VISCOUS DRAG AND LATERAL VISCOSITY HETEROGENEITIES - IMPLICATIONS FORINTRAPLATE DEFORMATION

We consider the effects of lateral Variations in the thickness of the lithosphere on surface topography, horizontal intraplate deformation a nd stress accumulation for plates that drift with respect to the highl y viscous lower mantle and transition zone. The lithosphere and upper mantle are described by means of a viscoelastic Maxwell theology. A fi nite element scheme allows us to deal with large viscosity contrasts i n the vertical and horizontal directions. Heterogeneous lithospheric s tructures appropriate a ''Mariana type'' subduction is modeled in a ve rtical cross sections. The dynamic topography and intraplate deformati on, maintained by constant horizontal forces applied at the edges of t he drifting plate, are extremely sensitive to lateral viscosity contra sts which interact with the upper mantle flow induced by the relative velocity with respect to the lower mantle. In concert with the other d riving forces of plate tectonics the mechanism considered in this pape r should be considered a valuable contributor to the evolution of back -arc basins and to the explanation of the largest angle of subduction in west-dipping slabs.