The influence of temperature and depth dependent viscosity on geoid and topography profiles from models of mantle convection

Abstract

We consider the geoid and topography profiles from convection models with depth and temperature-dependent viscosity models in a 2-D Cartesian geometry. We use a very low viscosity region in the top center of the geometry to generate plate-like viscosity models. We focus on subduction regions, which have narrow (100–200 km) trenches. The narrow trench is only a minor feature in the regional geoid. In depth-dependent viscosity models, we change the geometry of the weak zone around the ocean trench to study the impact of the weak zone on trench topography and the geoid. We also vary the vertical viscosity structure of the models. The weak zone geometry has very little effect on the surface velocity of the ocean plate or the large scale pattern of convection. The width of the weak zone, within a reasonable range, has little effect on topography and geoid profiles. There is a narrow trough in the geoid and topography profiles. This trough becomes wider and deeper with a high viscosity upper mantle. This may suggest that the high viscosity upper mantle is not suitable for our models. In temperature-dependent viscosity models, we vary the viscosity values for different layers. Temperature-dependent viscosity is also important to producing realistic subduction models; however, the activation energy must be weaker than laboratory estimates of olivine under mantle pressures and temperatures. From both depth and temperature-dependent viscosity models, the viscosity in the lower mantle should be at least ten times greater than in the upper mantle, consistent with previous studies of the geoid in regions of subduction. The topography and geoid profiles can match the observations when the viscosity increases with depth.