Cenozoic tectono-sedimentary evolution of the northern Turkana Depression (East African Rift System) and its significance for continental rifts

Abstract

Rifting is an emblematic facet of continental drift and its early stage (i.e., stretching phase) is pivotal to be fully characterized and understood because it then conditions all subsequent stages of the break-up cycle. Although the final architectures of rifted margins have been extensively documented and discussed over the past decades from a wide range of examples and from multiple surface and sub-surface data, the complete evolution of the stretching phase itself remains mostly illustrated from analogue and numerical models and more rarely from real case studies and field evidences. The northern Turkana Depression (Cenozoic East African Rift System) stands as a key natural example to investigate every stage of the evolution of a stretching phase, as it provides almost ∼30 Ma of rift evolution from its initiation to present-day. In this study, we combine field analyses and seismic reflection data to characterize every stage of this tectono-sedimentary evolution, with a focus on the, rarely observed, surface rupture initiation and tectonic decay. In particular, we show rifting initiates with the reactivation of non-optimal inherited structures, progresses with their abandonment during the development of long and wide grabens or half-grabens, and locally terminates with the development of a post-tectonic flexural sag, synchronously to the migration of the locus of brittle deformation to an adjacent area where starts a similar cycle. Our observations allow us to nuance commonly accepted generic rift evolution models, and to discuss possible geodynamic controls on fault evolution and strain migration. We propose a field-based alternate conceptual evolution model for rift systems sharing common characteristics with the northern Turkana Depression such as abundant inherited basement structures, thin and hot lithosphere and a low to moderate magmatism.