Role of Metasomatism in the Development of the East African Rift at the Northern Tanzanian Divergence: Insights From 3D Magnetotelluric Modeling

The Northern Tanzanian Divergence in the East Africa Rift is arguably the best place on Earth to study the controls on rifting of thick lithosphere. Here, where the East Africa Rift intersects the Tanzanian Craton and the Mozambique Belt, the relationships between volcanism, faulting, pre-existing structures and lithospheric thickness and composition can be observed. In this work, we carry out the first lithospheric-scale 3D magnetotelluric modeling of the Northern Tanzanian Divergence and combine the results with experimental electrical conductivity and petrology models to calculate mantle composition, which is also inferred in the craton from reanalysis of garnet xenocryst data. Our results show that metasomatic materials exist in the cratonic lithospheric mantle and the relatively undeveloped southern part of the rift zone. However, the lithospheric mantle of the Mozambique Belt and the more developed northern section of the rift is more resistive and does not contain metasomatic phases. Combined with geochemical data from erupted lavas, these results suggest that, in zones that have experienced voluminous Cenozoic magmatism, melting events have destroyed the metasomes and dehydrated the mantle. Since the presence of magma is a primary control of lithospheric strength, rifting may become limited as the lithospheric mantle becomes dehydrated and harder to melt. The motion of tectonic plates relies on a specific set of physical conditions. Continental breakup or rifting occurs when certain parts of the lithosphere are weak, and when stress applied to these regions is sufficient. Weaknesses in the lithosphere rely on its composition and pre-existing structures. We can image and analyze these features using the magnetotelluric method, a geophysical technique that maps electrical conductivity variations within the Earth. Our results show that compositionally weakening agents (metasomes) play an essential role in the development of the rift by making the mantle easier to melt. We also image some portions of the rift that do not contain such agents, suggesting that melts may have dried out these parts of the lithosphere, leaving a dry and resistive residue. This situation may indicate that melting in the region might be limited in the long run due to the absence of these materials. 3D magnetotelluric models of North Tanzanian Divergence are converted to water in mantle models to map metasomatism in the region Melting events in the Mozambique Belt caused metasomes to be destroyed and the lithospheric mantle to be dehydrated The rifting in the region might be limited if there is no supply of metasomatic material toward the rift zone