Publication Date

Spring 2016

Advisor(s) - Committee Chair

Nahid D. Gani (Director), Aaron Celestian, Xingang Fan

Degree Program

Department of Geography and Geology

Degree Type

Master of Science

Abstract

The Ethiopian Plateau, situated in the northernmost part of the East African Rift System, has been uplifted and incised by the Blue Nile River, creating the 1.5 km-deep Blue Nile Canyon, the only extensive canyon within the Nile drainage basin. Much of this canyon incision is related to the Ethiopian Plateau uplift, which is due to a combined tectonic influence linked to the East African Rift System and the Afar Mantle Plumerelated upwelling since the Oligocene. The Blue Nile Canyon incision followed this plume-related upwelling, which resulted in extensive flood basalt emplacement around ~30 Ma. This study employs low-temperature apatite (U-Th)/He thermochronology to better understand the timeline and mode of the Blue Nile Canyon incision and the landscape geodynamics of the Ethiopian Plateau. Neoproterozoic basement rocks and Mesozoic sedimentary rock samples were collected in a vertical transect along the Blue Nile Canyon for AHe dating. Euhedral apatite grains that are inclusion-free and fractureless were selected for dating. The Australian Scientific Instrument (ASI) Alphachron and Inductively Coupled Plasma Mass Spectrometer (ICP-MS) measured the concentrations of radiogenic isotopes uranium (U), thorium (Th), and helium (He) to obtain apatite helium (AHe) dates, providing (U-Th)/He cooling ages ranging from 64 to 460 Ma. These AHe dates are much older than the 30 Ma flood basalt event, thus older than the canyon incision. In this study, a cooling/thermal history is suggested by explaining the data dispersion by the radiation damage effect in the dated apatite grains, which is related to He retention with time. The apatite suite has been subjected to protracted cooling, longer residence time in the He partial retention zone, and partial resetting due to Mesozoic rift-related burial and subsidence from deposition of ~1.2 kmthick sedimentary rocks, the extensive emplacement of ~1 km-thick flood basalt at 30 Ma, and differential incision. Radiation Damage Accumulation and Annealing Model (RDAAM) time-temperature thermal models indicate a rapid cooling event, suggesting rapid incision after 10 Ma, which is explained by an increase in uplift of the Ethiopian Plateau, likely due to mantle delamination.

Disciplines

Earth Sciences | Geochemistry | Geophysics and Seismology

Available for download on Thursday, April 11, 2019

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