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 modelling 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.