Clustering of Lower Mantle Structure – Perm Anomaly

Cluster analysis of shear velocity (Vs) profiles in the lower mantle shows that profiles can be grouped into two families: a slow one comprising the two large low shear velocity provinces (LLSVPs, a.k.a. “superplumes”), and a geographically continuous cluster where velocities are faster than average. We summarize the results of this clustering using a “vote map” which assigns to each location on Earth a count of in how many tomographic models the point is assigned by cluster analysis to the slow cluster. This map is shown below. The clustering data for the entire lower mantle can be downloaded here: Clustering_LM

There is a single clear exception to this striking dichotomy: a slow velocity anomaly is found in all recent global tomographic Vs models, approximately centered beneath the city of Perm, Russia. Waveform modeling shows that this anomaly has Vs ~6% slower than average, is 900 km across and ~400 km tall. It is bounded by sharp lateral gradients in velocity, much like a superplume. It represents the first member of meso-scale structure in the lowermost mantle, intermediate between the LLSVPs and the much smaller ultra low velocity zones (ULVZs).