Morphology of Large- and Meso-scale structures in the Mid and Lower Mantle

Abstract:

Seismic tomography tells us that the large-scale structure of the lower mantle is dominated by a pair of Large Low Shear Velocity Provinces (LLSVPs), and contains regions of faster-than-average velocities associated with past subduction. These structures are thought to influence and be influenced by mantle convection, and are linked to the introduction/creation, survival, and destruction of chemical heterogeneity in the deep interior. In fact, while sinking slabs are ipso facto associated with compositional variations, the laterally abrupt velocity changes across the boundaries of the LLSVPs – among other lines of evidence – indicate that the LLSVPs also represent a domain compositionally distinct from ambient mantle. The discovery of meso-scales structures, seismically similar to the LLSVPs and exemplified by the Perm Anomaly, suggests that compositional variations in the lower mantle might be tomographically discernible even at smaller length scales. Quantifying the morphology and spatial distribution of these seismically imaged structures provides important clues to the chemical heterogeneity and dynamical state of the Earth’s deep interior.

In this presentation, we determine the morphology of the slower-than-average, faster-than-average, and neutral seismic regions, in order to provide quantitative estimates of the locations, volumes, and shapes of lower mantle structures, including, but not limited to the African and Pacific LLSVPs. We do so by carrying out a cluster analysis of VS profiles with a sliding window in depth, which allows us to map out the three-dimensional morphology of the LLSVPs that is consistent across tomographic models. Based on these analyses, we estimate the volume of the LLSVPs to be 5-9% of the mantle, which is several times larger than past estimates. We then catalog the broad range of boundary geometries for the LLSVPs and compare them to results from high-resolution waveform studies, where available. Finally, we identify meso-scale structures within and outside the LLSVPs, and demonstrate their effects on seismic waveforms.