Reactivation or non-reactivation? Analogue models of multiphase rifting and applications to the Turkana depression, East Africa

Abstract

The Turkana depression (Ethiopia-Kenya) is a tectonic basin related to the Cretaceous-Early Cenozoic development of the South Sudan and the Anza grabens filled with a thick sequence of Cretaceous-Paleogene sediments reaching up to a thickness of 6–8 km. These two NW-SE trending depressions, which likely resulted from NE-SW extension, were later affected by W-E extension related to the Cenozoic East African Rift System. The influence of crustal thinning related to these NW-SE grabens on later W-E-related extension is testified by the marked change in style of deformation from the narrow rift valleys in Kenya and Ethiopia, to a distributed, basin-and-range-style faulting in the Turkana depression. Despite some local scale reactivation is visible, large scale reactivation of pre-existing NW-SE structures in the Turkana depression is not obvious, as it is extensively masked by the sedimentary and volcanic cover; consequently, contrasting hypothesis on the possible role exerted by discrete pre-existing fabrics have been proposed in the literature. To address this controversy, we performed analogue models to investigate whether inherited structures, largely obscured by sediments in the Turkana depression, might have been reactivated during subsequent tectonic phases. We run 2-layer, brittle-ductile models deformed in two successive phases: a first phase of NE-SW extension, followed by W-E extension. Different models were subjected to different amount of bulk extension during the first phase to investigate the influence of this parameter (and the importance of first-phase structures) on later reactivation. Our models indicate that the amount of deformation in the initial tectonic phase is key for structure reactivation in subsequent tectonic phases: the larger the deformation in the first phase, the higher the probability of reactivation. Comparison of the experimental results with nature suggest that, despite some local fault reactivation, large-scale structures were likely not reactivated in the Turkana depression. These outcomes represent a useful tool to decipher fault reactivation, not only in the Turkana depression but also in tectonic basins worldwide, especially where thick sedimentary covers may mask tectonic structures.