Plate-kinematic Constraints from Mantle B�nard Convection

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General kinematic implications for plate tectonics are determined for Rayleigh-Benard convection of the mantle. The continuum of all possible configurations of Benard polygons is probed by large random samples of global configurations (450,000 to 54,000,000), for each of which the Euler poles are determined on the basis of viscous coupling across the asthenosphere. Two computationally related methods lead first, to Euler pole restrictions for fourteen plates, and second, to restrictions on the Benard cell configuration. Result No. 1: Euler poles occur in global “preference-patterns,” which are determined exclusively by the shape of the plate. The observational HS2-NUVEL1 model poles occur near regions preferred by Benard convection (Eurasia excluded); the agreement is best for the most accurate observational poles. Result No. 2: Seven specific mantle Benard cells are indicated by present-day plate motions. The upwelling centers correlate with hotspot domains; the major global subduction zones correlate with Benard model downwelling. This result is independent of the Euler pole accuracy used in its determination, and is consistent with the distribution of low seismic p-wave propagation velocities determined by tomography, and with shear-wave splitting analysis within the asthenosphere. Conclusions: The results suggest that the bulk mantle is divided into less than ten Benard convection cells globally (cf., Fohlmeister and Renka, 2002), each of which extends from the asthenosphere to the core-mantle boundary; turbulent flow, and other perturbations of the Benard kinematics appear to be limited. These primally poloidal flow kinematics provide basal shear forces as a major component in driving plate tectonics, and are specifically configured for the directions of plate motions. The Benard model is incomplete without a dynamic contribution from the lithosphere, which represents a separate convection layer of the distinct polar kinematics of rigid plates. The complete hybrid mechanism for driving plate tectonics includes lithospheric buoyancy dynamics, specifically from the subducting Pacific plate slabs to compensate for plate-slowing due to the “back-flow sector” of the Hawaiian convection cell, and collision-drag dynamics principally for smaller plates or continental margins.