Discrepancies in SKS-SKKS shear wave splitting: An indicator for contributions from lowermost mantle anisotropy

 In the Earth's lowermost mantle large-scale anomalies of partly continental size are assumed to play a key role for global geodynamic processes. In this context the Large Low Shear Velocity Provinces (LLSVPs) beneath Africa and the Pacific as well as areas with faster than average seismic velocities appear as most prominent features in seismic tomography models. However, the exact extensions as well as the interactions of these massive structures with the surrounding mantle material are still under debate.
Teleseismic data of core-refracted shear waves (SKS, SKKS, PKS) recorded at the large-aperture ScanArray network was analyzed to mainly studying the anisotropic structure directly beneath Fennoscandia. However, during the routine shear wave splitting analysis distinct discrepancies in the splitting parameters for SKS-SKKS pairs of the same event were observed at several permanent and temporary stations. Since both phases have very similar raypaths in the transition zone and shallower volumes beneath the receiver, this discrepancies are strong indicators for anisotropy in the lowermost mantle. West of our network, the observed splitting discrepancies correlate well with the northern edges of the African LLSVP in around 2700 km depth and potentially mirror a connection to the more meso-scale low-velocity anomaly beneath Iceland. In contrast, east of the network the splitting observations coincide with an area beneath Siberia that is dominated by a major fast velocity anomaly. This anomaly can be clearly identified in most global tomography models and is associated with material of a deeply subducted slab. Although it is quite difficult to separate the lowermost mantle signal from contributions of shallower volumes, in this presentation I will discuss a series of strong arguments that support the assumption of a deep anisotropic source.