North America's root is shifted

The lower part of the continental shelf has moved 850 kilometers to the southwest

The deep continent root of North America is less harsh and stable than thought © NASA
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Because of hard and stable: The deep root of the North American continent is not where it should be. Instead, this rock base is shifted by 850 kilometers to the southwest, as geologists discovered. But this disproves the common doctrine that these Kraton roots should actually remain under billions of billions, according to the researchers in the journal Nature Geoscience.

Earth's continents are not static: over hundreds of millions of years, they migrate, break apart, and rejoin. Nevertheless, the core areas of the continents are geologically very stable - at least in moderation. In 2014, geologists found indications that the plates themselves could warp and bend within the continents.

However, the deep roots of our land masses, the so-called cratons, have not changed significantly from the usual Annhame since their formation 2.5 to 3.8 billion years ago - so it was thought in any case so far. "The key to the conservation of the cratons lies in their strong and thick lithospheric roots, " explain Mikhail Kaban of the German Research Center for Geosciences GFZ and his colleagues. These rocks at the transition from the Earth's crust to the mantle were considered too hard and too light to be influenced by the convection currents of the mantle.

The continental crust below the landmass reaches deeper into the mantle © USGS

850 kilometers postponed

However, as Kaban and his colleagues have noted, the cratons are apparently less rock hard and resilient than previously thought. For example, the lower part of the root of the North American continent has shifted by 850 kilometers to the west-southwest, as the evaluation of data from gravity field measurements, topography, seismology and crustal structure revealed. "This result strongly suggests that there is a traction at the lower part of the kraton lithosphere, " the researchers say.

Where does this deformation of the so stable and hard craton come from? One reason is the composition of the rock in these deep foundations of the terrestrial landmasses: "The kratonische root below 175 kilometers of depth differs chemically, thermally and in their flow characteristics from the overlying remainder of the root", explains Kaban and his colleagues. "Thus, the Kraton proves to be not as hard and temperature-resistant, as previously thought." Display

Speed ​​of plate tectonics for North America M. Kaban / GFZ

Old view refuted

A model of the currents in the Earth's mantle below North America showed that the rock of the root of the Kraton root is drifted to the south-west at a depth of about 225 kilometers at a depth of about four millimeters per year. If this is reckoned back, then today's displacement of the kraton root could have started around 850 kilometers some 200 million years ago, as the researchers explain. This agrees well with the time when the Atlantic opened and thereby changed the currents in the mantle.

"Our observations contradict the conventional view of the Kratons as static, non-evolving geological structures, " Kaban and his colleagues say. Instead, there are decidedly more mechanical, chemical and thermal interactions between the millennium-old rock of the Craton and its upper mantle environment. (Nature Geoscience, 2015; doi: 10.1038 / ngeo2525)

(German Research Center for Geosciences GFZ, 08.09.2015 - NPO)