Crust 5.1-based inference of the Earth's dynamic surface topography: geodynamic implications

Abstract

SUMMAR YWe 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 ®eld 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 ®eld 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 pro®les 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 wholemantle circulation models that preclude lower mantle heterogeneity from maintaining signi®cant vertical stresses on the outer surface. These models are characterized by a signi®cant 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 antipodal Paci®c 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 5.1based 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.

Constraints on Seismic Models from Other Disciplines - Constraints on 3-D Seismic Models from Global Geodynamic Observables: Implications for the Global Mantle Convective Flow

“…This derivation would, however, require that the amplitudes of seismic velocity anomalies are well constrained and that all relevant mineralogical variables in the mantle (e.g., reference composition, temperature, and equation of state parameters) are sufficiently well known. An alternative method for inferring mantle density anomalies is based on the direct inversion of the convection-related geophysical observables (e.g., Corrieu et al, 1994;Deschamps et al, 2001;Forte and Mitrovica, 2001;Forte and Perry, 2000;Forte et al, 1993a,b;Karpychev and Fleitout, 2000;Panasyuk and Hager, 2000;Pari, 2001). An alternative method for inferring mantle density anomalies is based on the direct inversion of the convection-related geophysical observables (e.g., Corrieu et al, 1994;Deschamps et al, 2001;Forte and Mitrovica, 2001;Forte and Perry, 2000;Forte et al, 1993a,b;Karpychev and Fleitout, 2000;Panasyuk and Hager, 2000;Pari, 2001).…”

Density of the continental roots: compositional and thermal contributions

“…The CRUST5.1 model represents 5 ‡U5 ‡ averaged crustal data derived from seismic studies [17]. It was used earlier to calculate mantle and isostatic gravity anomalies and residual topography [24^26] as well as longwavelength dynamic surface topography [15,27,28]. Here, the most recent 2 ‡U2 ‡ global model of crustal structure, CRUST2.0, was used, which is supplemented by 1 ‡U1 ‡ sedimentary thickness data [29].…”

Nature of the crust-mantle transition zone and the thermal state of the upper mantle beneath Iceland from gravity modelling

“…The former is proportional to the total sum of anomalous masses but the latter is proportional to the vertical velocity gradient in the upwelling mantle flow multiplied by viscosity (e.g. Pari 2001). However, the last component may be neglected for the uppermost mantle which is confirmed, for example, by the fact that kernels for a dynamic topography are close to −1 near the surface (e.g.…”

“…Receiver functions have also been used to estimate the deep reflector below Iceland (Du & Foulger 1999, 2001Schlindwein 2001 pers. comm.).…”