BASAL DRAG AND LATERALLY VARYING LITHOSPHERE - IMPLICATIONS FOR SEA-LEVEL FLUCTUATIONS AND INTRAPLATE DEFORMATION

For plates drifting with respect to the highly viscous lower mantle an d transition zone, we study the potential implications of lateral vari ations in the thickness of the lithosphere on sea level fluctuations a nd surface topography, horizontal intraplate deformation and stress ac cumulation. The lithosphere and upper mantle are described by a viscoe lastic Maxwell rheology within the framework of a finite element schem e. Heterogeneous lithospheric structures, appropriate for cratons and ''Mariana type'' subductions, are modeled in 2D vertical cross section s. The whole set of geophysical and geological signatures is highly se nsitive to lateral viscosity contrasts which interact with the upper m antle flow induced by the relative velocity of the plates with respect to the lower mantle. In concert with the other driving forces of plat e tectonics, the mechanism considered in this paper can be a valuabie contributor to sea-level changes on geologic time scales. Viscous drag at the base of a laterally varying lithosphere can also contribute to the evolution of back-arc basins and to the explanation of the larges t angle of subduction in west-dipping slabs.