Mantle Heterogeneity and Flow from Seismic and Geodynamic Constraints

Model showing contoured slow shear velocity anomalies with corresponding density and thermal anomalies(Simmons et al., 2007). The density anomalies here are inferred assuming heterogeneity is due solely to temperature anomalies.
<p>
Images of mantle heterogeneity are most commonly in the form of seismic velocity since seismic waves are the most direct mantle probe. Although these static images provide general patterns of heterogeneity in the mantle, it is difficult to directly translate them to mantle flow for a variety of reasons. Some reasons include the inherent non-uniqueness of tomographic inversion and the uncertainties in the mineral physics parameters linking seismic velocity to density perturbations which are the driving force behind mantle flow. In attempts to overcome these obstacles, we have developed tomographic images of the mantle through simultaneous inversion of shear-wave constraints and a suite of convection-related observations including the global free-air gravity field, tectonic plate divergences, dynamic surface topography and the excess ellipticity of the core-mantle boundary. The convection-related observations are interpreted via viscous-flow response functions and density perturbations are internally linked to velocity heterogeneity with mineral physics constraints. This joint inversion procedure has allowed us to directly investigate many hypotheses regarding the style of mantle flow as well as the sources of mantle heterogeneity since the process effectively removes biases inherent to pure seismically-derived models. We conclude that temperature variations likely dominate shear-wave and density heterogeneity in the noncratonic mantle. However, notable compositional anomalies are detected, most strongly within the African superplume structures [Simmons et al. 2006, 2007, 2009]. Time-dependent flow calculations from the jointly-derived density models provide evidence that the (usually) minor compositional anomalies play an important dynamic role, particularly beneath the African plate. The static density models have also been used in dynamic flow calculations that predict anomalous flow patterns that coincide with known tectonic features including the New Madrid Seismic Zone [Forte et al., 2007], the Colorado Plateau [Moucha et al., 2008], and several features within the African plate [Forte et al., 2010]. Collectively, these observations lend support to the validity of jointly-derived images of mantle heterogeneity.
</p><p>References
</p><p>Forte, A. M., N. A. Simmons, R. Moucha, S. P. Grand, and J. X. Mitrovica, 2007, Descent of the ancient Farallon slab drives localized mantle flow below the New Madrid seismic zone, Geophys. Res. Lett., 34, doi: 10.1029/2006GL027895.
</p><p>Moucha, R., A. M. Forte, D. B. Rowley, J. X. Mitrovica, N. A. Simmons, and S. P. Grand, 2008. Mantle convection and the recent evolution of the Colorado Plateau and the Rio Grande Rift valley, Geology, 36, 439-442, doi:10.1130/G24577A.1.
</p><p>Simmons, N., A. Forte, and S. P. Grand, 2006. Constraining mantle flow with seismic and geodynamic data: A joint approach, Earth Planet. Sci. Lett., 246, 109-124, doi:10.1016/j.epsl.2006.04.003.
</p><p>
Simmons, N., A. Forte, and S. P. Grand, 2007, Thermochemical structure and components of the African superplume, Geophys. Res. Lett., 34, doi:10.1029/2006GL028009
</p><p>Simmons, N., A. Forte, and S. P. Grand, 2009, Joint seismic, geodynamic and mineral physical constraints on three-dimensional mantle heterogeneity: Implications for the relative importance of thermal versus compositional heterogeneity, Geophys. J. Int., 177, 1284-1304, doi:10.1111/j.1365-246X.2009.04133.x
</p><p>Acknowledgements: This work was supported by NSF grant EAR0309189.</p>

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