FAULT ZONE ARCHITECTURE AND IMPLICATIONS OF 2-D OBSERVATIONS ALONG THE BLUE NILE CANYON, ETHIOPIAN PLATEAU, EAST AFRICAN RIFT SYSTEM

Mantle plume activity, and isostatic rebound related to the extension of the East African Rift System, resulted in ~2 km of uplift over the past 30 Ma, creating the Ethiopian Plateau. A major tributary of the Nile River, the Blue Nile, simultaneously incised the plateau to form the Blue Nile Canyon, and exposed the Neoproterozoic basement, Mesozoic sedimentary rocks and Cenozoic to Quaternary volcanic rocks along the gorge.

Using Caine’s et al. (1996) approach for quantifying fluid flow potential in fault zones, two field surveys were conducted along the Blue Nile’s southwestern bend in 2004 and 2016. Field data was collected from the canyon walls based on damage zone and total fault zone widths. By dividing these variables, a mean architectural index (F_m) was determined for each fault. 26 faults from the first location had an average F_m value of 0.79, while the second location had a mean architectural index of 0.91. F_m values calculated in this study are categorized as combined conduit-barrier structures, inclined towards conductivity.

Lower hemisphere equal area stereographic projections and MOVE software were also used to plot strike and dip locations of these faults on the basis of lithologic units, explaining the effect of tectonic stresses on rock permeability. The results of this study point towards high permeability potential in the Jurassic limestone and sandstone units, and less potential in the Cenozoic volcanic and Neoproterozoic basement rocks.

The positive trending permeability potential based on preliminary field observations is expected for a heavily eroded extensional setting. The F_m values can be applied to understand the extensional tectonics, as well as watershed character of the Blue Nile. High subsurface conductivity can be applied to hydrocarbon and groundwater resourcing as well.

Although a 2-d outcrop analysis of the Blue Nile's incised walls provides highly positive results, the 3-d nature of the faults and damage zones must be considered to understand subsurface structures of these zones. By considering the mechanics of normal faulting and damage zones in sedimentary rocks, the F_m analysis of the fault zones has proven a viable preliminary study when evaluating permeability in rift settings.