Contrasting results on the magnitude of the dynamic component of topography motivate us to analyse the sources of uncertainties affecting long wavelength topography modelling. We obtain a range of mantle density structures from thermo-chemical interpretation of available seismic tomography models. We account for pressure, temperature and compositional effects as inferred by mineral physics to relate seismic velocity with density. Mantle density models are coupled to crustal density distributions obtained with a similar methodology. We compute isostatic topography and associated residual topography maps and perform instantaneous mantle flow modelling to calculate the dynamic topography. We explore the effects of proposed mantle 1-D viscosities and also test a 3D pressure- and temperature-dependent viscosity model. We find that the patterns of residual and dynamic topography are robust, with an average correlation coefficient (r) of respectively ∼0.74 and ∼0.71, upper-lower quartile ranges of 0.86–0.65 for residual topography and 0.83–0.62 for dynamic topography maps. The amplitudes are, on the contrary, poorly constrained. For the static component, the inferred density models of lithospheric mantle give an interquartile range of isostatic topography that is always higher than 100 m, reaching 1.7 km in some locations, and averaging ∼720 m. Crustal density models satisfying the same compressional velocity structure lead to variations in isostatic topography averaging 350 m, with peaks of 1 km in thick crustal regions, and always higher than 100 m. The uncertainties on isostatic topography are strong enough to mask, if present, the contribution of mantle convection to surface topography. For the dynamic component, we obtain a peak-to-peak dynamic topography amplitude exceeding 3 km for all our mantle density and viscosity models. These extremely high values would be associated with a magnitude of geoid undulations that is not in agreement with observations. Considering chemical heterogeneities in correspondence with the lower mantle Large Low Shear wave Velocity Provinces (LLSVPs) helps to decrease the peak-to-peak amplitudes of dynamic topography and geoid, but significantly reduces the correlation between synthetic and observed geoid. The correlation coefficients between all our residual and dynamic topography maps (a total of 220 and 198, respectively) is <0.55 (average = ∼0.19). The correlation slightly improves when considering only the very long-wavelength components of the maps (average = ∼0.23). We therefore conclude that a robust determination of dynamic topography is not feasible since current uncertainties affecting crustal density, mantle density and mantle viscosity are still too large. A truly interdisciplinary approach, combining constraints from the geological record with a multi-methodological interpretation of geophysical observations, is required to tackle the challenging task of linking the surface topography to deep processes.
Guerri, M., Cammarano, F., Tackley, P.J. (2016). Modelling Earth's surface topography: Decomposition of the static and dynamic components. PHYSICS OF THE EARTH AND PLANETARY INTERIORS, 261, 172-186 [10.1016/j.pepi.2016.10.009].
Modelling Earth's surface topography: Decomposition of the static and dynamic components
CAMMARANO, FABIO;
2016-01-01
Abstract
Contrasting results on the magnitude of the dynamic component of topography motivate us to analyse the sources of uncertainties affecting long wavelength topography modelling. We obtain a range of mantle density structures from thermo-chemical interpretation of available seismic tomography models. We account for pressure, temperature and compositional effects as inferred by mineral physics to relate seismic velocity with density. Mantle density models are coupled to crustal density distributions obtained with a similar methodology. We compute isostatic topography and associated residual topography maps and perform instantaneous mantle flow modelling to calculate the dynamic topography. We explore the effects of proposed mantle 1-D viscosities and also test a 3D pressure- and temperature-dependent viscosity model. We find that the patterns of residual and dynamic topography are robust, with an average correlation coefficient (r) of respectively ∼0.74 and ∼0.71, upper-lower quartile ranges of 0.86–0.65 for residual topography and 0.83–0.62 for dynamic topography maps. The amplitudes are, on the contrary, poorly constrained. For the static component, the inferred density models of lithospheric mantle give an interquartile range of isostatic topography that is always higher than 100 m, reaching 1.7 km in some locations, and averaging ∼720 m. Crustal density models satisfying the same compressional velocity structure lead to variations in isostatic topography averaging 350 m, with peaks of 1 km in thick crustal regions, and always higher than 100 m. The uncertainties on isostatic topography are strong enough to mask, if present, the contribution of mantle convection to surface topography. For the dynamic component, we obtain a peak-to-peak dynamic topography amplitude exceeding 3 km for all our mantle density and viscosity models. These extremely high values would be associated with a magnitude of geoid undulations that is not in agreement with observations. Considering chemical heterogeneities in correspondence with the lower mantle Large Low Shear wave Velocity Provinces (LLSVPs) helps to decrease the peak-to-peak amplitudes of dynamic topography and geoid, but significantly reduces the correlation between synthetic and observed geoid. The correlation coefficients between all our residual and dynamic topography maps (a total of 220 and 198, respectively) is <0.55 (average = ∼0.19). The correlation slightly improves when considering only the very long-wavelength components of the maps (average = ∼0.23). We therefore conclude that a robust determination of dynamic topography is not feasible since current uncertainties affecting crustal density, mantle density and mantle viscosity are still too large. A truly interdisciplinary approach, combining constraints from the geological record with a multi-methodological interpretation of geophysical observations, is required to tackle the challenging task of linking the surface topography to deep processes.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
