TIMING OF THE BLUE NILE CANYON INCISION: AN UNDERSTANDING FROM LOW-TEMPERATURE THERMOCHRONOLOGY FROM NORTHERN AND SOUTHERN SECTIONS OF THE RIVER

Despite being the deepest and most extensive canyon in the greater Nile drainage basin in eastern Africa, the timing of the Blue Nile canyon incision of the Ethiopian Plateau is poorly understood. For the last few decades, this timing has been a topic of scientific interest yet controversial due to the plateau’s complicated geologic setting and thermal response to surface topographic changes. The area was affected by a number of tectonic events namely Afar mantle plume, elevated landscape, continental rifting and sedimentation, flood basalt eruption, and shield and strombolian-type volcanisms.

To tease out the near-surface cooling history and incision timing of the Blue Nile, this study integrates and compares new apatite (U-Th)/He thermochronometric data from the southern section of the Blue Nile Canyon with our previous cooling age data collected from the northern section of the canyon. Similar to the northern section, we acquired single-grain apatite (U-Th)/He ages from Neoproterozoic crystalline basement rocks from the southern section. Our results reveal old cooling ages ranging from 56 to 460 Ma for a total of twelve samples collected from both sections. Interpretation of the canyon incision history from these cooling ages is complicated by the complex geologic history and scatter in single-grain ages. We address these complications by analyzing age-elevation relation, age-eU (effective Uranium) correlation and eU-grain radius relation to comprehend radiation damage effect that is likely linked to the variations in helium retentivity. Thermal modeling of these cooling ages yields a young incision where the onset of rapid erosion started at late Miocene, which is younger than the previously published, thermochronology-derived incision ages of the same area. Thus, this study brings fresh a perspective on the topographic evolution and the incision history of the Ethiopian Plateau.