Cluster analysis of global lower mantle tomography: A new class of structure and implications for chemical heterogeneity
Earth's lower mantle is dominated by a pair of antipodal large low shear velocity provinces (LLSVPs) that reach >1000 km up from the core-mantle boundary (CMB). These are separated by a ring of faster-than-average velocities thought to be related to subduction of oceanic lithosphere. How robustly does global tomography constrain velocity structure in the lower mantle, and are there other robust large scale features that have not been identified? We use cluster analysis to identify structures and seismic characteristics common to a set of recent global tomographic models which have been derived using different data sets, parameterizations, and theory behind approximations used in inversion. We detect a pronounced asymmetry in the velocity gradient with depth between seismically fast and slow regions in the lowermost 500 km of the mantle, suggesting the presence of compositional heterogeneity. We find that, in all models, there is a clear separation of lower mantle structure into one fast and two slow regions, and that the boundary of the regions is remarkably similar across models even on length scales as small as <1000 km. This inter-model similarity indicates that long wavelength features are not a consequence of lack of fine-scale resolution, but that they truly dominate the structure in the lowermost mantle. There is a single exception to this separation: an isolated slow anomaly ̃900 km across (at the CMB) and extending ̃500 km upward from the core-mantle boundary, which we call the "Perm Anomaly". Though it is far smaller than an LLSVP, waveform analysis confirms that this anomaly is robustly constrained and bounded by rapid lateral velocity gradients like those found around LLSVPs, suggesting that the nature and process of formation of both types of structures may be related.