Continental rifting initiates the transition to breakup when the crust is necked and deformation localizes at the rift axis. However, the slow crustal-stretching rates and >20-km deep mantle beneath many active rifts worldwide suggest that present-day breakup may not be imminent. High-resolution seismic data from the Turkana Rift Zone of the East African Rift System (EARS) reveal the rift’s subsurface structure. Here, we show that Turkana’s crystalline crust has thinned to ~13 km along the rift axis, revealing an active rift undergoing crustal necking. Onset of necking is constrained to ~4 Ma and facilitated the accumulation of Turkana’s world-famous fossil record of human evolution. Identification of necking in the EARS indicates that eastern Africa is primed for continental breakup. This study indicates that the East African Rift System in northern Kenya may be closer to breaking apart than previously thought and that its development played an important role in the preservation of the hominin fossil record.
Rowan et al. (Thu,) studied this question.