[S16-1] Large low shear velocity provinces and deep mantle structure

[S16-1-05] Constraining Mantle Viscosity and Thermochemical Structure Using the Geoid in 3-D Mantle Convection Models with Plate Motion History

Wei Mao, ^○Shijie Zhong, Mingming Li (Department of Physics, University of Colorado)

The long-wavelength geoid anomalies are one of the most important geophysical observations that have implications on mantle viscosity structure, mantle dynamics, and true polar wander. The geoid shows strong degree-2 anomalies with positive anomalies over the center Pacific and Africa, and it also displays significant positive anomalies over subduction zones at shorter wavelengths (i.e., degree 4 and higher harmonics). Previous studies have demonstrated that the degree-2 geoid anomalies correlate strongly with the long-wavelength lower mantle seismic anomalies, particularly, the large low shear velocity provinces (LLSVP) in the lower mantle, while the shorter wavelength geoid highs over subduction zones are related to subducted slabs. Using the observed seismic structure as a proxy of mantle buoyancy in mantle flow models, these studies suggest that the lower mantle needs to be 30-100 times more viscous than the upper mantle in order to explain the geoid anomalies. Significant progress has been made in the last decade in modeling time evolution of mantle structure in 3-D global mantle convection models with realistic plate motion history and mantle properties. These convection models explain many essential features of seismic structure including the LLSVPs. However, it remains unclear how these models may account for the geoid anomalies. In this study, we compute the geoid and dynamic topography anomalies from the time-dependent mantle convection models. We find that in addition to the viscosity contrast between the lower and upper mantles, the geoid is also sensitive to the lower mantle viscosity and thermochemical nature of the LLSVPs. Using the geoid observation, we compute a large number of convection models with different parameters to place constraints on not only viscosity contrast but also the viscosity itself. We also seek to place constraints on the LLSVPs.