Crust 5.1-based inference of the Earth's dynamics surface topography: Geodynamic implications

Abstract

We employ the recently published Crust 5.1 model of global crustal structure (Mooney et al. 1998) to estimate the topographic contributions of isostatically compensated crustal loads and, in turn, infer a long-wavelength field for Earth's dynamic surface topography. Our inference of dynamic surface topography is characterized by peak-to-peak variations of the order of 4 km. We argue that the square root of age variation of the oceanic bathymetry has a dynamic origin. Remarkably, continental regions are the site of deep (approximately 1.5 km) dynamic surface topography depressions. The power spectrum of the field is dominated by spherical harmonic degrees ℓ = 1, 4 and 5, suggesting the importance of continent-ocean differences, rather than deep lower mantle heterogeneity, to its origin. We model the Crust 5.1-based inference of dynamic surface topography within the context of seismic tomography-based internal loading theories. We compute the depth dependences of dynamic surface topography response functions for various viscosity profiles that we consider in our analyses. Our best descriptions of the Crust 5.1-based inference achieve total variance reductions of the order of 70 per cent in the spherical harmonic degree range ℓ = 1-8. These are obtained for whole-mantle circulation models that preclude lower mantle heterogeneity from maintaining significant vertical stresses on the outer surface. These models are characterized by a significant increase in viscosity across the depth of the 660 km seismic discontinuity, by a factor of at least 50 relative to the average viscosity of the upper mantle. We also consider results of a layered circulation model but, in this case, we are unable to reconcile constraints provided by large-scale superswell topography. We propose that the anti-podal Pacific and African superswells are dynamically maintained by positively buoyant lower mantle superplumes imparting on the outer surface a large-scale, low-amplitude, predominantly degree 2 pattern of vertical stresses. Finally, we note that the Crust 5.1-based inference of dynamic surface topography provides useful geodynamic constraints on the nature of deep subcontinental structure: optimal descriptions of the data require deep subcontinental keels composed of anomalously dense material.