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Understanding the dynamics of tectonic plates is one of the most important problems in geophysics today. Convection in the Earth's mantle is generally assumed to drive global plate motions, but the details of that linkage remain obscure. Bouyancy forces associated with subduction of cool, dense lithosphere at zones of plate convergence are thought to provide significant driving force, but the relative magnitudes of other driving and resisting forces are less clear. Over the past decade different numerical approaches have been developed in order to model self-consistently plate tectonics together with mantle convection, in an attempt to better understand what causes plate motions and, most importantly, plate motion changes. One approach consists in trying to generate plate tectonics from mantle dynamics. Specifically, efforts have been focused on developing numerical models of convection in a highly viscous fluid that allow the generation of narrow, weak boundaries separating broader, stiffer regions in the upper thermal boundary layer. Although those models are capable to reproduce some peculiarities of plate kinematics (i.e. vorticity and divergence in amounts compatible with observations), they require the implementation of mantle rheologies that appear somewhat unusual, when compared to results from laboratory experiments performed on olivine. Here we take a different approach.
Building on recent substantial, yet separate progress in modeling lithosphere dynamics and mantle convection, we couple global lithosphere models, featuring among others realistic plate configurations and the brittle behavior of the upper lithosphere, with high-resolution (more than 100 million grid points) 3-D circulation models of Earth's mantle. We demonstrate that such approach allows obtaining an accurate budget of forces driving and resisting plate motions. Furthermore we make two predictions of plate motion changes for the convergent systems Nazca/South America and India/Eurasia that are explicitly tied to processes occurring in the upper, brittle lithosphere. In both cases numerical results compare remarkably well with the recent paleomagnetic and geodetic records of plate motions, suggesting that the faulted nature of lithosphere needs to be implemented through a brittle rheology in global coupled models of mantle convection/lithosphere dynamics. Finally, we speculate on the possibility that global plate motions represent one additional constraint to the amount of heat conducted into the Earth's mantle from the core.
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