A Uniform Database of Shear Wave Splitting Measurements for the Tanzania Craton and the Adjacent Segments of the East African Rift System and Its Tectonic Implications

Abstract:

Splitting of SKS, SKKS, and PKS (“XKS”) phases is a robust tool for unraveling past and present mantle dynamic processes. Due to the diversity in the data processing procedure and the ranking criteria of the measurements employed by different research groups, significant discrepancies exist in the resulting splitting parameters obtained at the same seismic stations. Additionally, due to the steep angle of incidence of the XKS ray paths, the splitting measurements have a limited vertical resolution, leading to lasting debates regarding the geodynamic implications of the measurements. In this study, by taking advantage of recent developments in measuring techniques and anisotropy source depth estimation, and availability of recently recorded broadband seismic data, we conduct a systematic investigation of upper mantle anisotropy beneath the Archean Tanzania Craton and the surrounding segments of the Cenozoic East African Rift System. A total of 1963 pairs of measurements, including 323 from PKS, 430 from SKKS, and 1210 from SKS, were obtained at 256 stations. Among the stations only one station demonstrates a clear azimuthal dependence of the splitting parameters, suggesting that a single layer of anisotropy with a horizontal axis of symmetry is sufficient to explain the vast majority of the splitting measurements. Spatial coherency analysis of the splitting parameters led to an optimal depth of 150-200 km, indicating that the observed anisotropy is mostly from the rheologically transitional layer between the lithosphere and asthenosphere. Furthermore, systematic spatial distributions of the splitting times and the absolute difference between the fast orientations and the direction of absolute plate motion suggest that the observed anisotropy is mostly associated with APM-induced simple shear and asthenospheric flow deflected by the relatively deep root of the Tanzania Craton.