Geoid anomalies and dynamic topography from time-dependent, spherical axisymmetric mantle convection

Geoid anomalies and dynamic topography are two important diagnostics of mantle convection. We present geoid and topography results for several time-dependent convection models in spherical axisymmetric geometry for Rayleigh numbers between 10(6) and 10(7) with depth-dependent viscosity and mixtures of bottom and internal heating. The models are strongly chaotic, with boundary layer instabilities erupting out of both thermal boundary layers. In some instances, instabilities from one boundary layer influence the development of instabilities in the other boundary layer. Such coupling between events at the top and bottom of the mantle has been suggested to play a role in a mid-Cretaceous episode of enhanced volcanism in the Pacific. These boundary layer instabilities produce large temporal variations in the geoid anomalies and dynamic topography associated with the convection. The amplitudes of these fluctuations depend on the detailed model parameters, but fluctuations of 30-50% relative to the time-averaged geoid and topography are common, The convective planform is strongly sensitive to the specific initial conditions, Convection cells with larger aspect ratio tend to have larger fractional fluctuations in their geoid and topography amplitudes, because boundary layer instabilities have more time to develop in long cells. In some instances, we observe low-amplitude topographic highs adjacent to the topographic lows produced by cold downwellings. We discuss applications of these results to several situations, including the temporal variability of hotspots such as Hawaii, the topography of subduction zone outer rises, and the topography of coronae on Venus. (C) 1998 Elsevier Science B.V. All rights reserved.