At a subduction zone, one tectonic plate submerges below the neighbouring plate. Researchers have now used simulations to determine the decisive factors preventing both plate boundaries sinking into the earth’s interior. Under suitable conditions, unilateral asymmetric subduction occurs at tectonic plate boundaries. This involves one of the two plate edges sliding below the other. A study by a Swiss-American team of researchers has now shown that water plays the decisive role in asymmetric subduction.

New oceanic crust is formed continuously by magma upwelling at the central oceanic ridge. The age of the crust increases progressively, moving from there towards the outermost edge of the ocean floor. As a rule, when the crust reaches a maximum age of about 200 million years, it has cooled to such an extent that it is heavy enough to sink down gradually into the earth’s mantle. In this process it then slides under an adjacent younger crustal plate. A subduction zone develops.

Against the convection flows

In the current models, crustal or lithosphere plates, which consist of the crust and the upper parts of the earth’s mantle, are propelled by convection flows in the earth’s mantle. At the point where two lithosphere plates collide, two convection flows also collide against one another. Taras Gerya, Research Associate at the Institute of Geophysics, James Connoly, Professor at the Institute for Mineralogy and Petrology of the Department of Earth Sciences of ETH Zurich, and David Yuen, Professor at the University of Minnesota, have now studied the question of why both plate edges do not plunge downwards at such plate boundaries instead of what occurs. Namely, that only one of the two lithosphere plates submerges and is then overridden by the other. At least this is the picture presented by seismic tomographies and the measured velocities of the plates at the earth’s various subduction zones.

Using numerical models, the researchers simulated the processes occurring at a subduction zone. They did this by using a computer to perform 71 experiments with two hypothetical oceanic crustal plates of different ages. In the computer simulations they systematically varied parameters such as the absence or presence of water, the rate at which the water moves out of the minerals and pore cavities under the prevailing pressure and temperature conditions, the age of the plates, the plastic strength of the crustal and mantel rocks, and the extent of the water-bearing weakness zone at which subduction begins.

Water as a lubricant

If an old oceanic lithosphere plate is pushed under a younger one and subducted, the water-bearing mantle rock and the subducted crust become partially molten at great depth. The molten material rises towards the surface and collects under the overlapping plate. The material thickens the plate and causes volcanism at the surface. What is known as an island arc is formed, Japan being a typical example.

However, the overriding plate is stretched at the same time. A new central oceanic ridge, at which new crust is formed, gradually comes into being at the elongation point. The researchers have now shown that these processes and sustained unilateral subduction develop only if the plate strength is very high. An additional decisive factor is a permanent water-bearing weakness zone on the surface of the submerging piece of crust, through which water can be released from the rocks and minerals. Taras Gerya stresses that “Water is a kind of lubricant and decouples the overriding and submerging plates from one another.” He says water enables asymmetric unilateral subduction of the kind observable on planet Earth. This is why subduction of this type can occur only on planets on which water also exists. Gerya says that otherwise plate tectonic processes would necessarily take place symmetrically everywhere.

Literature

Gerya, T., Connolly, J. & Yuen, D.: Why is terrestrial subduction one-sided?, Geology 36/2008. DOI: 10.1130/G24060A.