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- Title
Reconciling Observations and Predictions of Earth's Dynamic Topography.
- Authors
Richards, Fred; Hoggard, Mark; Ghelichkhani, Sia; Lau, Harriet; Austermann, Jacqueline
- Abstract
While the bulk of topography on Earth is generated and maintained by variations in thethickness and density of crust and lithosphere, a significant time-variable contribution isexpected to result from convective flow in the underlying mantle. For over three decades, thisdynamic topography has been calculated numerically from inferred density structure andradial viscosity profiles. These models predict ±2 km of long wavelength (i.e., ∼ 20,000 km)dynamic topography with minor contributions at wavelengths shorter than ∼ 5,000 km.Recently, observation-based studies have revealed that, at the longest wavelengths, dynamictopography variation is approximately half that predicted, with ±1 km amplitudesrecovered at shorter wavelengths. This significant discrepancy between predictions andobservations suggests that current knowledge of our planet’s internal structure isincomplete. However, if Earth models can be found that are compatible with these newconstraints, a significantly improved understanding of mantle dynamics is withinreach. Most numerical models excise the upper ∼ 300 km of Earth’s mantle and are thus unableto reconstruct the short wavelength and fast rates of vertical motion observed in manylocations. However, residual depth observations strongly anticorrelate with asthenosphericshear wave velocity anomalies suggesting a close link between surface deflections anddensity anomalies immediately beneath the lithosphere. Through conversion of upper mantleshear wave velocities to temperature and density using a calibrated anelasticityparameterization, we show that observed shorter wavelength (i.e., ≤ 5,000 km) dynamictopography is largely generated by ±150 ∘C temperature anomalies in a low-viscosityasthenospheric channel. Inclusion of this anelastically-corrected density structure inwhole-mantle instantaneous flow models also reduces long wavelength discrepancy betweenpredictions and observations of dynamic topography. Residual mismatch is further reduced ifthe basal 300 km of large low shear wave velocity regions in the deep mantle areassumed to be basaltic in composition and therefore negatively buoyant. Importantly,these thermochemical models of mantle convection simultaneously improve fit tocore-mantle boundary topography and the geoid, contradicting previous studies thatsuggest modest long wavelength dynamic topography cannot be reconciled withgeoid observations using physically reasonable Earth models. This work providesstrong evidence for large basaltic regions in the deepest mantle, corroborating recentresults from inverse modelling of body tides, normal modes and seismic velocities.
- Subjects
EARTH tides; TOPOGRAPHY; VERTICAL motion; SEISMIC wave velocity; CONVECTIVE flow; EARTH topography; SURFACE waves (Seismic waves); SEISMIC waves
- Publication
Geophysical Research Abstracts, 2019, Vol 21, p1
- ISSN
1029-7006
- Publication type
Article