The influence of lower mantle viscosity on plate velocity timedependence in numerical mantle convection models
Abstract
Plate boundary locations, and therefore plate dimensions,can change significantly over time periods that are relatively short compared to the mantle overturn timescale. 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 timedependence of the mean surface velocity, individual plate velocities and heat flow in models featuring different mantle viscosity profiles.. The threedimensional 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 timedependent 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 3×3×1 solution domain geometry, we compare the timedependence of the convection obtained in cases where plate geometry is able to evolve, with cases where the geometry remains fixed. We investigate timedependence in three distinct viscosity stratification models. We compare the timedependence of the plate velocities and mantle heat flux in these calculations and examine the consistency of the results in a pair of larger geometry calculations (obtained with 6×6×1 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 all global measures of the vigor of the convection. However, the presence of a high viscosity lower mantle dampens the timedependence of the convection, even over periods in which the plate geometry evolves considerably.
 Publication:

AGU Fall Meeting Abstracts
 Pub Date:
 December 2007
 Bibcode:
 2007AGUFM.T23E..08L
 Keywords:

 8120 Dynamics of lithosphere and mantle: general (1213);
 8121 Dynamics: convection currents;
 and mantle plumes;
 8125 Evolution of the Earth (0325);
 8155 Plate motions: general (3040);
 8162 Rheology: mantle (8033)