Morphology of seismically slow lower-mantle structures

Large low shear velocity provinces (LLSVPs), whose origin and dynamic implication remain enigmatic, dominate the lowermost mantle. For decades, seismologists have created increasingly detailed pictures of the LLSVPs through tomographic models constructed with different modeling methodologies, data sets, parametrizations and regularizations. Here, we extend the cluster analysis methodology of Lekic $\textit{et al.}$, to classify seismic mantle structure in five recent global shear wave speed ($\textit{V}_S$) tomographic models into three groups. By restricting the analysis to moving depth windows of the radial profiles of $\textit{V}_S$, we assess the vertical extent of features. We also show that three clusters are better than two (or four) when representing the entire lower mantle, as the boundaries of the three clusters more closely follow regions of high lateral $\textit{V}_S$ gradients. Qualitatively, we relate the anomalously slow cluster to the LLSVPs, the anomalously fast cluster to slab material entering the lower mantle and the neutral cluster to ‘background’ lower mantle material. We obtain compatible results by repeating the analysis on recent global $\textit{P}$-wave speed ($\textit{V}_P$) models, although we find less agreement across $\textit{V}_P$ models. We systematically show that the clustering results, even in detail, agree remarkably well with a wide range of local waveform studies. This suggests that the two LLSVPs consist of multiple internal anomalies with a wide variety of morphologies, including shallowly to steeply sloping, and even overhanging, boundaries. Additionally, there are indications of previously unrecognized meso-scale features, which, like the Perm anomaly, are separated from the two main LLSVPs beneath the Pacific and Africa. The observed wide variety of structure size and morphology offers a challenge to recreate in geodynamic models; potentially, the variety can result from various degrees of mixing of several compositionally distinct components. Finally, we obtain new, much larger estimates of the volume/mass occupied by LLSVPs— 8.0 per cent ±0.9 ($\mu$ ± 1$\sigma$) of whole mantle volume and 9.1 per cent ±1.0 ($\mu$ ± 1$\sigma$) of whole mantle mass—and discuss implications for associating the LLSVPs with the hidden reservoir enriched in heat producing elements.