convergent plate margins Convergent margins are one of the three types of plate boundary envisaged in plate tectonics, and are arguably the most complex of the three. They occur where two lithospheric plates are colliding, and their detailed structure depends on the natures of the two plates involved.
There are three possibilities: ocean-ocean, ocean- continent, and continent-continent. The differences between them arise as a result of the differences in strength and density of oceanic and continental lithosphere. Oceanic lithosphere is relatively strong and has almost the same density as the underlying asthenosphere. At a convergent boundary it therefore tends to bend rather than break, and is relatively easily subducted. Continental lithosphere, on the other hand, is lighter and therefore difficult to subduct. It is also relatively weak compared with oceanic lithosphere, and its yield strength is less than the force needed to subduct it. Continental lithosphere will therefore tend to break up rather than be subducted. (A consequence of this is that continental rocks, once created, generally remain at the surface of the Earth; so the area of continental rocks tends to increase with time.)
The simplest of the three margins is the ocean-ocean margin (Fig. 1a). Both plates are strong and can bend elastically with only minor fracturing. Either is relatively easy to subduct, and a subduction zone is readily formed. Which of the two plates actually becomes the subducting one may depend on detailed differences in density (the older plate being the denser) and perhaps also on details of the local stress regime and mantle convection pattern. Once subduction begins, a classic ocean-ocean subduction zone is established, exemplified by those of the western Pacific such as the Mariana Trench (between the Pacific and Philippine plates) or the Tonga-Kermadec Trench (between the Pacific and Indo-Australian plates). The plate boundary is a narrow zone lying in a deep-sea trench, and can often be localized to a single, active décollement fault. In plan view, earthquake epicentres associated with the subduction zone may occupy a band several hundred kilometres wide, but in three dimensions the earthquake foci are seen to be mostly confined to a narrow, dipping Benioff zone that lies along and within the subducting plate.
The next simplest convergent boundary is ocean- continent. Here, too, the oceanic plate is readily subducted, and the continental one simply overrides it (Fig. 1b). Such convergent margins are exemplified by those of the eastern Pacific between the South American and Nazca plates. The subducted side is similar to that in an ocean-ocean margin; again there is a narrow plate boundary zone and a narrow dipping zone of earthquakes. However, because of the weakness of continental lithosphere, the continental plate may undergo various forms of deformation, including both extensional and compressional faulting and folding. Continent-continent convergent margins are the most complex. They are exemplified by the great fold-mountain belts and associated regions, such as the Himalayan mountains and Tibetan plateau of the India-Asia continental convergence. Neither plate is readily subducted, and initially it is probable that both are strongly deformed. At least one plate becomes shortened and thickened through thrust faulting and folding, and the other is underthrust at a shallow angle, though without being subducted (Fig. 1c). As convergence continues, the thickening lithosphere rises; large, regional blocks may be extruded sideways between the converging plates, and eventually the margin may become so thick and high that the weak continental crust begins to collapse under its own weight. Although the boundary between the two plates may be localized along a single thrust front, active deformation goes on over a broad zone many hundreds or even thousands of kilometres wide, including important zones of strike-slip and tensional deformation, as well as thrust faulting and folding.
Roger Searle
Bibliography
Kearey, P. and and Vine F. J. (1996) Global tectonics. Blackwell Science, Oxford.
