Along‐Axis Variations of Rift Width in a Coupled Lithosphere‐Mantle System, Application to East Africa

Narrow and wide rifts are end‐member expressions of continental extension. Within the framework of passive rifting, the transition from wide to narrow rifts requires lowering the geothermal gradient. Reconciling this view with observational evidence for narrow rift zones in regions underlain by sublithospheric hot plume material, such as the eastern branch of the East African Rift, requires invoking preexisting weak zones for strain to localize in a warm crust. Based on thermomechanical numerical models, we show that along‐rift width variations can develop spontaneously as a consequence of spatial variations of the geotherm over an evolving mantle plume impinging a lithosphere subjected to ultraslow extension. The eastern branch of the East African Rift, with a narrow Kenya segment underlain by a mantle plume head and widening to the north and south in the colder regions of the Turkana depression and North Tanzania divergence, is in agreement with this numerical prediction.

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