The influence of lateral permeability of the 660-km discontinuity on geodynamic models of mantle flow. Annemarie G. Muntendam-Bos 1, Ondrej Cadek 2, Wim Spakman 3 1) SRON Netherlands Institute for Space Research, EOS Division, Utrecht. 2) Department of Geophysics, Charles University, Prague, Czech Republic. 3) Faculty of Earth Sciences, Utrecht University.

2 Outline: ➢ Geoid perturbations due to mass anomalies in the Earth ➢ Modelling mantle flow ➢ The controversy: Whole-mantle versus layered flow ➢ Laterally variable layering of the flow ➢ Implications for geoid fit and dynamic topography ➢ Conclusions & Future work

3 Geoid perturbations due to mass anomalies in the Earth (1): The geoid is an equipotential surface: If our planet was completely covered by water its surface elevation would correspond to the observed geoid. The topography of the geoid is +/- 100 meters. The topography is a clear representation of the lateral variations in the Earth's density. These density variations drive mantle flow: Lighter rocks flow up, while heavier rocks sink down; all very-very slowly (cm/year)

4 Geoid perturbations due to mass anomalies in the Earth (2):

5 Modelling mantle flow Basic equations describing steady-state motion driven by density anomalies in a fluid are 1) Continuity equation: 2) Poisson's equation: 3) Navier-Stokes equation: The total stress for a Newtonian fluid is described as: Boundary conditions: Core-Mantle boundary => free-slip; no radial flux Base of the lithosphere => no slip Density model: Scaled form seismic tomographic velocity model of the Earth. Explanation of variables:

6 The controversy: Whole-mantle versus layered flow Recently, partially layered mantle flow models have been proposed! 660-km discontinuity: ➢ spinel transforms into perovskite & magnesiow üstite ➢ endothermic reaction (absorbs heat) ➢ inhibits flow 2 significant phase transition exist within the Earth: 410-km discontinuity: ➢ olivine tranforms into spinel ➢ exothermic reaction (produces heat) ➢ facilitates flow

7 Laterally variable layering of the flow (1) There is regional tomographic evidence for slabs lying atop and plumes broadening against the 660- km discontinuity, as well as slabs and plumes penetrating the interface. We devellop a model for the permeability of the 660-km discontinuity reflecting these phenomena.

8 Laterally variable layering of the flow (2) Model based on two tomographic depth slices at app. 500 km and app. 810 km depth A tomographic velocity is provided for each 0.6 °x0.6° grid cell. For each grid cell the velocity at 500 km depth is compared to the velocity at 810 km depth and the degree to which they compare is computed (their correlation). Complete correlation (=1): The size and sign of the velocities are identical. Complete anti-correlation (=-1): The sizes of the velocities are identical, but they have opposite signs. Implication: For throughgoing slabs and plumes we obtain a possitive correlation, for flatlying slabs and broadening plumes we obtain a negative correlation (anti- correlation).

9 Laterally variable layering of the flow (3)

10 Implications for geoid fit and dynamic topography (1) Geoid Gravity field

11 Implications for geoid fit and dynamic topography (2) Topography of the 660-km interface Dynamic surface topography

12 Conclusions: Future work: ➢ Imposing more realistic boundary conditions: e.g. Plate motions at the surface. ➢ Imposing lateral viscosity variations in the upper 300 km of the mantle. ➢ Comparing the predicted geoid & gravity field to the observed. ➢ Consider a more layered viscosity profile of the Earth's mantle. ➢ First to take into consideration the lateral variations in permeability of the 660-km discontinuity. ➢ Our model clearly mimics the evidence for flat lying and throughgoing slabs. ➢ The lateral variations improve/deteriorate the fit of the predicted geoid to the observed. ➢ The lateral variations improve/deteriorate the fit of the predicted gravity field to the observed.