Constraints on upper-mantle viscosity
from the flow-induced pressure gradient
across the Australian continental keel

Christopher Harig¹ and Shijie Zhong and Frederik J Simons²

1 Dept. of Geological Sciences
University of Colorado
Boulder CO 80309, USA

2 Geosciences Department
Princeton University
Princeton NJ 08544, USA

Abstract

The thickness of continental lithosphere varies considerably from tectonically active to cratonic regions, where it can be as thick as 250-300km. Embedded in the upper mantle like a ship, when driven to move by a velocity imposed at the surface, a continental keel is expected to induce a pressure gradient in the mantle. We hypothesize that the viscosity of the asthenosphere or the shear coupling between lower lithosphere and asthenosphere should control this pressure effect and thus the resulting dynamic topography. We perform three-dimensional finite-element calculations to examine the effects of forcing a continental keel by an imposed surface velocity, with the Australian region as a case study. When the upper mantle is strong, but still weaker than the lower mantle, positive dynamic topography is created around the leading edge, and negative dynamic topography around the trailing edge of the keel, which is measurable by positive and negative geoid anomalies, respectively. For a weak upper mantle the effect is much reduced. We analyze geoidal and gravity anomalies in the Australian region by spatiospectral localization using Slepian functions. The method allows us to remove a best-fit estimate of the geographically localized low spherical-harmonic degree contributions. Regional geoid anomalies thus filtered are on the order of +/-10 m across the Australian continent, with a spatial pattern similar to that predicted by the models. The comparison of modeled and observed geoid anomalies places constraints on mantle viscosity structure. In a two-layer model the viscosity ratio between upper and lower mantle remains insufficiently constrained. The addition of a third, weak, upper-mantle layer — an asthenosphere — amplifies the effects of keels. Our three-layer models, with lower-mantle viscosity of 3x10²² Pa s, suggest that the upper mantle (asthenosphere) is 300 times weaker than the lower mantle, while the transition zone (400-670 km depths) has a viscosity varying between 10²¹ and 10²² Pa s.

Figures

1. Figure 01 Cartoon illustrating the mass balance argument in the analytical treatment
   
2. Figure 02 Tomographic keel depth, EGM96 geoid and Slepian function coverage of the Australian continent
   
3. Figure 03 Example of Slepian filtering technique for a low maximum bandwidth
   
4. Figure 04 Examples of model output
   
5. Figure 05 Mantle velocity at 200 km depth for two model cases
   
6. Figure 06 Example model fits
   
7. Figure 07 Color-shaded images of misfit between filtered model cases and observed geoid
   

Frederik Simons