At equator, the Great Rift Valley is divided into two main parts, described in Wikipedia: https://en.wikipedia.org/wiki/Great_Rift_Valley. The western rift, called Albertine Rift and the eastern rift, called Gregory Rift, together span 9° from east to west along equator. The convection rolls responsible for the formations are shown here:
Each convection roll spans 1.5° from east to west, making the effects on the surface quite understandable. The rifting process is therefore subject to three upwelling sites, whereas the outer margins are marked by downwelling division lines between mantle convection rolls.
Note: A tectonic plate is 120 km thick structure, with a brittle upper part and ductile lower part. The transformation from the rigid structure of tectonic plate, to the flowing nature of mantle material takes place 120 km below the surface. Therefore, adiabatic temperature gradient is found below 120 km depth (described in: Volcanoes: A Planetary Perspective: Written by Peter Francis, 1993 Edition, Publisher: Clarendon).
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