DYNAMICAL CONSEQUENCES ON FAST SUBDUCTING SLABS FROM A SELF-REGULATING MECHANISM DUE TO VISCOUS HEATING IN VARIABLE VISCOSITY CONVECTION

We have studied 2-D time-dependent convection for a theology which is both non-Newtonian and temperature-dependent. Strong effects associate d with viscous heating are found in the downwelling sheets, which are heated on both sides with an intensity around O(10(2)) times the chond ritic value. The magnitude of viscous heating increases strongly with the subduction speed. The slab interior is weakened by viscous heating and slab breakoff then takes place. This process provides a self-regu lating mechanism for governing the speed of intact stabs able to reach the deep mantle. Timescales associated with viscous heating are quite short, a few million years. Internal heating by radioactivity decreas es the amount of shear heating.