THE ROLE OF SHEAR HEATING IN LUBRICATING MANTLE FLOW

Abstract

The role of viscous heating in mantle flows has been studied within the framework of a steady-state, two-dimensional cartesian model with a composite non-Newtonian and Newtonian rheology. There is temperature-dependence in the upper-mantle rheology and temperature-and pressure-dependent rheology in the lower mantle. We have considered adiabatic and viscous heating, using the extended Boussinesq approximation. We have selected as the only control variable the temperature at the core-mantle boundary (CMB). We have compared systematically both the Boussinesq and extended-Boussinesq solutions for the same rheology and same amount of internal heating by varying the temperature at the CMB from 2000 K to around 10,000 K. Viscosity contrasts in the system increase with the vigor of convection and reach up to five orders in magnitude. Both the vigor of convection and the global viscous heating increases systematically with the temperature at the CMB. The rms velocity shows a power-law type dependence on the temperature at the CMB with a power-law exponent of 1.5. For high CMB temperatures, greater then 3500 K, a focused concentration of shear heating promotes the development of high strain-rate and low viscosity zones next to descending flows, which is primarily non-Newtonian in character. In the high CMB temperature regime, much more pronounced shear zones are developed along the downwelling for models with viscous heating present than for the Boussinesq flows, illustrating the important role played by mechanical heating on lubricating limbs in mantle circulation.