Structural evolution of continental rifting, quantitative analysis of fault populations: insights from the central Kenya rift, central and north main Ethiopian rift

The aim of the current research is to provide new insights into the evolution of normal faults and their implications on continental rift evolution in order to inform our understanding of the north to south rift propagation of the east African rift system (EARS). Therefore, three different rift segments representing different stages of continental rifting have been selected along the EARS, namely; the central Kenya rift representing an early stage of continental rifting, the central main Ethiopian rift (CMER) as an intermediate stage and the northern Ethiopian rift (NMER) exemplifying a late stage. Digital Elevation Model DEM with 30m horizontal resolution and Google Earth images are the main data used for this study. A large dataset of fault size attributes of the present-day fault geometry was built from 2130 faults scarps that were identified and manually mapped from DEM surface of the three areas, in order to conduct a quantitative and statistical analysis on scaling relations of fault populations, spatial strain distribution and to assess the role of pre-existing structures on fault development. Estimations of extensional strain along the three rift segments revealed a general progressive increase of strain from south to north. In the central Kenya rift, fault length and throw populations exhibited a power-law distribution, the fractal dimension of fault throw populations showed a decrease with increasing strain, while the fractal dimension of fault length populations remained almost constant, which may imply that the fault system in this central Kenya rift develop in accordance to near constant length fault growth model. Analysis of the spatial distribution of strain exhibits a rather distributed faulting deformation in the southernmost part of the central Kenya rift, and more localized deformation on rift flanks towards the north. As for the central main Ethiopian rift (CMER) and the northern main Ethiopian rift (NMER), the cumulative distributions for length and displacement populations fit to both exponential and log-normal functions, which is a function of limited crustal layers thickness. The observed decrease of average fault lengths and increasing average fault throws and D/L ratio in the NMER as opposed to the CMER may have occurred after reaching the maximum fault length at an earlier stage of development, which would again be in line with the coherent fault model. The domain of deformation moved from being rather localized at and near rift margins in the southern segment (CMER) to a more distributed domain of faulting deformation across the northern rift segment (NMER). This is in contrast to domains of deformation observed in the central Kenya rift as we move from south to north. The possible influence of the underlying Precambrian basement structures on the orientation of Cenozoic fault on the surface has been investigated and discussed in the light of existing experimental models, and that suggests the presence of basement influence in the central Kenya rift, whereas such influence is not as evident within the CMER and NMER segments. Generally, in the present thesis, the quantitative and statistical analysis of scaling properties of fault populations in the EARS demonstrates how these properties can provide new insights into the evolution of different stages of continental rifting.