Self‐consistent generation of tectonic plates in time‐dependent, three‐dimensional mantle convection simulations 2. Strain weakening and asthenosphere

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Abstract
In a previous paper [Tackley, 2000b] it was shown that a combination of temperature‐dependent viscosity and viscoplastic yielding is sufficient to give rudimentary plate tectonic‐like behavior in three‐dimensional models of mantle convection. Here the calculations are extended to include two complexities that have been suggested as being important in localizing deformation at plate boundaries: strain weakening and the presence of a low‐viscosity asthenosphere. Introducing an asthenosphere by reducing the viscosity by a factor of 10 where material reaches a solidus (1) dramatically improves plate quality, even if the asthenosphere is restricted to regions around spreading centers, (2) gives good, smoothly evolving platelike behavior over a wide range of yield stress values spanning an order of magnitude, and (3) gives bimodal stable solutions over a range of yield stresses; either immobile‐lid or plate behavior may be obtained, depending on initial condition. By contrast, introducing an asthenosphere by using depth‐dependent viscosity has a much smaller effect on system behavior. Increasing strain weakening (1) improves localization at spreading centers but leads to an increasingly complex network of spreading centers fragmenting the plates, (2) weakens convergent zones and can make downwellings highly episodic, and (3) does not lead to pure transform boundaries in these calculations. Time‐dependent “damage” evolution and instantaneous strain‐rate weakening give very similar results. All cases that display platelike behavior have very long wavelength mantle heterogeneity, consistent with Earth's.