Normal fault reactivation during multiphase extension: Analogue models and application to the Turkana depression, East Africa

We present crustal scale physical analogue models of multiphase rifting to provide new constraints on the role exerted by faults formed during early extension on structures developed during later episodes of rifting. Our laboratory experiments (sand mixture is used to simulate the upper crust and PDMS-Corundum mixture is used to simulate the lower crust) are inspired by the Turkana depression in the East African Rift System, a natural case of rift zone developed through multiple extensional phases. The models reproduce a first rifting phase with NE-SW extension direction followed by a phase of W-E trending extension. In the experiments, we varied the amount of extension during the initial phase, which caused variations in the development and prominence of sets NW-SE normal faults, representing pre-existing faults to be potentially reactivated during the later rifting phase. Our model results indicate that the amount of extension in the early phase is critical in controlling the reactivation: the greater the amount of extension in the 1st-phase, the more important early faults are, and the easier these 1st-phase normal faults are reactivated during the 2nd-phase. At a local scale, fault reactivation may result in faults with a typical zigzag pattern, whose importance increases with the increase in the amount of extension during the early phase. Extrapolation of model results to the Turkana depression suggests that large-scale fault reactivation is unlikely to have occurred in the region, possibly implying that the Cretaceous-Early Paleogene extension was limited in the central part of Turkana depression. Results have been also extrapolated to other regions interested by multiphase rifting, such as basins in the North Sea rift.