A novel description of plate tectonics reveals the stability of underlying mantle flows

27.06.2013|Potsdam:
For about 250 million years, the upward and downward currents in the mantle have remained very stable. These convection currents in semi-liquid rock below the earth's crust are responsible for the movement of tectonic plates and could now be related to the movement patterns on the earth's surface, as reported in the current issue of Nature (498, June 2013). At the same time there were indications that the anomalies of wave propagation found during the observation of seismic waves in the mantle are associated with these patterns.

Continental plates are driven by convection in the underlying earth's mantle, however, the convection pattern cannot be determined directly via the plate movements. "The idea was therefore to extract information that is indicative of the underlying mantle flow field from reconstructions of plate motions in the geological past," explains co-author Bernhard Steinberger of the GFZ German Research Centre for Geosciences. For this purpose, two reference points – dipoles - were determined, which describe the average movement of the plates away from one hemisphere to the other. Furthermore, "quadrupoles" were determined: with them, the plate movements from two opposing points (divergent poles) towards two further points located at a 90 degree angle (convergent poles) can be described. Dipole and quadrupole are parameters in a mathematical method that is often used to describe physical observations, e.g. concerning the Earth's magnetic field. For the first time, this method has now been applied to plate tectonics. This approach could be chosen because presently the dipoles and quadrupoles of the plate movements coincide respectively very well with the corresponding quantities of the underlying driving forces caused by currents in the mantle and the lithosphere plates being subducted into the mantle. It stands to reason, therefore, to expect a correspondingly good agreement in the past, as there is no way to determine past mantle flow directly from observations.

This new description of plate tectonics showed amazing results. The convection pattern in the mantle has been found to be very stable: "At present, the convergent poles agree well with large-scale downward currents in the mantle, and the divergent poles with upward currents," explains GFZ researcher Steinberger. "During the last 250 million years, the convergent dipole has always been in the region of Asia, while the divergent quadrupoles have always been located in the areas of current eastern Africa and the eastern Pacific. We conclude that the underlying up- and down-flows were also correspondingly stable."

Another finding of the study is that the velocity anomalies in the mantle of the shear waves caused by earthquakes observed by global seismology match these patterns well. The divergent quadrupoles are located above the eastern edge of so-called Large Low Shear Velocity Provinces (LLSVPs) in the lowermost mantle. Steinberger: "Our results therefore provide a further indication that the LLSVPs were also stable for at least 250 million years in their current location, and may actively influence mantle convection."

Clinton P. Conrad, Bernhard Steinberger and Trond H. Torsvik,
"Stability of active mantle upwelling revealed by net characteristics of plate tectonics", Nature, 498, 27 June 2013, doi: 10.1038/nature12203