Evolution of large-scale structure in Earth's mantle
Colloquium
Max Rudolph
The viscosity of the mantle affects every aspect of the thermal and compositional evolution of Earth's interior. Radial variations in viscosity can affect the sinking of slabs, the morphology of plumes, and the rate of convective heat transport and thermal evolution. I will present the results of new inferences of the depth-variation in mantle viscosity. The lowermost mantle contains two antipodal Large Low Shear Velocity provinces, which are likely to be different in composition and somewhat more dense than the ambient lower mantle. Using full-spectrum mantle tomographic models that jointly constrain shear- and compressional wavespeeds as well as density, we examine the effect of lowermost mantle density variations on inferences of the viscosity profile. We find some evidence for a reduction in viscosity below the mantle transition zone and an increase in viscosity in the mid mantle, near 1000 km depth. We show that global models of mantle circulation that include such a low-viscosity channel or 'second asthenosphere' can reproduce some observations of slab stagnation within and below the mantle transition zone.