2.5 Mountain Building Along Andean-type Margins: Converging Plate Tectonics

The first stage in the development of an Andean type mountain belt occurs prior to the formation of the subduction zone. At this time, the continental margin is a passive margin. It is not a plate boundary, but a part of the same plate as the adjoining oceanic crust.

The east coast of the United States provides a present-day example of a passive continental margin. In such settings, deposition of sediment on the continental shelf produces a thick platform of shallow water sandstones, limestones, and shales. Beyond the continental shelf, turbidity currents deposit sediments on the floor of the deep ocean basin. In this environment, three distinct structural elements of a developing mountain belt gradually take form: volcanic arcs, accretionary wedges, and forearc basins.

How does a volcanic Arc begin to build? Recall that as Ooeanic lithosphere descends into the mantle, increasing temperatures and pressures drive volatiles, mostly water, from the crustal rocks. These mobile fluids migrate upward into the wedge-shaped piece of mantle located between the subducting slab and the upper plate. Once the sinking slab reaches a depth of about 100 kilometers, 60 miles, these water-rich fluids reduce the melting point of hot mantle rock sufficiently to trigger some melting. Partial melting of mantle rock, principally peridotite, generates primary magmas, with basaltic composition. Because they are less dense than the rocks from which they originated, these newly formed basaltic magmas will buoyantly rise. Upon reaching the base of the continental crust, which consists of low-density, rocky components, these basaltic magmas typically collect, or pond. However, recent volcanism at modern arcs, the eruptions of Mount Etna for example, indicates that some magma must reach the surface.

In order to continue to ascend, magma bodies must remain buoyant relative to the crust. In subduction zones, this is generally achieved through magmatic differentiation, in which heavy iron-rich minerals crystallize and settle out, leaving the remaining melt enriched in silica and other light components. Hence, through magmatic differentiation, a comparatively dense basaltic magma can generate low-density, buoyant melts that have an andesitic, intermediate, or even a rhyolitic, felsic, composition.

Volcanism along continental arcs is dominated by the eruption of lava and pyroclastic materials of andesitic composition, whereas lesser amounts of basaltic and rhyolitic rocks may be generated. Because water driven from the subducting plate is necessary for melting, these mantle derived magmas are enriched in water and other volatiles, the gaseous component of magmas. It is these gas-laden andesitic magmas that produce the explosive eruptions that characterize continental volcanic arcs and mature island arcs.

Emplacement of plutons. Thick continental crust greatly impedes the ascent of magma. A high percentage of the magma that intrudes the crust never reaches the surface. Instead, it crystallizes at depth to form plutons. The emplacement of these massive igneous bodies will metamorphose the host rock by the process called contact metamorphism.

Eventually, uplifting and erosion exhume these igneous bodies and associated metamorphic rocks. Once they are exposed at the surface, these massive structures are called batholiths. Composed of numerous plutons, batholiths form the core of the Sierra Nevada in California and are prevalent in the Peruvian Andes. Most batholiths are composed of intrusive igneous rocks with an intermediate to felsic composition, such as diorite and granodiorite, although granites have been observed. Granite is sparse in the batholiths along the western margin of North America, but significant amounts occur in the core of the Appalachian Mountains.

How does an accretionary wedge develop? During the development of volcanic arcs, 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 over-riding plate. The resulting chaotic accumulation of deformed and thrust faulted sediments and scraps of ocean crust is called an accretionary wedge. The processes that deform these sediments have been likened to what happens to a wedge of soil as it is scraped and pushed in front of an advancing bulldozer.

Some of the sediments that comprise an accretionary wedge are muds that accumulated on the ocean floor and were subsequently carried to the subduction zone by plate motion. Other materials are derived from the adjacent volcanic arc and consist of volcanic ash and other pyroclastic materials, as well as sediments eroded from these elevated landforms.

Some subduction zones have minimal accretionary wedges or lack them all together. The Mariana Trench, for example, lacks an accretionary wedge, partly because of the distance to a significant source region. Another proposed explanation for the lack of an accretionary wedge is that much of the available sediment is subducted. By contrast, the Cascadia subduction zone has a large accretionary wedge. Here the Juan de Fuca plate has a three km, two mile, thick mantle of sediments, contributed mainly by the Columbia River.

Prolonged subduction, in regions where sediment is plentiful, may thicken an accretionary wedge enough so 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 region. The resulting wedge emerges to form the island of Barbados.

Not all the available sediment becomes part of the accretionary wedge. Some is subducted to great depths. As these sediments descend, pressure steadily increases, but the temperatures within the sediments remain relatively low, because they are in contact with the cool plunging plate. This activity generates a suite of high pressure, low temperature metamorphic minerals. Because of their low density, some of the subducted sediments and associated metamorphic components will buoyantly rise toward the surface. This backflow tends to mix and turn the sediment within the accretionary wedge. Thus an accretionary wedge evolves into a complex structure consisting of faulted and folded sedimentary rocks and scraps of oceanic crust that may be intermixed with metamorphic rocks formed during the subduction process. The unique structure of accretionary wedges has greatly aided geologists in their effort to piece together the events that have generated our modern continents.

What is a forearc basin? As the accretionary wedge grows upward, it tends to act 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 volcanic arc. This region, which is composed of relatively undeformed layers of sediment and sedimentary rocks, is called a forearc basin. Subsidence, and continued sedimentation in forearc basins, can generate a sequence of horizontal sedimentary strata that is several kilometers thick.

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