Rift nucleation, rift propagation and the creation of basement micro-plates within active rifts

In this contribution we study the dynamics of rift nucleation and the interaction of propagating rift segments. We use the East African Rift System (EARS) as a case study with special emphasis on the Albertine rift system situated within the western branch of the EARS with the 5000 m high Rwenzori horst, a basement block that was captured by two rift segments. Fieldwork in the Rwenzori mountains shows that the range is not only affected by range parallel faults but that major fault sets cut across the Rwenzoris. In order to understand these complex fault patterns and the dynamics of the process that leads to the capturing of basement blocks within the extending rift system we developed a two-dimensional visco-elasto-plastic spring model. The model shows that propagating rift faults commonly capture micro-plates with shapes similar to the Rwenzori mountains. These micro-plates rotate during successive rift opening in a clockwise or anti-clockwise sense depending on the direction of the rift segment overstep. A left-lateral overstep as is the case for the Albertine rift system produces a clockwise rotation of the associated micro-plate indicating that the Rwenzori mountains rotate clockwise. During the rotation of the micro-plate the local stress field at the rift segment tips is perturbed so that the rifts turn 70 to 90° and propagate towards each other to capture the plates completely. This perturbed stress field produces an extensional fault set that is oriented almost parallel to the far field extension direction and explains the range cutting faults that we find in the Rwenzori mountains. The large-scale geometry of the rift system seems to be strongly controlled by rigid cratons in the lithosphere leading to a curvature of the western branch of the EARS and a sinistral wrench component in the lake Albert rift north of the Rwenzori mountains. Capturing of micro-plates of various sizes within the rift system is a common phenomenon and leads to strong perturbations of the local stress field during micro-plate rotation. The resulting structures are normal and strike-slip faults that cut through the micro-plates and local reverse faults.