EFFECT OF DEPTH-DEPENDENT VISCOSITY ON THE PLANFORM OF MANTLE CONVECTION

LITHOSPHERIC plate motions at the Earth's surface result from thermal convection in the mantle(1). Understanding mantle convection is made d ifficult by variations in the material properties of rocks as pressure and temperature increase from the surface to the core. The plates the mselves result from high rock strength and brittle failure at low temp erature near the surface. In the deeper mantle, elevated pressure may increase the effective viscosity by orders of magnitude(2-5). The infl uence of depth-dependent viscosity on convection has been explored in two-dimensional numerical experiments(6-8), but planforms must be stud ied in three dimensions. Although three-dimensional planforms can be e lucidated by laboratory fluid dynamic experiments(9,10), such experime nts cannot simulate depth-dependent rheology. Here we use a three-dime nsional spherical convection model(11,12) to show that a modest increa se in mantle viscosity dth depth has a marked effect on the planform o f convection, resulting in long, linear downwellings from the upper su rface boundary layer and a surprisingly 'red' thermal heterogeneity sp ectrum, as observed for the Earth's mantle(13). These effects of depth -dependent viscosity may be comparable to the effects of the plates th emselves.