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BPT Report Summary

Project ID: 267631
Funded under: FP7-IDEAS-ERC
Country: Norway


Plate tectonics and the Wilson cycle were recognized as key elements of geodynamics in the 1960’s. Plate tectonic theory was extremely successful in providing a framework for understanding deformation and volcanism at plate boundaries, and allowed us to understand how continent motions through time are a natural result of heat escaping from Earth's deep interior. Plate Tectonics is as fundamentally unifying to the Earth Sciences as Darwin’s Theory of Evolution is to Life Sciences. However, it is an incomplete theory without a clear understanding of how plate tectonics and mantle plumes interact and lacks a generally accepted mechanism that explains plate tectonics in the framework of mantle convection. These are the fundamental problems we set out to resolve in the project, and we have for the first time developed a global absolute plate motion model for the entire Phanerozoic (back to 540 Ma), which have allowed to unravel the link between surface and deep Earth processes, and answer issues like “When, where and how do plumes develop?” and “What is the origin, stability and time-scales for the Large Low Shear-wave Velocity Provinces in the lowermost mantle?”. Situated just above the core-mantle boundary, two large low shear-wave velocity provinces situated beneath Africa and the Pacific - dubbed TUZO and JASON - are identified as long-lived thermochemical piles, probably both denser and hotter in the lowermost parts, and the residual geoid is largely a result of buoyant upwellings overlying them.

The conceptual Earth we have developed in the project is a simple and stable degree-2 planet, and the lower mantle is dominated by sinking slabs and rising thermochemical plumes. Subduction zones show a predominantly large-scale pattern, especially the “ring of fire” circling the entire Pacific. Therefore, slabs sinking all the way to the lowermost mantle also relate to long-wavelength lower-mantle structure dominated by degree-2. Plumes rise from the margins of TUZO and JASON — the plume generation zones — which can be described as loci of an intermittent or continuous upward flux of hot and buoyant material from the core-mantle boundary. On the surface, this flux is witnessed by the catastrophic emplacement of large igneous provinces (LIPs) contributing to episodic biotic extinction events, and volumetrically lesser kimberlites and hotspot volcanoes, of which a few lie on tracks departing from LIPs. LIPs have also contributed to continental breakup and therefore punctuated plate tectonics by creating new plate boundaries and oceanic gateways through geological time.
All LIPs are probably not sourced by plumes from the deepest mantle. Based on global tomography models there are exceptions such as the 15 Ma Columbia River Basalt. Needless to say, our model that most LIPs, kimberlites, and hotspots are predominantly sourced by deep mantle plumes from the margins of TUZO and JASON, has generated debate in the literature. Among the most fervent opponents have been the representatives of the “Andersonian movement” and mantle modellers disagreeing with the interpretation of TUZO and JASON as mantle structures having distinct chemical properties. Our Earth model is very different from an “Andersonian Earth”, where slabs are often halted by the 660 km discontinuity and only punch through after sufficient accumulation, plumes do not exist, and hotspot volcanism is only linked to lithosphere tensile stresses, cracking, and decompression melting. Whole mantle tomography and the reconstructed locations of LIPs and kimberlites in relation to the tomography of the lowermost mantle are clearly at odds with such a planet. But interestingly, the “Andersonian Earth” includes ancient low-velocity regions in the deepest mantle, which are comparable with TUZO and JASON. In our view, these could be primordial thermochemical piles that possibly formed during early magma ocean crystallization (or shortly afterward), perhaps by magmatic segregations of Fe-rich peridotitic or komatiitic materials. It is still unclear, though, why lower-mantle structures similar to today would already have existed back in the Hadean (> 4 Ga), and at the moment we can only state that the stability of TUZO and JASON back to about 540 Ma is consistent with data, and that in some numerical models, it is certainly possible to maintain such piles throughout Earth history.

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