Linking deep mantle structures to Earth surface processes
A model for absolute plate motion and true polar wander on Earth for the past 540 million years is developed by CEED researchers. The model reconstructs continents in longitudes in such a way that large igneous provinces and kimberlites are positioned above the plume generation zones in the Earth’s deep mantle. This provides a framework to understand how the mantle interacts with plate tectonics.
The globe shows a reconstruction of the continents in Late Devonian where Laurussia (including North America, Greenland, Scandinavia & England) was separated from Gondwana (South America) by the Rheic Ocean, and Siberia by the Ægir Sea. These continents are positioned in latitude from paleomagnetic data but their longitude is calibrated in such a way that kimberlites (green circles) fall directly above the plume generation zones in the deep mantle. Figure: T.H. Torsvik/CEED.
The research team lead by professor Trond Helge Torsvik from Centre for Earth Evolution and Dynamics (CEED), University of Oslo (UiO), published June 2 the research on the absolute plate motion and true polar wander in the article Deep mantle structure as a reference frame for movements in and on the Earth in the prestigious American journal - Proceedings of the National Academy of Sciences (PNAS).
The article may impact the current understanding of Earth’s geologic history, including proposed scenarios for mantle convection in the deep past and the long-term stability of mantle reservoirs in the lowermost mantle.
The research is conducted by the Centre for Earth Evolution and Dynamics (CEED), UiO but also involves researchers from other institutions.
Some Earth facts:
Mantle is the layer between the crust and the outer fluid core of the Earth. About 2,900 kilometers thick and constitutes ca. 84% of the Earth's volume.
Large igneous provinces (LIPs) are large surface expressions, in the form of igneous rocks of overwhelmingly of basaltic affinity, and with catastrophically rapid dissipation of large quantities of internal heat.
Plume generation zones are narrow loci of upward fluxes of hot and buoyant material (mantle plumes) from the core mantle boundary at the margins of two large thermochemical reservoirs ("Tuzo and Jason").
Kimberlite is a volatile-rich ultramafic igneous rock best known for sometimes containing diamonds. It is named after the town of Kimberley in South Africa.
Almost 320 million years ago between the late Paleozoic and early Mesozoic Eras a supercontinent called Pangea was formed. Since Pangea was born and later dispersed, the majority of large igneous provinces and diamond-bearing rocks (kimberlites) near Earth’s surface can be sourced to plumes erupting from the margins of two large thermochemical reservoirs (Tuzo and Jason) at the core-mantle boundary.
This correlation suggests that the two nearly antipodal deep mantle structures, situated beneath Africa and the Pacific Ocean, have remained stable for at least 300 million years.
Seeking to test whether this remarkable surface to core-mantle boundary correlation was sustainable prior to the supercontinent Pangea, Torsvik and CEED colleagues, as well as international collaborators, had to develop a new plate tectonic model before the Pangea supercontinent. It is this model which is now published in the prestigious PNAS journal.
A framework model
– We had to develop a new model for absolute tectonic plate motion that maintained the connection back 540 million years and that complied with known geological and tectonic constraints such as opening and closing oceans and building mountains, explains professor Trond H. Torsvik which also is the research director of the CEED centre.
The new model by Torsvik and co-workers is all about finding a way to navigate continents in longitude (meridians). Historically, navigation as a tool was transformed in the mid-eighteenth century by the invention of a robust and accurate sea-going chronometer that allowed mariners to calibrate longitude against an arbitrary prime meridian (zero degrees longitude) through the Royal Observatory in Greenwich. In combination with latitude derived from the positions of the sun or stars people traveling on the sea could for the first time reliably calculate their position on the globe.
Earth scientists have been in a comparable position of having no way of calculating the longitudes of continents in deep time. With the new model, CEED scientists and their collaborators are now able to navigate continents in longitude (latitude is derived from paleomagnetic data) back to the dawn of the Paleozoic (540 million years ago).
True polar wander
However, the issue is complicated. One challenge the research team had to deal with was the Earth's rotation axis (with respect to which latitude is defined) is not fixed to the deep mantle and the stable mantle reservoirs therein.
The team identified six phases of true polar wander ― motion of the spin axis relative to the deep mantle ― during the Paleozoic Era along a great circle centered at the equator ~11° east (or 169°W) of Greenwich, approximately the longitudes at which also the two thermochemical piles in the deep mantle are centered. The city of Oslo would therefore be a better choice as the Earth’s prime meridian according to this research article.
– The new plate tectonic model, in which Tuzo and Jason appear to have been stable for at least 540 million years, suggest that a very early origin of these deep mantle structures is a viable hypothesis, and our approach can potentially be extended to the assembly of the previous supercontinent Rodinia about one billion years ago, explains professor Torsvik. The model is a model for only the continents and the next step in improving it will be developing a global model for the entire lithosphere, including synthetic oceanic lithosphere. This is challenging but essential for assessing whether the new model is tectonically and geodynamically plausible.
About the CEED centre
The Centre for Earth Evolution and Dynamics (CEED) was established in 2013 at University of Oslo, Department of Geosciences. The centre is a Centre of Excellence funded by the Norwegian Research Council.
The centre is dedicated to research of fundamental importance to the understanding of our planet and the vision is to develop an Earth model that explains how mantle processes interact with plate tectonics and trigger massive volcanism and associated environmental and climate changes throughout Earth history.
Torsvik, T.H., Van der Voo, R., Doubrovine, P.V., Burke, K., Steinberger, B., Ashwal, L.D., Trønnes, R.G., Webb, S.J., & A.L. Bull, (2014). Deep mantle structure as a reference frame for movements in and on the Earth. Proceedings of the National Academy of Sciences. Doi:10.1073/pnas.1318135111