2.3 Dynamics at Subduction Zones: Back Arc Spreading at Convergent Margins

Because subduction zones form where two plates are converging, it is natural to assume that large compressional forces are at work to deform the plate margins. Indeed, this is the case along many convergent plate boundaries. However, convergent margins are not always regions dominated by compressional forces.

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Extension and Back Arc spreading
Along some convergent plate margins, the overlying plates are under tension, which causes stretching and thinning of the crust. But how do extensional processes operate where two plates are moving together?

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The age of the subducting oceanic slab is thought to play a significant role in determining the dominant forces acting on the overriding plate. Recall that when a relatively cold, dense slab subducts, it does not follow a fixed path into the asthenosphere. Rather, it sinks vertically as it descends, causing the trench to retreat, or roll back. As the subducting plate sinks, it creates a flow, slab suction, in the asthenosphere that pulls the upper plate toward the retreating Trench. Visualize what would happen if you were sitting in a Lifeboat near the Titanic as it sank! As a result, the overriding plate is under tension and may be elongated and thinned. If tension is maintained long enough, a Backarc Basin may form.

Recall that thinning and rifting of the lithosphere results in upwelling of hot mantle rock and accompanying decompression melting. Continued extension initiates a type of seafloor spreading that generates new ocean crust, thereby increasing the size of a developing backarc basin. Active backarc basins are found behind the Mariana and Tonga islands, whereas inactive basins are exemplified by the South China Sea and the Sea of Japan. Backarc spreading that formed the Sea of Japan is thought to have lifted a small piece of continental crust from Asia. Gradually, this crustal fragment migrated seaward along the retreating trench. Seafloor spreading, in turn, created the oceanic crust that floors the Sea of Japan.

Compressional Regimes. At some subduction zones, compressional forces are dominant. This appears to be the case in the central Andes, where an episode of deformation began about 30 million years ago. During this span, the western margin of South America has been actively over-running the subducting Nazca Plate at a rate of about 3 centimeters per year. Put another way, the South American Plate has been advancing toward the Peru-Chile trench at a rate faster than the trench has been retreating, because of slab sinking.

Thus, in the case of the Andes, the descending slab of oceanic lithosphere serves as a “wall” that resists the westward motion of the South American Plate. The resulting tectonic forces have shortened and thickened the western margin of South America. It is important to note that continental crust is generally weaker than oceanic crust, hence most deformation occurs in the continental blocks. In this region, the Andean crustal block has its greatest thickening, about 70 km (40 miles), and a mountainous topography that occasionally exceeds 6000 meters (20000 feet) in elevation.