Birth of the East African Rift System: Nucleation of magmatism and strain in the Turkana Depression

Abstract

© 2019 Geological Society of America. The Turkana Depression of northern Kenya and southern Ethiopia contains voluminous plume-related basalts that mark the onset of the Paleogene-recent East African Rift System (EARS) at ca. 45 Ma. Thus, the Turkana Depression is crucial to understanding the inception of intracontinental rifting. However, the precise chronology of early rift-basin formation in Turkana is poorly constrained. We present apatite fission-track and (U-Th-Sm)/He thermochronology data from basement rocks from the margins of the north-south-trending Lokichar Basin that constrain the onset of rift-related cooling. Thermal history modeling of these data documents pronounced Eocene to Miocene denudational cooling of the basinbounding Lokichar fault footwall. These results, along with ~7 km of Paleogene to middle Miocene syn-rift strata preserved in the Lokichar fault hanging wall, suggest that formation of the Lokichar Basin began as early as ca. 45-40 Ma. Preexisting lithospheric heterogeneities inherited from earlier Mesozoic rifting and Eocene plume magmatism likely facilitated the broadly concurrent nucleation of strain in the Turkana Depression, up to ~15 m.y. earlier than EARS initiation elsewhere. Late Paleogene extension in the Lokichar Basin and other parts of Turkana significantly predate the Miocene creation of pronounced plume-related topography in East Africa, suggesting that other mechanism(s), such as far-field stresses or mantle basal drag, likely played a critical role during EARS inception.