Mantle Anisotropy in NW Namibia From XKS Splitting: Effects of Asthenospheric Flow, Lithospheric Structures, and Magmatic Underplating

The presence of the Etendeka flood basalts in northwestern Namibia is taken as evidence for the activity of the Tristan da Cunha mantle plume during the continental breakup between Africa and South America. We investigate seismic anisotropy beneath NW Namibia by splitting analysis of core‐refracted teleseismic shear waves (XKS phases) to probe mantle flow and lithospheric deformation related to the tectonic history of the region. We present the results of the joint splitting analysis of XKS data collected from 34 onshore stations and 12 ocean‐bottom seismometers. The fast polarization directions (FPDs) are consistent with a model that combines the effects of lithospheric deformation and large‐scale mantle flow due to the NE motion of the African plate. The dominantly NNW‐SSE‐oriented FPDs in the northern part are likely caused by shallow lithospheric structures. Our observations do not show any strong evidence of a pervasive effect of the Tristan da Cunha mantle plume.

Plain Language Summary: The geology of Northwest Namibia is characterized by the presence of flood basalts, originating from magma sourced in the Earth's mantle. The source magma of these flood basalts was produced during the passage of the African plate over a mantle plume, more than 80 million years ago, contemporaneous with the onset of the breakup of the South American plate from the African plate. The role of the mantle plume in the continental breakup can be examined by a seismological technique named shear wave splitting analysis. The mantle flow induces direction‐dependent physical properties, that is, seismic anisotropy, which causes a shear wave to split into two different components traveling at different speeds. The leading component is polarized in a direction representing the direction of the flow in the earth's mantle. Except for the northern part, the polarization direction of the fast shear wave is consistent with the model of mantle flow caused by the NE motion of the African plate and deformations in the lithosphere. The results of our study do not show any direct evidence for the direct impact of the mantle plume on the mantle beneath our region of study.

Key Points: Upper mantle anisotropy beneath NW Namibia is a combined effect of the present‐day motion of the African Plate and lithospheric structures. No significant direct effect of the Tristan da Cunha mantle plume is observed in shear wave splitting measurements. Localized shearing in the lithosphere and crustal underplating are likely the main causes of the lateral variations in seismic anisotropy.

https://doi.org/10.14470/KP6443475642

https://doi.org/10.14470/1N134371

https://doi.org/10.7914/SN/IU

https://doi.org/10.18715/SKS_SPLITTING_DATABASE

https://www.geophysik.uni-frankfurt.de/64002762/Software