Journal Article

A Thermo‐Compositional Model of the African Cratonic Lithosphere

Mooney, W. D., Kaban, M. K., Finger, N.‐P., Thomas, M., Tesauro, M.

2022

Overview

Recently, the continually increasing availability of seismic data has allowed high‐resolution imaging of lithospheric structure beneath the African cratons. In this study, S‐wave seismic tomography is combined with high resolution satellite gravity data in an integrated approach to investigate the structure of the cratonic lithosphere of Africa. A new model for the Moho depth and data on the crustal density structure is employed along with global dynamic models to calculate residual topography and mantle gravity residuals. Corrections for thermal effects of an initially juvenile mantle are estimated based on S‐wave tomography and mineral physics. Joint inversion of the residuals yields necessary compositional adjustments that allow to recalculate the thermal effects. After several iterations, we obtain a consistent model of upper mantle temperature, thermal and compositional density variations, and Mg# as a measure of depletion, as well as an improved crustal density model. Our results show that thick and cold depleted lithosphere underlies West African, northern to central eastern Congo, and Zimbabwe Cratons. However, for most of these regions, the areal extent of their depleted lithosphere differs from the respective exposed Archean shields. Meanwhile, the lithosphere of Uganda, Tanzania, most of eastern and southern Congo, and the Kaapvaal Craton is thinner, warmer, and shows little or no depletion. Furthermore, the results allow to infer that the lithosphere of the exposed Archean shields of Congo and West African cratons was depleted before the single blocks were merged into their respective cratons. Plain Language Summary Cratons are the ancient cores of continents that, with few exceptions, are underlain by a cold, strong lithospheric root with a thickness of about 250 km. The physical properties of lithospheric roots, principally temperature and composition, shed light on the origin and evolution of the most ancient portions of the Earth's lithosphere, the Precambrian cratons. We use an iterative method to process S‐wave seismic tomography and satellite gravity data to calculate the thermal and compositional state of the lithosphere. Our results reveal great diversity in the thickness and physical properties of the African lithosphere. The West African, northern Congo, and Zimbabwe cratons are underlain by relatively cold, thick and chemically depleted lithosphere. In contrast, the Uganda, Tanzania, southern Congo, and Kaapvaal cratons are warmer, thinner and have a less depleted (or non‐depleted) composition, indicating either refertilization (metasomatism) or formation in a non‐depleted state. These results document the formation of the Africa continent during the past 3.7 Ga from a diverse collection of cratons, each with a unique evolutionary history. Key Points A new Moho map was constructed from available seismic data to improve thermo‐compositional modeling of the African cratonic lithosphere Lithosphere of the West African, central to northern Congo, and Zimbabwe cratons is cold, up to 250 km thick, and chemically depleted Hot, thin (<200 km) and mostly undepleted lithosphere of Uganda, Tanzania, southern Congo, and Kaapvaal cratons indicates refertilization

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: African cratons / Composition / Corrections / Cratons / Density / Depletion / depth to Moho / Dynamic models / Gravity / Gravity data / integrated modeling / Juveniles / Lithosphere / lithosphere composition / Modelling / Moho / Physical properties / Physics / Precambrian / Resolution / Satellites / Seismic data / Seismic tomography / Seismological data / Temperature / Tomography / Upper mantle / upper mantle temperature and density