P- and S-wave boundaries of the north-western African Large Low Shear Velocity Province.

Abstract:

Tomographic images of Earth's lowermost mantle are dominated by two large, nearly antipodal regions of strongly reduced seismic velocities. These areas, known as the Large Low Shear Velocity Provinces (LLSVPs), are located beneath the Pacific and Africa. This study focusses on the African LLSVP and mapping its northern and western boundaries using P- and S-waves and aims to infer the structural changes across this boundary. Tomographic models agree on the location and large-scale features of the lateral boundaries of the African LLSVP. However how to define the boundary of a LLSVP, commonly interpreted as a thermochemical pile, is still debated. Different tomographic models also disagree on the westward extent of the African LLSVP and its connection to the potential source region of the Icelandic hotspot. We analyse data from deep earthquakes in South America ( 5.5 magnitude and 100km depth) collected at broadband receivers across Africa and Europe to provide extensive coverage of the African LLSVP beneath the Atlantic ocean. The data-set provides the largest lateral coverage of the expected north-western boundary. Using P_diff and PcP travel-time residuals we develop a model of the position of the western and north-western boundaries of the African LLSVP and detect smaller scale features located in the vicinity of the boundary. P_diff results combined with P and PcP data allow us to constrain the height of the African LLSVP and the sharpness of the boundaries, allowing better characterisation of this dominant feature of the lowermost mantle.