Cratons host the oldest rocks on the Earth, they described as regions with no deformation for a long period of time, cold and thick lithosphere (up to 200 km), depleted lithospheric mantle with a dry cratonic root. Recent studies from geochemistry and geophysical observations show that cratonic mantle can experiment modification (metasomatism, delamination) and the evolution of the lithospheric mantle is more complex than we generally think. Here we show recent results from a multi-observable thermochemical tomography (MQ_MTT_2020) across Central and southern Africa. This joint inversion uses geophysical and geochemical datasets,to constrain the present-day thermochemical structure (temperature and major elements composition) of the lithospheric and sub-lithospheric mantle. Knowledge of the physical state and present-day thermochemical structure of the mantle is key to understanding the relationships between internal Earth dynamics, surface observables (e.g. topography, gravity) and the location of mineral and energy resources. Central and southern Africa is a geologically intriguing region made up of several cratonic blocks (Kalahari, Tanzanian and Congo) surrounding by orogenic belts. Our model provides a high-resolution model of the thermochemical structure on this continental scale in comparison with models derived from seismological observations. More than the simple observations lithospheric thickness versus hot spot tracks and volcanism, our model show that main cratonic domains (Kalahari, Tanzanian and Congo) know different chemical evolutions. In this presentation, we will discuss number of robust features that carry important implications for supporting or disproving current evolutionary models for this region and Precambrian shields.
Plain text summary
In this presentation, we discuss our latest results about cratonic mantle properties across Central and Southern Africa. Cratons are well-knowns hosting the oldest rock on the Earth and generally show thick and cold lithosphere (> 200 km) with a depleted composition (higher magnesium number used as a proxy). In the last 20 years, several studies revise the evolution of cratonic mantle. Here we looked at the thermochemical structure of the lithospheric and sub-lithospheric mantle by using a joint modelling scheme. Our Joint inversion approach specifically designed to retrieve the physical state (e.g. temperature distribution, compositional structure) inverting key datasets with complementary sensitivities to the main fields of interest. To link geophysical observations and geodynamics, we need to have an idea of the physical properties of mantle minerals. A full thermodynamic model and an internally-consistent thermodynamic database are used to guaranteeing that our models will not violate fundamental thermodynamic relations. More details on the modelling can be found in the two papers by Afonso et al. (2013) about 3D multi-observables probabilistic inversion. To explore the model space representative of geophysical observable and sensitive to the thermochemical structure, the joint inversion scheme used here requires three kinds of datasets. First, surface observable (elevation, surface heat flow, geoid) extract from global models and require to solve 1D isostatic and heat transfer problems. Then seismic observable (crustal models: layering thickness, velocity, dispersion curves: sensitivity to velocity variations) extracted from global models. To address the problem of non-uniqueness of compositional space, datasets of mantle samples used here as prior information. We generally represent the chemical composition of the mantle with five major elements representing in term of oxides (SiO2, Al2O3, FeO, CaO, MgO) and we consider here than the mantle is a peridotite. In Central and southern Africa, we can identify three cratonic domain. Kalahari craton (Kapvaal in South Africa+ Zimbabwe), is one of the most studies in the world and is generally used a reference in the study of cratons. You need to keep in mind that the main interest in the cratonic domain is mainly the presence of diamantiferous pipes and potential relationship with kimberlitic volcanism. The second most cratonic domain studied in Africa is the Tanzania craton located in the East Africa rift region with continental rifting. Finally, we have the Congo craton, one of the largest cratonic domain in Africa but also one of the less studied. Our thermochemical tomography allows us to build a 3D model of the thermal structure of the mantle beneath central and southern Africa. We can see, for example, the asthenospheric upwelling beneath the Tanzania craton. A comparison of geotherms between mantle xenolith (a piece of the mantle) and geotherms derived from our thermochemical tomograph show that the present-day thermal structure can be different as the thermal recording by the mantle sampled during a kimberlitic eruption. Our tomography allows us to quantify the lateral chemical composition variation (magnesium number used as a proxy). We can observe that if Kahalari and Tanzania depleted as expected, the Congo craton show a more diffuse signal. This difference suggests that another process is mapped in the thermochemical structure.
A journey in cratonic mantle: example of Central and Southern Africa