Viscous heating: a potential mechanism for the formation of the ultralow velocity zone

The detection of several ultralow velocity zones (ULVZ) on the core-mantle boundary (CMB) together with the recent estimates of the lower-mantle solidus would suggest that some sort of melting may occur inside the upwellings in the deep mantle. We have studied the possibilities of viscous heating as a mechanism for producing favorable conditions leading to melting inside deep-mantle upwellings. A two-dimensional model in an aspect ratio 3 box with temperature- and pressure-dependent rheology and two major phase transitions has been employed. Both viscous and adiabatic heating have been included and various amounts of internal heating have been applied. Viscous heating can heat up the interior of the deep-mantle plumes to around 100 degrees above the CMB temperature. This amount of temperature rise is sufficient to cause melting inside upwellings in the deep lower mantle. We propose here that viscous heating plays an important role in producing conditions leading to the formation of the ULVZ. Massive melting within rising plumes might have been more prevalent in the past. (C) 1999 Elsevier Science B.V. All rights reserved.