COMPOSITIONAL VS THERMAL BUOYANCY AND THE EVOLUTION OF SUBDUCTED LITHOSPHERE

We formulate 2-D Cartesian finite element models that explore the fate of compositionally defined lithosphere as it encounters a viscosity i ncrease at the boundary between the upper and lower mantle. Subducted lithosphere is represented as a cold, stiff, layered composite of dens er eclogite underlain by more buoyant harzburgite. Slabs impinging on a lower mantle 30 and 100 times more viscous than the upper mantle thi cken and fold strongly as they penetrate the lower mantle. Approximate ly a factor of two thickening occurs via pure shear just above the dis continuity, with additional enhancement due to folding by over a facto r of two. No separation of the individual slab components occurs at th e discontinuity, and direct comparison with model in which composition al buoyancy is explicitly ignored indicates that slab evolution is lar gely controlled by the thermal buoyancy. These results are at odds wit h hypotheses about slab evolution in which the compositional buoyancy contributions lead to component separation and the formation of slab m egaliths or a compositionally layered upper mantle.