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Julian Lowman, University of Toronto Scarborough (Canada)
Andrew Gait, University of Manchester (United Kingdom)
Carl Gable, Los Alamos National Laboratory (United States)
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Plate boundary locations, and therefore plate dimensions, change significantly over time periods that are relatively short compared to the mantle overturn time-scale. However, the influence of plate geometry evolution on mantle convection has not been widely studied in 3D convection models. We examine the effect of mobile plate boundaries on the time-dependence of the mean surface velocity and individual plate velocities in models featuring different mantle viscosity profiles.
The three-dimensional Cartesian geometry models feature evolving plate geometries, high Rayleigh numbers, periodic boundary conditions, and multiple plates with dynamically determined motion. Plate motion is determined by specifying that each plate move rigidly with a velocity that results in a net shear stress of zero at the base of the thick, viscously defined, lithosphere. This condition ensures that the specified plate motion neither drives nor resists the buoyancy driven flow. The time-dependent plate velocities determine the evolution of the plate geometry.
Plate boundaries evolve as triple junctions and are moved with a velocity that is equal to the average of the velocity of the three surrounding plates. In a 3x3x1 solution domain geometry, we compare the time-dependence of the convection obtained in cases where plate geometry is able to evolve, with cases where the geometry remains fixed. We investigate time-dependence in three distinct viscosity stratification models and compare the time-dependence of the plate velocities in these calculations. In addition, the consistency of the results in a pair of larger geometry calculations is examined (i.e., calculations obtained with 6x6x1 Cartesian geometry solution domains featuring a minimum of 9 plates).
We find that an evolving plate geometry results in more rapid and more dramatic variations in the observed plate velocities. However, the presence of a high viscosity lower mantle dampens the time-dependence of the convection, even over periods in which the plate geometry evolves considerably.
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