Impacts of lithospheric rheology on surface topography

Dynamic topography is the surface expression of asthenospheric convection by means of vertical deflection of the lithosphere and includes the topographic responses to mantle flow patterns such as upwelling plumes and downwelling slabs. Dynamic topography depends on the properties of the convective anomaly, but the modulation due to properties of the lithosphere are less well understood. This work aims to investigate the impacts of the lithosphere through numerical and analytical approaches using purely viscous rheology and compare our results with analogue experiments. Unlike the free-slip top boundary condition used in traditional numerical models, we apply a more realistic, stress-free surface boundary condition, which can avoid artifacts of the free-slip approximation. This is particularly important for thick, high viscosity lithosphere due to the absence of plate bending resistant forces and an effective no-slip boundary at the base of the lithosphere. We find that a smaller viscosity, thinner lithosphere generates higher and narrower topography than the larger viscosity, thicker case. A stratified lithosphere with a weak lower crust (a decoupling layer) also creates higher topography than the case of a homogeneous lithosphere, which can be understood by the lowered averaged viscosity. We then explore the role of elasticity and edge-driven topography using viscoelastic rheology and examine the differences in lithospheric deformation, stress level and distribution. We find that viscoelastic rheology can increase the uplift rates on Maxwell timescales due to the additional elastic component compared to purely viscous rheology. We also find that mantle driven uplifted topography creates extensional stress in the lithosphere while tectonic edge-driven uplift generates much lower compressional stress. Such signatures may be helpful to detect the dominant contribution to regional topography by jointly examining geodetic, geological, and seismic data.