Study of the western edge of the African large low shear velocity province

It is well established that there is a large low shear velocity province (LLSVP) in the lowermost mantle beneath Africa, extending from beneath the southeastern Atlantic Ocean to the southwestern Indian Ocean. The detailed 3-D geometry of the LLSVP is crucial to the understanding of how this prominent lower mantle feature developed and evolved. Most studies have concentrated on mapping the southern and eastern edges of African LLSVP using the Kaapvaal array at South Africa. Here we use data from recently deployed arrays in the western Mediterranean to study its northwestern edge and evaluate the sharpness of its boundaries. Travel time and waveform modeling of S and SKS phases suggest the existence of sharp low velocity anomalies in the lowermost mantle beneath the eastern Atlantic Ocean, which agrees with global tomography models. However, the S-SKS differential times vary up to 6 s across the array. To match the large travel time variations and waveforms, the existence of a slow velocity structure with a sharp top is required. The structure has a 3.5% reduction in shear wave velocity and a height of ∼600 km above the core-mantle boundary, which is lower in topography than the southern and eastern part of the African LLSVP. Further 3-D synthetic waveforms and modeling calculations demonstrate the existence of these sharp boundaries for the northwestern portion of the African LLSVP. The strong lateral variation in both travel times and waveforms suggests that this part of the African LLSVP may be as complex as the mid-Pacific LLSVP. To explain the observed sharp top and large dome-like structure, a thermochemical origin of the African LLSVP is supported. Key Points The geometry of the western edge of the African LLSVP was developed A sharp top exists with the velocity gradient larger than 3.5% per 200 km The observations support that the African LLSVP is thermal-chemical