intraplate tectonics

intraplate tectonics Since the development of the theory of plate tectonics in the 1960s it has been recognized that this theory, although a very good approximation to the behaviour of much of the Earth's lithosphere, cannot be strictly applied to some regions. The boundaries of some plates are not sharp but are spread out over hundreds of kilometres, and the interiors of several plates have significant deformation. The existence of this intraplate deformation is revealed by the global pattern of recent shallow earthquakes (Fig. 1). Deep earthquakes (below 30 km depth) which are associated with the lower parts of subduction zones are not included in this map. Although the majority of earthquakes occur at the boundaries of plates (in particular the ‘ring of fire’ surrounding the Pacific Ocean), a significant number of earthquakes, including many damaging ones, occur elsewhere. Other evidence for the existence of intraplate deformation comes from the direct observation of crustal movements by geodetic means.

Continental intraplate tectonics

Many parts of the continental lithosphere do not deform. These cratons or continental shields are parts of the continental lithosphere, usually well within the interior of the plates, that have been unaffected by plate-boundary or mantle-plume activity for tens to hundreds of millions of years. As a result these regions, for example, central Canada and Siberia, have cooled at depth, thickening the lithosphere, and in consequence are relatively strong and resistant to deformation. Surrounding the cratons are areas of younger continental lithosphere, which are warmer and therefore weaker and more easily deformed. The most significant intraplate deformation takes place within the Alpine–Himalayan belt, a band of intense seismic activity running from Albania and Greece through the Caucasus and Iran to Tibet and south-east Asia. The width of the zone varies from a few hundred kilometres in Greece to over 1000 km in Tibet. The Eurasian plate to the north and the African, Arabian and Indian plates to the south are converging at up to 40 mm yr⁻¹, and so the predominant style of deformation is thrusting. Strike-slip and extensional faulting are, however, also observed. With the exception of smaller regions, such as central Turkey and the Tarim basin in Tibet, which appear to be rigid and have sharp plate-like boundaries, the plate motion in the Alpine–Himalayan belt is accommodated pervasively throughout the zone. A contrasting situation exists in the Basin and Range of the Western USA, where extensional intraplate deformation takes place despite the existence of an abrupt strike-slip plate boundary to the west.

Although the continental part of the African plate is mostly regarded as cratonic, extension is occurring across the East African Rift, which runs south from the Red Sea to roughly the northern border of South Africa. The situation here is that the African craton has been heated from underneath by a plume of hot, upwardly convecting mantle material. This has resulted in uplift of the continental lithosphere, which has been weakened and has eventually collapsed under its own weight and riven apart. In the Red Sea and the Gulf of Aden, rifting has taken place and a new plate boundary has been formed at which oceanic crust is being created, but the southern arm has not rifted completely. This style of deformation, in which continental rifting does not always lead to the formation of a plate boundary but still results in significant deformation, is widespread throughout geological history and is responsible for many of the old sedimentary basins in which much of the Earth's oil and gas reserves are found.

A third style of continental intraplate tectonics is represented by large, isolated earthquakes such as the 1811–12 New Madrid (eastern USA) events of about magnitude 8 and the even larger 1755 Lisbon (Portugal) earthquake. Because they are infrequent, such events are not well understood. Although each one results in a significant relative displacement of parts of the Earth's crust, how such events lead to long-term tectonic motion is unclear. Smaller isolated intraplate earthquakes also exist, but their contribution to tectonic movement is negligible.

Oceanic intraplate tectonics

Because the oceanic lithosphere is made up of olivine-rich material, in contrast to the quartz- and feldspar-rich continental lithosphere, the oceanic plates are (at a given temperature) much stronger and less likely to break than the continents. The outcome of this is that wide zones of deformation within the oceanic lithosphere near to plate boundaries simply do not occur. The main intraplate tectonic activity in the oceans takes the form of hot-spot volcanism. At the hot-spot, a rising convective plume of hot mantle material causes melting in the upper asthenosphere. Much of the melt never reaches the surface, but some is erupted to form a submarine volcano which may eventually grow into an island. Because the plates are moving in relation to the hot-spots, over a long period of time a chain of oceanic volcanoes will be formed. Only those close to the plume will be active; older volcanoes further away will erode and subside as the plate cools, forming a chain of seamounts. A change of direction of a seamount chain is the result of a change in the relative movement of the plume and the oceanic plate. A good example of this is the Hawaii–Emperor chain in the Pacific Ocean, which has a pronounced kink corresponding to a change in relative plate velocity 43 Ma ago.

Although intraplate volcanism is a sign of tectonic activity, it does not result in large-scale deformation of the lithosphere. The only current evidence of significant intraplate oceanic deformation comes from the equatorial Indian Ocean within the Indo-Australian plate. Here, the existence of widespread seismicity and folding of sea-floor rocks indicates that deformation at a rate less than 1 cm year⁻¹, less than half of that at the slowest-spreading current mid-ocean ridge, has taken place during the past 10–20 Ma. Geodetic measurements demonstrate that this intraplate movement is an ongoing phenomenon.

Peter Clarke