Understanding Landscape Exhumation from Apatite He Age Data Dispersion: An Example from the Ethiopian Plateau, East Africa

Abstract:

The Blue Nile, a major tributary of the Nile River, extensively incised the long-wavelength Ethiopian Plateau developed within the East African Rift System, exposing various rock formations ranging in age from Neoproterozoic to Recent. The 1.6 km deep Blue Nile Canyon, the only deep and extensive canyon in the greater Nile drainage basin, characterizes the plateau, which has an average elevation of 2.5 km. The time line of the plateau uplift is still not well understood, and has been highly contested, with some suggesting early (before 20 Ma) and others arguing late (after 10 Ma) rise of the plateau. A recent modeling of deep-mantle convection beneath the African continent showed that the Ethiopian Plateau started to rise rapidly around 10 Ma. Quantifying the incision history of the Ethiopian Plateau is critical in unraveling the causes and timing of plateau’s elevation gain.

Low temperature thermochronology provides a rigorous analytical tool in determining the cooling history to reconstruct canyon incision. From apatite (U-Th)/He ages measured from Neoproterozoic basement rocks and Mesozoic sedimentary strata exposed along the valley wall of the Blue Nile canyon, we investigate rock cooling history of the Ethiopian Plateau induced by this canyon incision. Although our apatite He dates show strong data dispersion, both within and among samples and ranging from 58 to 460 Ma, these ages positively correlate with apatite effective uranium concentrations. Hence, this age dispersion is likely linked to changes in helium retention and diffusivity related to differential accumulation of radiation damage. Our inverse thermal model simulations determine the best-fit cooling history that shows the onset of major canyon incision after 30 Ma with regional exhumation accelerated after 15 Ma. These observations are also supported by our quantitative geomorphic data extracted from the Blue Nile tributaries that correlates with mantle tomography data.