On the amplitude of dynamic topography at spherical harmonic degree two

Abstract

Two large, seismically slow regions in the lower mantle beneath Africa and the Pacific Ocean are sometimes referred to as "superplumes". This name evokes images of large-scale active upwellings. However, it remains unclear whether these features are real or represent collections of multiple regular mantle plumes. Here, we investigate the implications of these upwellings for dynamic topography. We combine detailed measurements of oceanic residual topography from Hoggard et al. (2016) with continental constraints derived from CRUST1.0 to produce a global model expanded in spherical harmonics. Observed dynamic topography is subsequently compared to predictions derived from mantle flow following Steinberger (2016) using tomographic density models. Results yield relatively good overall agreement and amplitude spectra with similar slopes, except for degree two (i.e. > 10, 000 km wavelengths) where predicted amplitude is more than two times as large and is dominated by contributions from the lower mantle. Predictive models suggest two large-scale uplifted regions above the "superplumes" that are barely seen in the observed topography. We suggest that this mismatch can only partly be reconciled by altering the seismic velocity to density conversion factor or by including the effects of lower mantle chemical heterogeneity. In addition, it may be important to consider more significiant revisions to the lower mantle flow patterns, such as those possibly induced by different radial viscosity profiles and laterally-varying or anisotropic lower mantle viscosity.

On the Scales of Dynamic Topography in Whole‐Mantle Convection Models

“…The dynamic topography spectra that we obtained (Figure 7) are comparable to those derived by recent inversions of present-day mantle structures (Yang & Gurnis, 2016) to simultaneously fit the long-wavelength geoid, free-air gravity anomalies, gravity gradients, and residual topography point data (Hoggard et al, 2016), and of a high-resolution upper mantle tomography model (Steinberger et al, 2017). More recently, Steinberger et al (2017) used a crustal model to estimate present-day continental residual topography and also found a limited degree-2 residual topography amplitude. The discrepancy between numerical models of dynamic topography and observation-derived present-day dynamic topography remains debated (e.g., Hoggard et al, 2017Hoggard et al, , 2016Molnar et al, 2015;Steinberger et al, 2017;Watkins & Conrad, 2018;Yang & Gurnis, 2016;Yang et al, 2017).…”

“…These processes produce dynamic topography over a range of spatial scales that cannot yet be taken into account in 3-D global models because of computational limitations. The discrepancy between numerical models of dynamic topography and observation-derived present-day dynamic topography remains debated (e.g., Hoggard et al, 2017Hoggard et al, , 2016Molnar et al, 2015;Steinberger et al, 2017;Watkins & Conrad, 2018;Yang & Gurnis, 2016;Yang et al, 2017). The dynamic topography spectra that we obtained (Figure 7) are comparable to those derived by recent inversions of present-day mantle structures (Yang & Gurnis, 2016) to simultaneously fit the long-wavelength geoid, free-air gravity anomalies, gravity gradients, and residual topography point data (Hoggard et al, 2016), and of a high-resolution upper mantle tomography model (Steinberger et al, 2017).…”

“…Pulses of mantle plume activity or upper mantle vigorous convection at a maximal rate of However, most global mantle convection models do not predict significant dynamic topography for wavelengths smaller than 5,000 km (Hoggard et al, 2016). Recently, Steinberger et al (2017) showed that considering uppermost mantle density heterogeneities from a high-resolution near-surface tomography model, in combination with a global S wave model, produces intermediate scales of dynamic topography (500 to 10 4 km). In addition, the spatial variability of the seismic velocity to density conversion factor necessary to predict the mantle flow is uncertain and chemical effects are generally ignored in the uppermost mantle.…”

A Neogene history of mantle convective support beneath Borneo

“…Despite more than 30 years of geodynamic modeling, there is considerable debate about the amplitude and wavelength of present-day dynamic topography (see e.g. Steinberger , 2007;Spasojevic & Gurnis, 2012;Steinberger et al, 2017;Müller et al, 2018). A significant example of this lack of consensus concerns the history of regional epeirogeny across southeast Asia.…”

“…According to Stokes' law, the resultant mass excess generates and maintains large-scale downwelling within the viscously deformable mantle, which produces and maintains surface draw-down (compare Figure 1c and d). It is important to emphasize that this topic is a rapidly evolving one and that several alternative dynamic topographic models predict that a region encompassing Borneo underwent significant Neogene uplift (see, e.g., Steinberger et al, 2017;Müller et al, 2018).…”