Large long wavelength dynamic topography with small amplitude free air anomaly

Abstract:

The amplitude of convection-induced long wavelength dynamic topography is uncertain. One advocacy of small amplitude dynamic topography invokes the observed small free air anomaly and GTR (free air gravity-topography ratio). We investigate this problem by inversing mantle structure constrained by long wavelength geoid, free air anomaly, gravity gradient and recently compiled high accuracy residual topography. Although geoid, free air anomaly and gravity gradient essentially include the same information, they are sensitive to mantle structure at different depth. Residual topography provides direct constraints on mantle convection. However, previous estimates of residual topography are usually with large uncertainty. We use recently compiled high accuracy point measurements of residual topography to constrain our inversion. We include lateral viscosity variation (temperature dependent viscosity, shallow viscosity heterogeneity, LVV) in our inversion since previous forward models suggests that LVV affects the surface gravitational and topography signals significantly. Our results indicate that considering LVV in the inversion does not change the misfit significantly, in contrary to previous forward models, suggesting (1) the over-fitting in previous 1D viscosity profile inversions or (2) temperature induced lateral viscosity variation are balanced by other effects at long wavelength features (2-8 degree). Although the inverted mantle structure is non-unique, all models predict larger than ~ 1.0 km amplitude long wavelength dynamic topography, consistent with the long wavelength components of the recent residual topography observations. Our analysis indicates that the relationship between free air gravity and topography depends on sphere harmonic degrees (wavelength). Thus the small amplitude free air gravity anomaly does not require small amplitude dynamic topography. We propose that despite GTR is quite helpful in deciphering the compensation depth of topography, it should not be used to estimate compensation depth based on global distributed observations on Earth.