Large- and Meso-scale Structure of Low Shear Velocity Provinces

Abstract:

Structure of the lower mantle is dominated by a pair of Large Low Shear Velocity Provinces (LLSVPs), which global shear velocity (V_S) tomographic models image with remarkable consistency. The LLSVPs are spatially associated with the locations of hotspots, eruptions of large igneous provinces, and the Earth’s rotation axis. Toward their base, they appear to be bounded by laterally sharp velocity gradients, which suggest that they are compositionally distinct from the surrounding mantle. Recently, the discovery of the Perm Anomaly – a much smaller, isolated, structure otherwise similar to an LLSVP – motivates the study of meso-scale structures in the lower mantle both within and outside the LLSVPs.

Here, we investigate how large and meso-scale structure of LSVPs varies with height above the core-mantle boundary. We do so by carrying out two re-analyses of V_S tomographic models: 1.) Cluster analysis of V_S profiles with depth, which allows us to map out the three-dimensional morphology of the LLSVPs that is consistent across tomographic models and present new estimates for their volumes; and, 2.) Analysis of the width and skewness of Vs anomaly distributions within LLSVPs at several depths, and their implications for the thickness of the thermal boundary layer, LLSVP temperature profiles, and predicted seismic attenuation. Based on these analyses, we show differences between the African and Pacific LLSVPs, identify meso-scale structures within and outside the LLSVPs, and relate these to inferences of lower mantle attenuation and anisotropy, location of Ultra-Low Velocity Zones, and locations of hotspots and large igneous provinces.