Investigating the Kinematics and Dynamics of Strain Localization in the Northern Western Branch of the East African Rift System

Abstract

Continental rifts are thought to be formed by repeated episodes of magmatism, reactivation of pre-existing structures, or a combination of both. Continental rifts are evidence of a divergent plate boundary, where tectonic plates are breaking apart. The northern Western Branch of the East African Rift System (EARS) is an active continental rift that forms part of the divergent plate boundary between the Nubian plate and Victoria microplate, comprising magma-rich Lakes George-Edward graben in southwestern Uganda and the magma-poor Albertine and Rhino grabens in northwestern Uganda. The physics of strain localization in the northern Western Branch is not well understood. This PhD study investigated the kinematics and dynamics of strain localization in the northern Western Branch through two projects: 1) investigating the contribution of deep melt to rifting generated from two small-scale convection mechanisms beneath the northern Western Branch using the finite element software Advanced Solver for Planetary Evolution, Convection, and Tectonics (ASPECT); and 2) calculating the present-day Victoria microplate motion, strain rates, and geodetic fault slip rates to assess deformation zones using Global Navigation Satellite System (GNSS) data from within the Victoria microplate and geodetic inversion modeling with TDEFNODE software. Modeling results from project 1 indicate that rifting in the northern Western Branch is likely influenced by deep melt that migrates northward from the Kivu Rift and deep melt that migrates westward along the Aswa shear zone from the Kenyan Rift. Regarding project 2, we find that the Victoria microplate is rotating counterclockwise at 0.0623±0.0293 deg/My, consistent with previous studies, but with the Euler pole ~376 km northwest of earlier work. Calculated strain rates along the Victoria microplate boundaries range from 7.6×10⁻⁸ to 1.36×10⁻⁷ y-1, while calculated fault slip rates along the eastern NWB faults range between 1.93 and 2.34 mm/y. Consistent with geologic observations of slip directions, our kinematic model indicates that Victoria Plate rotation is accommodated, in part, by oblique-slip on northern Western Branch border faults.