Crustal and lithospheric mantle thinning variations beneath the East African Rift System

Knowledge gaps remain regarding strain localization mechanisms that may initiate the transition to late-phase rifting and break-up along active rifts hosted in thick continental lithosphere. Here, we conduct a multiscale study of the East African Rift System (EARS) using one of the highest-resolution seismic tomography models of Africa, along with a comprehensive compilation of lithospheric structure models from active- and passive-source seismic studies. We estimate mantle temperatures and the lithosphere-asthenosphere boundary using this model. Combined with estimated crustal thicknesses, we estimate the thinning factors of the crust and lithospheric mantle, as well as the deformation decoupling parameter, defined as the ratio of the lithospheric mantle to the crustal thinning factors. Our results reveal intriguing magnitudes of lithospheric mantle necking beneath both the eastern (magma-rich) and western (magma-poor) branches. However, the eastern branch ubiquitously exhibits commensurate crustal and lithospheric mantle thinning, whereas the western branch exhibits a complex mix of segments with coupled-thinning and those with significantly decoupled thinning. We note the key influence of inheritance, lithospheric foundering and delamination, and melt-induced ductile strain on decoupling of lithospheric thinning. The dearth of necked crust and the common occurrence of necked lithospheric mantle along the EARS suggest that necking of the lithospheric mantle probably precedes crustal necking along evolving rifts, which, where observed, manifest a weakened lithosphere primed to progressively transition to the later phases of continental extension.