserves an accretionary wedge (Coast Ranges), a forearc basin (Great Valley), and the roots of an Andean-style mountain belt (Sierra Nevada).

• accretionary wedge: A large wedge-shaped mass of sediment that accumulates in subduction zones. Here, sediment is scraped from the subducting oceanic plate and accreted to the overriding crustal block.

• forearc basin: The region located between a volcanic arc and an accretionary wedge where shallow-water marine sediments typically accumulate.