Seismic anisotropy in the lowermost mantle near low shear velocity provinces: A comparison of the Perm Anomaly and the African LLSVP

Abstract:

The seismic velocity structure of the lower mantle is dominated by two large, nearly antipodal low shear velocity structures, known as the African and Pacific LLSVPs (Large Low Shear Velocity Provinces). Recent work on the interpretation of tomographic images of the lowermost mantle has identified a smaller region of low shear velocities beneath Eurasia, dubbed the Perm Anomaly. The nature, origin, and dynamics of these low shear velocity anomalies remain obscure; an important outstanding question is how these structures interact with mantle flow. Observations of seismic anisotropy in the lowermost mantle can help to shed light on mantle dynamics in the vicinity of low shear velocity anomalies, and recent work by several groups has produced evidence for strong anisotropy in D" along the boundaries of the African LLSVP. Here we interrogate the anisotropic structure of the lowermost mantle in the vicinity of the Perm Anomaly using SKS-SKKS splitting discrepancies measured at 18 long-running broadband stations in Europe. We selected stations for which the upper mantle anisotropy signal beneath the receiver has been well characterized and is simple enough to remove, isolating any contribution to splitting from the lowermost mantle. SK(K)S phases from earthquakes in the southwest Pacific subduction zones sample the lowermost mantle surrounding the Perm Anomaly, while phases from earthquakes in South America and Scotia sample the northernmost portion of the African LLSVP, allowing for a straightforward comparison between the two structures. Preliminary results indicate lowermost mantle anisotropy in both regions, with clear evidence for well-constrained discrepancies in SKS-SKKS splitting for the same event-station pairs measured at several different stations. Comparisons between observations of lowermost mantle anisotropy and the geometry of low shear velocity anomalies deduced from tomographic models shed light on the spatial relationship between the anomalies' edges and strong anisotropy, and on the dynamics of the mantle in and around these anomalous regions.