Lithospheric Imaging in the West African Craton Using Receiver Function Modelling

Abstract

The lithospheric structure beneath the west African craton (WAC) is presented in this study to understand the oldest parts of the continent. The shear velocity structure, associated composition (felsic/intermediate/mafic) and nature of crust transition (sharp or flat) at depth provide the link between the age these oldest parts of the continents formed and reworked in, formulating models of their evolution. This study estimates the crust and uppermost mantle velocity structure using joint inversion of the Rayleigh wave group velocity dispersion and receiver functions data calculated from 8 broadband seismic stations. The results show a significant variation of crustal properties in the Precambrian WAC. The shear wave velocity (Vs) at depth reveals a 42–47 km thick crust of felsic-to-intermediate composition near the boundary of the Precambrian old lithosphere and ~ 39 km thick felsic crust in the cratonic interior. The thick crust near the margins is also synchronous with a thick (~ 10–21 km) lower crust layer with high Vs (4.0–4.3 km/s). Contrarily, the thin crust accommodates a thin (~ 4–6 km) high Vs lower crust layer in the cratonic interior. This high Vs layer is often interpreted as the laminated base of the crust, providing insight into the thickness of the Precambrian lithosphere. Its presence as a thick layer at the base in the Proterozoic crust, or its absence (or thinning) in the Archean crust, is linked with the evolution of the continents. This high-velocity base is dissolved, reworked and delaminated over time, forming a thin felsic stabilised crust. We also observed higher uppermost mantle Vs in WAC, similar to the other Precambrian cratons (≥ 4.5 km/s).